Patent Publication Number: US-10763590-B2

Title: Slot antenna

Description:
This is a continuation of International Application No. PCT/JP2016/083623, with an international filing date of Nov. 4, 2016, which claims priority of Japanese Patent Application No. 2015-217657 filed Nov. 5, 2015, and Japanese Patent Application No. 2016-179540 filed Sep. 14, 2016, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a slot array antenna. 
     2. Description of the Related Art 
     An antenna device including one or more antenna elements (hereinafter also referred to “radiating elements”) finds its use in various applications, e.g., radar and communication systems. In order to radiate electromagnetic waves from an antenna device, it is necessary to supply electromagnetic waves (e.g., radio-frequency signal waves) to each antenna element, from a circuit which generates electromagnetic waves (“feed”). Such feed is performed via a waveguide. A waveguide is also used to send electromagnetic waves that are received at the antenna elements to a reception circuit. 
     Conventionally, feed to an antenna device has often been achieved by using a microstrip line(s). However, in the case where the frequency of an electromagnetic wave to be transmitted or received by an antenna device is a high frequency, e.g., above 30 gigahertz (GHz), a microstrip line will incur a large dielectric loss, thus detracting from the efficiency of the antenna. Therefore, in such a radio frequency region, an alternative waveguide to replace a microstrip line is needed. 
     It is known that using a hollow waveguide, instead of a microstrip line, to feed each antenna element allows the loss to be reduced even in frequency regions exceeding 30 GHz. A hollow waveguide, also known as a hollow metallic waveguide, is a metal body having a circular or rectangular cross section. In the interior of a hollow waveguide, an electromagnetic field mode which is adapted to the shape and size of the body is created. For this reason, an electromagnetic wave is able to propagate within the body in a certain electromagnetic field mode. Since the body interior is hollow, no dielectric loss problem occurs even if the frequency of the electromagnetic wave to propagate increases. However, by using a hollow waveguide, it is difficult to dispose antenna elements with a high density, because the hollow portion of a hollow waveguide needs to have a width which is equal to or greater than a half wavelength of the electromagnetic wave to be propagated, and the body (metal wall) of the hollow waveguide itself also needs to be thick enough. 
     Patent Documents 1 to 3, and Non-Patent Documents 1 and 2 disclose waveguiding structures which guide electromagnetic waves by utilizing an artificial magnetic conductor (AMC) extending on both sides of a ridge-type waveguide.
     [Patent Document 1] International Publication No. 2010/050122   [Patent Document 2] the specification of U.S. Pat. No. 8,803,638   [Patent Document 3] European Patent Application Publication No. 1331688   [Non-Patent Document 1] Kirino et al., “A 76 GHz Multi-Layered Phased Array Antenna Using a Non-Metal Contact Metamaterial Waveguide”, IEEE Transaction on Antennas and Propagation, Vol. 60, No. 2, February 2012, pp 840-853   [Non-Patent Document 2] Kildal et al., “Local Metamaterial-Based Waveguides in Gaps Between Parallel Metal Plates”, IEEE Antennas and Wireless Propagation Letters, Vol. 8, 2009, pp 84-87   

     SUMMARY 
     The present disclosure provides a novel slot antenna which has low losses and which is capable of downsizing. 
     A slot antenna according to an implementation of the present disclosure includes: a first electrically conductive member having a first electrically conductive surface; a second electrically conductive member having a second electrically conductive surface opposing the first electrically conductive surface; a waveguide member between the first electrically conductive member and the second electrically conductive member, the waveguide member having an electrically conductive waveguide face of a stripe shape opposing the first electrically conductive surface, the waveguide member extending in a first direction along the first electrically conductive surface; and an artificial magnetic conductor extending on both sides of the waveguide member, between the first electrically conductive member and the second electrically conductive member. The first electrically conductive member has one or more slots. At least one of the slot or slots is a complex slot having a pair of vertical portions and a lateral portion that interconnects the pair of vertical portions. The lateral portion of the complex slot opposes the waveguide face, and intersects the first direction. 
     These general and specific aspects may be implemented using a system, a method, and a computer program, and any combination of systems, methods, and computer programs. 
     Additional benefits and advantages of the disclosed embodiments will be apparent from the specification and Figures. The benefits and/or advantages may be individually provided by the various embodiments and features of the specification and drawings disclosure, and need not all be provided in order to obtain one or more of the same. 
     In accordance with an embodiment of the present disclosure, a slot antenna which has low losses and which is small in size can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically showing an exemplary construction for a slot array antenna  201  including a ridge waveguide. 
         FIG. 2  is a perspective view schematically showing the construction of a slot array antenna  200  according to an illustrative embodiment of the present disclosure. 
         FIG. 3A  is a diagram schematically showing a construction of a cross section through the center of a slot  112 , taken parallel to the XZ plane. 
         FIG. 3B  is a diagram schematically showing another exemplary construction of a cross section through the center of a slot  112 , taken parallel to the XZ plane. 
         FIG. 4  is a perspective view schematically showing a slot array antenna  200 , illustrated so that the spacing between a first conductive member  110  and a second conductive member  120  is exaggerated. 
         FIG. 5  is a diagram showing an exemplary range of dimension of each member in the structure shown in  FIG. 3A . 
         FIG. 6A  is a diagram for explaining the aperture shape of a slot  112 . 
         FIG. 6B  is a diagram for explaining the aperture shape of a slot  112 . 
         FIG. 6C  is a diagram for explaining the aperture shape of a slot  112 . 
         FIG. 7A  is a perspective view showing an exemplary construction of a slot antenna  200  including horns. 
         FIG. 7B  is an upper plan view showing a first conductive member  110  and a second conductive member  120  shown in  FIG. 7A , each viewed from the +Z direction. 
         FIG. 8A  is a cross-sectional view showing an exemplary structure in which only a waveguide face  122   a , defining an upper face of the waveguide member  122 , is electrically conductive, while any portion of the waveguide member  122  other than the waveguide face  122   a  is not electrically conductive. 
         FIG. 8B  is a diagram showing a variant in which the waveguide member  122  is not formed on the second conductive member  120 . 
         FIG. 8C  is a diagram showing an exemplary structure where the second conductive member  120 , the waveguide member  122 , and each of the plurality of conductive rods  124  are composed of a dielectric surface that is coated with an electrically conductive material such as a metal. 
         FIG. 8D  is a diagram showing an exemplary structure in which dielectric layers  110   b  and  120   b  are respectively provided on the outermost surfaces of conductive members  110  and  120 , a waveguide member  122 , and conductive rods  124 . 
         FIG. 8E  is a diagram showing another exemplary structure in which dielectric layers  110   b  and  120   b  are respectively provided on the outermost surfaces of conductive members  110  and  120 , a waveguide member  122 , and conductive rods  124 . 
         FIG. 8F  is a diagram showing an example where the height of the waveguide member  122  is lower than the height of the conductive rods  124 , and a portion of a conductive surface  110   a  of the first conductive member  110  that opposes the waveguide face  122   a  protrudes toward the waveguide member  122 . 
         FIG. 8G  is a diagram showing an example where, further in the structure of  FIG. 8F , portions of the conductive surface  110   a  that oppose the conductive rods  124  protrude toward the conductive rods  124 . 
         FIG. 9A  is a diagram showing an example where a conductive surface  110   a  of the first conductive member  110  is shaped as a curved surface. 
         FIG. 9B  is a diagram showing an example where also a conductive surface  120   a  of the second conductive member  120  is shaped as a curved surface. 
         FIG. 10  is a perspective view showing an implementation where two waveguide members  122  extend in parallel upon the second conductive member  120 . 
         FIG. 11A  is an upper plan view of an array antenna including 16 slots in an array of 4 rows and 4 columns, as viewed in the Z direction. 
         FIG. 11B  is a cross-sectional view taken along line B-B in  FIG. 11A . 
         FIG. 12A  is a diagram showing a planar layout of waveguide members  122 U in a first waveguide device  100   a.    
         FIG. 12B  is a diagram showing another exemplary planar layout of waveguide members  122 U in the first waveguide device  100   a.    
         FIG. 13  is a diagram showing a planar layout of a waveguide member  122 L in a second waveguide device  100   b.    
         FIG. 14A  is a diagram showing another exemplary shape of a slot. 
         FIG. 14B  is a diagram showing another exemplary shape of a slot. 
         FIG. 14C  is a diagram showing another exemplary shape of a slot. 
         FIG. 14D  is a diagram showing another exemplary shape of a slot. 
         FIG. 14E  is a diagram showing another exemplary shape of a slot. 
         FIG. 14F  is a diagram showing another exemplary shape of a slot. 
         FIG. 15  is a diagram showing a planar layout where the four kinds of slots  112   a  through  112   d  shown in  FIGS. 14A through 14F  are disposed on a waveguide member  122 . 
         FIG. 16  is a diagram showing a driver&#39;s vehicle  500 , and a preceding vehicle  502  that is traveling in the same lane as the driver&#39;s vehicle  500 . 
         FIG. 17  is a diagram showing an onboard radar system  510  of the driver&#39;s vehicle  500 . 
         FIG. 18A  is a diagram showing a relationship between an array antenna AA of the onboard radar system  510  and plural arriving waves k. 
         FIG. 18B  is a diagram showing the array antenna AA receiving the k th  arriving wave. 
         FIG. 19  is a block diagram showing an exemplary fundamental construction of a vehicle travel controlling apparatus  600  according to the present disclosure. 
         FIG. 20  is a block diagram showing another exemplary construction for the vehicle travel controlling apparatus  600 . 
         FIG. 21  is a block diagram showing an example of a more specific construction of the vehicle travel controlling apparatus  600 . 
         FIG. 22  is a block diagram showing a more detailed exemplary construction of the radar system  510  according to this Application Example. 
         FIG. 23  is a diagram showing change in frequency of a transmission signal which is modulated based on the signal that is generated by a triangular wave generation circuit  581 . 
         FIG. 24  is a diagram showing a beat frequency fu in an “ascent” period and a beat frequency fd in a “descent” period. 
         FIG. 25  is a diagram showing an exemplary implementation in which a signal processing circuit  560  is implemented in hardware including a processor PR and a memory device MD. 
         FIG. 26  is a diagram showing a relationship between three frequencies f 1 , f 2  and f 3 . 
         FIG. 27  is a diagram showing a relationship between synthetic spectra F 1  to F 3  on a complex plane. 
         FIG. 28  is a flowchart showing the procedure of a process of determining relative velocity and distance according to a variant. 
         FIG. 29  is a diagram concerning a fusion apparatus in which a radar system  510  having a slot array antenna and a camera  700  are included. 
         FIG. 30  is a diagram illustrating how placing a millimeter wave radar  510  and a camera  700  at substantially the same position within the vehicle room may allow them to acquire an identical field of view and line of sight, thus facilitating a matching process. 
         FIG. 31  is a diagram showing an exemplary construction for a monitoring system  1500  based on millimeter wave radar. 
         FIG. 32  is a block diagram showing a construction for a digital communication system  800 A. 
         FIG. 33  is a block diagram showing an exemplary communication system  800 B including a transmitter  810 B which is capable of changing its radio wave radiation pattern. 
         FIG. 34  is a block diagram showing an exemplary communication system  800 C implementing a MIMO function. 
     
    
    
     DETAILED DESCRIPTION 
     &lt;Findings Forming the Basis of the Present Disclosure&gt; 
     Prior to describing embodiments of the present disclosure, findings that form the basis of the present disclosure will be described. 
     For applications in which thinness is required of an antenna and a waveguide (e.g., onboard millimeter wave radar applications), antenna structures which allow themselves to be thin are broadly adopted. Gain and directivity characteristics are the performance factors that are required of an antenna. Gain determines a detection range of a radar. Directivity characteristics determines a region of detection, angular resolution, and degree of image suppression. For example, to each antenna element (radiating element) of an array antenna which includes a plurality of antenna elements, a signal wave (e.g., a signal wave of a radio frequency) is supplied via a feeding network. The method of supplying a signal wave differs depending on the performance that is required of the array antenna. For example, when maximization of gain is desired, an approach (hereinafter referred to as “standing-wave series feed”) may be taken in which a standing wave is created on a feeding network, and a radio frequency signal is supplied to antenna elements which are inserted in series to the feeding network. 
     A ridge waveguide which is disclosed in the aforementioned Patent Document 1 and Non-Patent Document 1 is provided in a waffle iron structure which is capable of functioning as an artificial magnetic conductor. A ridge waveguide in which such an artificial magnetic conductor is utilized based on the present disclosure (which hereinafter may be referred to as a WRG: Waffle-iron Ridge waveguide) is able to realize an antenna feeding network with low losses in the microwave or the millimeter wave band. Moreover, use of such a ridge waveguide allows antenna elements to be disposed with a high density. 
       FIG. 1  is a perspective view schematically showing an exemplary construction for a slot array antenna  201  including a ridge waveguide. The slot array antenna  201  shown in the figure includes a first conductive member  110  and a second conductive member  120  opposing the first conductive member  110 . For ease of understanding,  FIG. 1  schematically shows a slot array antenna  201 , illustrated so that the spacing between a first conductive member  110  and a second conductive member  120  is exaggerated. The surface of the first conductive member  110  is composed of an electrically conductive material. The first conductive member  110  includes a plurality of slots  112  as radiating elements. On the second conductive member  120 , a waveguide member (ridge)  122  having an electrically-conductive waveguide face  122   a  opposing a slot row consisting of a plurality of slots  112 , and a plurality of conductive rods  124  are provided. The plurality of conductive rods  124  are disposed on both sides of the waveguide member  122 , constituting an artificial magnetic conductor together with the conductive surface of the second conductive member  120 . Electromagnetic waves are unable to propagate in the space existing between the artificial magnetic conductor and the conductive surface of the first conductive member  110 . Therefore, while propagating in a waveguide which is created between the waveguide face  122   a  and the conductive surface of the first conductive member  110 , an electromagnetic wave (signal wave) excites each slot  112 . As a result, an electromagnetic wave is radiated from each slot  112 . The following description will be based on an orthogonal coordinate system in which the width direction of the ridge  122  defines the X axis direction, the direction that the ridge  122  extends defines the Y axis direction, and a direction which is perpendicular to the waveguide face  122   a , i.e., the upper face of the ridge  122 , defines the Z axis direction. 
     In the construction shown in  FIG. 1 , the aperture shape of each slot  112  as viewed from the Z direction extends linearly along the X direction. Such a slot shape may be referred to as an “I shape” in the present specification. When such an I-shaped slot  112  is used, an electric field is induced in the central portion of the slot  112  at transmission or reception, this electric field oscillating along a direction (i.e., the Y direction) which is perpendicular to the inner peripheral surface. By using such slots  112  as antenna elements, transmission or reception of electromagnetic waves can be performed. 
     The example of  FIG. 1  illustrates that there is one waveguide member  122  and one row of slots  112  (slot row); however, pluralities of them may exist. For example, a slot array antenna that includes a plurality of waveguide members  122  which are arrayed in parallel and a plurality of slot rows corresponding to the respective waveguide members  122  may be constructed. 
     In the case where I-shaped slots as shown in  FIG. 1  are used, the large slot dimension along the X direction may not allow the slots to be disposed with a high density, especially when a plurality of slots are to be arrayed along the X direction. In order for the I-shaped slots to function as antenna elements, the dimension of the longitudinal direction of each slot (i.e., the X direction in the example of  FIG. 1 ) needs to be greater than a half of a wavelength λo in free space of an electromagnetic wave for transmission/reception. Accordingly, the distance between the centers of two adjacent slots along the X direction is also greater than a half of the wavelength λo. In the case where a horn (described later) is to be provided around each slot for improved directivity characteristics, the intervals between slots will be even larger. Due to such constraints, use of I-shaped slots will increase the device dimension in the longitudinal direction of the slots. 
     The inventors have accomplished the technique according to the present disclosure by considering the above facts. In an embodiment of the present disclosure, each slot may have a shape composed of a pair of vertical portions and a lateral portion that interconnects the pair of vertical portions. Since a shape in which a lateral portion connects the centers of a pair of vertical portions may resemble the alphabetical letter “H”, such a shape may be referred to as an “H shape”. More generally, a slot having a shape in which a lateral portion interconnects a pair of vertical portions may herein be referred to as a “complex slot”. Each slot is disposed so that its lateral portion opposes the waveguide face of a waveguide member, and yet intersects (e.g. being orthogonal to) the direction that the waveguide member extends. Such construction allows to reduce the dimension of the slot along the X direction. As a result, a plurality of slots can be disposed at a high density along the X direction, for example. Without being limited to a slot array antenna having a plurality of slots, the technique according to the present disclosure is also applicable to a slot antenna that only has one slot. For example, in order to realize an antenna device with a wide directivity, a single complex slot (e.g., an H-shaped slot) may be used as an antenna element. 
     Hereinafter, more specific exemplary constructions for slot antennas according to embodiments of the present disclosure will be described. Note however that unnecessarily detailed descriptions may be omitted. For example, detailed descriptions on what is well known in the art or redundant descriptions on what is substantially the same constitution may be omitted. This is to avoid lengthy description, and facilitate the understanding of those skilled in the art. The accompanying drawings and the following description, which are provided by the present inventors so that those skilled in the art can sufficiently understand the present disclosure, are not intended to limit the scope of claims. 
     In the present specification, an antenna device having at least one slot as an antenna element may be referred to as a “slot antenna”. In particular, a slot antenna having a plurality of slots as antenna elements will be referred to as a “slot array antenna”. A “slot array antenna” may also be referred to as a “slot antenna array”. Although the following description will mainly illustrate slot array antennas each having a plurality of antenna elements, the present disclosure also encompasses any slot antenna that has a single slot, as mentioned above. 
     Embodiments 
     First, an exemplary fundamental construction for a slot antenna according to an embodiment of the present disclosure will be described. 
     In the slot antenna according to an embodiment of the present disclosure, electromagnetic waves can be guided by utilizing stretches of artificial magnetic conductor that are provided on both sides of a waveguide member; thus, electromagnetic waves can be radiated from or allowed to impinge on one or more slots that are made in the conductive member. The use of artificial magnetic conductor restrains radio frequency signals from leaking on both sides of the waveguide member (e.g., a ridge having an electrically-conductive waveguide face). 
     An artificial magnetic conductor is a structure which artificially realizes the properties of a perfect magnetic conductor (PMC), which does not exist in nature. One property of a perfect magnetic conductor is that “a magnetic field on its surface has zero tangential component”. This property is the opposite of the property of a perfect electric conductor (PEC), i.e., “an electric field on its surface has zero tangential component”. Although no perfect magnetic conductor exists in nature, it can be embodied by an artificial structure, e.g., an array of conductive rods. An artificial magnetic conductor functions as a perfect magnetic conductor in a specific frequency band which is defined by its structure. An artificial magnetic conductor restrains or prevents an electromagnetic wave of any frequency that is contained in the specific frequency band (propagation-restricted band or prohibited band) from propagating along the surface of the artificial magnetic conductor. For this reason, the surface of an artificial magnetic conductor may be referred to as a high impedance surface. 
     As disclosed in Patent Documents 1 and 2 and Non-Patent Documents 1 and 2, an artificial magnetic conductor can be realized by a plurality of electrically conductive rods which are arrayed along row and column directions. The electrically conductive rods do not need to be disposed with a specific period in clearly defined rows and columns, so long as they have a one-dimensional or two-dimensional distribution. Such rods are portions (projections) that protrude from an electrically conductive member, and may also be referred to as posts or pins. A slot array antenna according to one embodiment of the present disclosure includes a pair of opposing electrically conductive members (electrically conductive plates). One conductive plate has a ridge protruding toward the other conductive plate, and stretches of an artificial magnetic conductor extending on both sides of the ridge. An upper face (i.e., its electrically conductive face) of the ridge opposes, via a gap, a conductive surface of the other conductive plate. An electromagnetic wave of a frequency which is contained in the propagation-restricted band of the artificial magnetic conductor propagates along the ridge, in the space (gap) between this conductive surface and the upper face of the ridge. 
       FIG. 2  is a perspective view schematically showing the construction of a slot array antenna  200  (which hereinafter may also be referred to as a “slot antenna  200 ”) according to an illustrative embodiment of the present disclosure.  FIG. 2  shows XYZ coordinates along X, Y and Z directions which are orthogonal to one another. The slot array antenna  200  shown in the figure includes a plate-like first conductive member  110  and a plate-like second conductive member  120 , which are in opposing and parallel positions to each other. The first conductive member  110  has a plurality of slots  112  which are arrayed along a first direction (the Y direction). A plurality of conductive rods  124  are arrayed on the second conductive member  120 . 
     Note that any structure appearing in a figure of the present application is shown in an orientation that is selected for ease of explanation, which in no way should limit its orientation when an embodiment of the present disclosure is actually practiced. Moreover, the shape and size of a whole or a part of any structure that is shown in a figure should not limit its actual shape and size. 
       FIG. 3A  is a diagram schematically showing the construction of a cross section through the center of a slot  112 , taken parallel to the XZ plane. As shown in  FIG. 3A , the first conductive member  110  has a conductive surface  110   a  on the side facing the second conductive member  120 . The conductive surface  110   a  has a two-dimensional expanse along a plane which is orthogonal to the axial direction (Z direction) of the conductive rods  124  (i.e., a plane which is parallel to the XY plane). Although the conductive surface  110   a  is shown to be a smooth plane in this example, the conductive surface  110   a  does not need to be a smooth plane, but may be curved or include minute rises and falls, as will be described later. 
       FIG. 4  is a perspective view schematically showing the slot array antenna  200 , illustrated so that the spacing between the first conductive member  110  and the second conductive member  120  is exaggerated for ease of understanding. In an actual slot array antenna  200 , as shown in  FIG. 2  and  FIG. 3A , the spacing between the first conductive member  110  and the second conductive member  120  is narrow, with the first conductive member  110  covering over the conductive rods  124  on the second conductive member  120 . 
     See  FIG. 3A  again. The plurality of conductive rods  124  arrayed on the second conductive member  120  each have a leading end  124   a  opposing the conductive surface  110   a . In the example shown in the figure, the leading ends  124   a  of the plurality of conductive rods  124  are on the same plane. This plane defines the surface  125  of an artificial magnetic conductor. Each conductive rod  124  does not need to be entirely electrically conductive, so long as it at least includes an electrically conductive layer that extends along the upper face and the side face of the rod-like structure. Although this electrically conductive layer may be located at the surface layer of the rod-like structure, the surface layer may be composed of an insulation coating or a resin layer with no electrically conductive layer existing on the surface of the rod-like structure. Moreover, each second conductive member  120  does not need to be entirely electrically conductive, so long as it can support the plurality of conductive rods  124  to constitute an artificial magnetic conductor. Of the surfaces of the second conductive member  120 , a face  120   a  carrying the plurality of conductive rods  124  may be electrically conductive, such that the electrical conductor interconnects the surfaces of adjacent ones of the plurality of conductive rods  124 . Moreover, the electrically conductive layer of the second conductive member  120  may be covered with an insulation coating or a resin layer. In other words, the entire combination of the second conductive member  120  and the plurality of conductive rods  124  may at least include an electrically conductive layer with rises and falls opposing the conductive surface  110   a  of the first conductive member  110 . 
     On the second conductive member  120 , a ridge-like waveguide member  122  is provided among the plurality of conductive rods  124 . More specifically, stretches of an artificial magnetic conductor are present on both sides of the waveguide member  122 , such that the waveguide member  122  is sandwiched between the stretches of artificial magnetic conductor on both sides. As can be seen from  FIG. 4 , the waveguide member  122  in this example is supported on the second conductive member  120 , and extends linearly along the Y direction. In the example shown in the figure, the waveguide member  122  has the same height and width as those of the conductive rods  124 . As will be described later, however, the height and width of the waveguide member  122  may be different from those of the conductive rod  124 . Unlike the conductive rods  124 , the waveguide member  122  extends along a direction (which in this example is the Y direction) in which to guide electromagnetic waves along the conductive surface  110   a . Similarly, the waveguide member  122  does not need to be entirely electrically conductive, but may at least include an electrically conductive waveguide face  122   a  opposing the conductive surface  110   a  of the first conductive member  110 . The second conductive member  120 , the plurality of conductive rods  124 , and the waveguide member  122  may be parts of a continuous single-piece body. Furthermore, the first conductive member  110  may also be a part of such a single-piece body. 
     The waveguide face  122   a  of the waveguide member  122  has a stripe shape extending along the Y direction. In the present specification, a “stripe shape” means a shape which is defined by a single stripe, rather than a shape constituted by stripes. Not only shapes that extend linearly in one direction, but also any shape that bends or branches along the way is also encompassed by a “stripe shape”. Note that any portion that undergoes a change in height or width may be provided on the waveguide face  122   a . Such cases still fall under the meaning of “stripe shape” so long as the shape includes a portion that extends in one direction as viewed from the normal direction of the waveguide face  122   a . A “stripe shape” may also be referred to a “strip shape”. The waveguide face  122   a  does not need to extend linearly along the Y direction in regions opposing the plurality of slots  112 , but may be bending or branching along the way. 
     On both sides of the waveguide member  122 , the space between the surface  125  of each stretch of artificial magnetic conductor and the conductive surface  110   a  of the first conductive member  110  does not allow an electromagnetic wave of any frequency that is within a specific frequency band to propagate. This frequency band is called a “prohibited band”. The artificial magnetic conductor is designed so that the frequency of a signal wave to propagate in the slot array antenna  200  (which may hereinafter be referred to as the “operating frequency”) is contained in the prohibited band. The prohibited band may be adjusted based on the following: the height of the conductive rods  124 , i.e., the depth of each groove formed between adjacent conductive rods  124 ; the width of each conductive rod  124 ; the interval between conductive rods  124 ; and the size of the gap between the leading end  124   a  and the conductive surface  110   a  of each conductive rod  124 . 
     In the present embodiment, the entire first conductive member  110  is composed of an electrically conductive material, and each slot  112  is an aperture which is made in the first conductive member  110 . However, the slots  112  are not limited to such a structure. For example, in a construction where the first conductive member  110  includes an internal dielectric layer and an outermost electrically conductive layer, apertures which are made only in the electrically conductive layer and not in the dielectric layer would also function as slots. 
     The slot antenna  200  is used for at least one of transmission and reception of an electromagnetic wave having a central wavelength λo in free space. Herein, λg is a wavelength of an electromagnetic wave of the wavelength λo when propagating in a waveguide extending between the first conductive member  110  and the waveguide member  122 . The waveguide extending between the first conductive member  110  and the waveguide member  122  is open at both ends. The slot interval is set to an integer multiple (typically x1) of the wavelength λg of an electromagnetic wave in the waveguide, for example. Bumps, dents, or other structures may be provided on the waveguide face  122   a  and/or the conductive surface  110   a  that is immediately above it. Such structures will cause the spacing (gap) between the waveguide face  122   a  and the conductive surface  110   a  to be enlarged or reduced as compared to any adjacent site. Moreover, the waveguide face  122   a  may include a site(s) that enlarges the width (i.e., dimension along the X direction) (referred to as a “broad portion(s)”), or a site(s) that reduces the width (referred to as a “narrow portion(s)”), as compared to any adjacent site. Any such bump, dent, broad portion, or narrow portion changes at least one of capacitance and inductance of the waveguide (path) along the Y direction. Such structures may be referred to as “additional elements” in the present specification. By appropriately providing such additional elements, it is possible to adjust the phase of the electromagnetic wave that propagates, and alter its wavelength λg. When additional elements are provided, λg can be made greater or smaller than the wavelength λ R  of an electromagnetic wave in a ridge waveguide lacking any such structures. In the present embodiment, however, λg is equal to λ R  because the waveguide face  122   a  is flat. 
     Although not shown in  FIG. 4 , choke structures may be provided near both ends of the waveguide member  122  along the Y direction. A choke structure may typically be composed of: an additional transmission line having a length of approximately λg/4; and a row of plural grooves having a depth of about λo/4, or plural rods having a height of about λo/4, that are disposed at an end of that additional transmission line. The choke structures confer a phase difference of about 180° (Π) between an incident wave and a reflected wave, thereby restraining electromagnetic waves from leaking at both ends of the waveguide member  122 . Instead of the second conductive member  120 , such choke structures may be provided on the first conductive member  110 . 
     Although not shown, the waveguiding structure in the slot antenna  200  has a port (opening) that is connected to a transmission circuit or reception circuit (i.e., an electronic circuit) not shown. The port may be provided at one end or an intermediate position (e.g., a central portion) of the waveguide member  122  shown in  FIG. 4 , for example. A signal wave which is sent from the transmission circuit via the port propagates through the waveguide extending upon the ridge  122 , and is radiated through each slot  112 . On the other hand, an electromagnetic wave which is led into the waveguide through each slot  112  propagates to the reception circuit via the port. At the rear side of the second conductive member  120 , a structure including another waveguide that is connected to the transmission circuit or reception circuit (which in the present specification may also be referred to as a “distribution layer”) may be provided. In that case, the port serves to couple between the waveguide in the distribution layer and the waveguide on the waveguide member  122 . 
     Note that the interval between the centers of two adjacent slots may have a different value from that of the wavelength λg. This will allow a phase difference to occur at the positions of the plurality of slots  112 , so that the azimuth at which the radiated electromagnetic waves will strengthen one another can be shifted from the frontal direction to another azimuth in the YZ plane. Thus, with the slot antenna  200  shown in  FIG. 4 , directivity within the YZ plane can be adjusted. 
     A plurality of such antennas, each including a waveguide which has a plurality of slots made therein, may be arrayed along a second direction (e.g., the X direction perpendicular to the first direction) that intersects the first direction, i.e., the direction in which the slots are arrayed. An array antenna including a two-dimensional array of such plural slots on a plate-like conductive member may also be called a flat panel array antenna. Such an array antenna includes: a plurality of slot rows which are parallel to one another; and a plurality of waveguide members. The plurality of waveguide members each have a waveguide face, these waveguide faces respectively facing the plurality of slot rows. In accordance with desired antenna performance, the aforementioned additional elements may be formed as appropriate on the plurality of waveguide faces. Depending on the purpose, the plurality of slot rows which are parallel to one another may vary in length (i.e., in terms of length between the slots at both ends of each slot row). A staggered array may be adopted such that, between two adjacent rows along the X direction, the positions of the slots are shifted along the Y direction. Depending on the purpose, the plurality of slot rows and the plurality of waveguide members may not be parallel, but may be angled. 
     &lt;Example Dimensions, Etc. of Each Member&gt; 
     Next, with reference to  FIG. 5 , the dimensions, shape, positioning, and the like of each member will be described. 
       FIG. 5  is a diagram showing an exemplary range of dimension of each member in the structure shown in  FIG. 3A . The slot array antenna is used for at least one of the transmission and the reception of an electromagnetic wave of a predetermined band (referred to as the operating frequency band). In the following description, λo denotes a wavelength (or, in the case where the operating frequency band has some expanse, a central wavelength corresponding to the center frequency) in free space of an electromagnetic wave (signal wave) propagating in a waveguide extending between the conductive surface  110   a  of the first conductive member  110  and the waveguide face  122   a  of the waveguide member  122 . Moreover, λm denotes a wavelength (shortest wavelength), in free space, of an electromagnetic wave of the highest frequency in the operating frequency band. The end of each conductive rod  124  that is in contact with the second conductive member  120  is referred to as the “root”. As shown in  FIG. 5 , each conductive rod  124  has the leading end  124   a  and the root  124   b . Examples of dimensions, shapes, positioning, and the like of the respective members are as follows. 
     (1) Width of the Conductive Rod 
     The width (i.e., the size along the X direction and the Y direction) of the conductive rod  124  may be set to less than λo/2 (preferably less than λm/2). Within this range, for any signal wave with a free-space wavelength of λo or more, resonance of the lowest order can be prevented from occurring along the X direction and the Y direction. Since resonance may possibly occur not only in the X and Y directions but also in any diagonal direction in an X-Y cross section, the diagonal length of an X-Y cross section of the conductive rod  124  is also preferably less than λo/2 (and more preferably less than λm/2). The lower limit values for the rod width and diagonal length will conform to the minimum lengths that are producible under the given manufacturing method, but is not particularly limited. 
     (2) Distance from the Root of the Conductive Rod to the Conductive Surface of the First Conductive Member 
     The distance from the root  124   b  of each conductive rod  124  to the conductive surface  110   a  of the first conductive member  110  may be longer than the height of the conductive rods  124 , while also being less than λo/2 (preferably less than λm/2). When the distance is λo/2 or more, for any signal wave with a free-space wavelength of λo, resonance may occur between the root  124   b  of each conductive rod  124  and the conductive surface  110   a , thus reducing the effect of signal wave containment. 
     The distance from the root  124   b  of each conductive rod  124  to the conductive surface  110   a  of the first conductive members  110  corresponds to the spacing between the first conductive member  110  and the second conductive member  120 . For example, when a signal wave of 76.5±0.5 GHz (which belongs to the millimeter band or the extremely high frequency band) propagates in the waveguide, the wavelength of the signal wave is in the range from 3.8934 mm to 3.9446 mm. Therefore, λm equals 3.8934 mm in this case, so that the spacing between the first conductive member  110  and the second conductive member  120  can be set to less than a half of 3.8934 mm. So long as the first conductive member  110  and the second conductive member  120  realize such a narrow spacing while being disposed opposite from each other, the first conductive member  110  and the second conductive member  120  do not need to be strictly parallel. Moreover, when the spacing between the first conductive member  110  and the second conductive member  120  is less than λo/2 (preferably less than λm/2), a whole or a part of the first conductive member  110  and/or the second conductive member  120  may be shaped as a curved surface. On the other hand, the first and second conductive members  110  and  120  each have a planar shape (i.e., the shape of their region as perpendicularly projected onto the XY plane) and a planar size (i.e., the size of their region as perpendicularly projected onto the XY plane) which may be arbitrarily designed depending on the purpose. 
     Although the conductive surface  120   a  is illustrated as a plane in the example shown in  FIG. 3A , embodiments of the present disclosure are not limited thereto. For example, as shown in  FIG. 3B , the conductive surface  120   a  may be the bottom parts of faces each of which has a cross section similar to a U-shape or a V-shape. The conductive surface  120   a  will have such a structure when each conductive rod  124  or the waveguide member  122  is shaped with a width which increases toward the root. Even with such a structure, the device shown in  FIG. 3B  can function as the slot antenna according to an embodiment of the present disclosure so long as the distance between the conductive surface  110   a  and the conductive surface  120   a  is less than a half of the wavelength λo or λm. 
     (3) Distance L 2  from the Leading End of the Conductive Rod to the Conductive Surface 
     The distance L 2  from the leading end  124   a  of each conductive rod  124  to the conductive surface  110   a  is set to less than λo/2 (preferably less than λm/2). When the distance is λo/2 or more, for any electromagnetic wave with a free-space wavelength of λo, a propagation mode that reciprocates between the leading end  124   a  of each conductive rod  124  and the conductive surface  110   a  may occur, thus no longer being able to contain an electromagnetic wave. Note that, among the plurality of conductive rods  124 , at least those which are adjacent to the waveguide member  122  (described later) do not have their leading ends in electrical contact with the conductive surface  110   a . As used herein, the leading end of a conductive rod not being in electrical contact with the conductive surface means either of the following states: there being an air gap between the leading end and the conductive surface; or the leading end of the conductive rod and the conductive surface adjoining each other via an insulating layer which may exist in the leading end of the conductive rod or in the conductive surface. 
     (4) Arrangement and Shape of Conductive Rods 
     The interspace between two adjacent conductive rods  124  among the plurality of conductive rods  124  has a width of less than λo/2 (preferably less than λm/2), for example. The width of the interspace between any two adjacent conductive rods  124  is defined by the shortest distance from the surface (side face) of one of the two conductive rods  124  to the surface (side face) of the other. This width of the interspace between rods is to be determined so that resonance of the lowest order will not occur in the regions between rods. The conditions under which resonance will occur are determined based by a combination of: the height of the conductive rods  124 ; the distance between any two adjacent conductive rods; and the capacitance of the air gap between the leading end  124   a  of each conductive rod  124  and the conductive surface  110   a . Therefore, the width of the interspace between rods may be appropriately determined depending on other design parameters. Although there is no clear lower limit to the width of the interspace between rods, for manufacturing ease, it may be e.g. λo/16 or more when an electromagnetic wave in the extremely high frequency band is to be propagated. Note that the interspace does not need to have a constant width. So long as it remains less than λo/2, the interspace between conductive rods  124  may vary. 
     The arrangement of the plurality of conductive rods  124  is not limited to the illustrated example, so long as it exhibits a function of an artificial magnetic conductor. The plurality of conductive rods  124  do not need to be arranged in orthogonal rows and columns; the rows and columns may be intersecting at angles other than 90 degrees. The plurality of conductive rods  124  do not need to form a linear array along rows or columns, but may be in a dispersed arrangement which does not present any straightforward regularity. The conductive rods  124  may also vary in shape and size depending on the position on the second conductive member  120 . 
     The surface  125  of the artificial magnetic conductor that are constituted by the leading ends  124   a  of the plurality of conductive rods  124  does not need to be a strict plane, but may be a plane with minute rises and falls, or even a curved surface. In other words, the conductive rods  124  do not need to be of uniform height, but rather the conductive rods  124  may be diverse so long as the array of conductive rods  124  is able to function as an artificial magnetic conductor. 
     Furthermore, each conductive rod  124  does not need to have a prismatic shape as shown in the figure, but may have a cylindrical shape, for example. Furthermore, each conductive rod  124  does not need to have a simple columnar shape, but may have a mushroom shape, for example. The artificial magnetic conductor may also be realized by any structure other than an array of conductive rods  124 , and various artificial magnetic conductors are applicable to the waveguide structure according to the present disclosure. Note that, when the leading end  124   a  of each conductive rod  124  has a prismatic shape, its diagonal length is preferably less than λo/2. When the leading end  124   a  of each conductive rod  124  is shaped as an ellipse, the length of its major axis is preferably less than λo/2 (and more preferably less than λm/2). Even when the leading end  124   a  has any other shape, the dimension across it is preferably less than λo/2 (and more preferably less than λm/2) even at the longest position. 
     (5) Width of the Waveguide Face 
     The width of the waveguide face  122   a  of the waveguide member  122 , i.e., the size of the waveguide face  122   a  along a direction which is orthogonal to the direction that the waveguide member  122  extends, may be set to less than λo/2 (preferably less than λm/2, e.g., λo/8). If the width of the waveguide face  122   a  is λo/2 or more, for any electromagnetic wave with a free-space wavelength of λo, resonance will occur along the width direction, which will prevent any WRG from operating as a simple transmission line. 
     (6) Height of the Waveguide Member 
     The height (i.e., the size along the Z direction in the example shown in the figure) of the waveguide member  122  is set to less than λo/2 (preferably less than λm/2). The reason is that, if the distance is λo/2 or more, the distance between the root  124   b  of each conductive rod  124  and the conductive surface  110   a  will be λo/2 or more. Similarly, the height of the conductive rods  124  (especially those conductive rods  124  which are adjacent to the waveguide member  122 ) is set to less than λo/2 or less than λm/2. 
     (7) Distance L 1  Between the Waveguide Face and the Conductive Surface 
     The distance L 1  between the waveguide face  122   a  of the waveguide member  122  and the conductive surface  110   a  is set to less than λo/2 (preferably less than λm/2). If the distance is λo/2 or more, for any electromagnetic wave with a free-space wavelength of λo, resonance will occur between the waveguide face  122   a  and the conductive surface  110   a , which will prevent functionality as a waveguide. In one example, the distance is λo/4 or less. In order to ensure manufacturing ease, when an electromagnetic wave in the extremely high frequency band is to propagate, the distance L 1  is preferably λo/16 or more, for example. 
     The lower limit of the distance L 1  between the conductive surface  110   a  and the waveguide face  122   a  and the lower limit of the distance L 2  between the conductive surface  110   a  and the leading end  124   a  of each rod  124  depends on the machining precision, and also on the precision when assembling the two upper/lower conductive members  110  and  120  so as to be apart by a constant distance. When a pressing technique or an injection technique is used, the practical lower limit of the aforementioned distance is about 50 micrometers (μm). In the case of using an MEMS (Micro-Electro-Mechanical System) technique to make a product in e.g. the terahertz range, the lower limit of the aforementioned distance is about 2 to about 3 μm. 
     (8) Arraying Interval and Size of Slots 
     The distance (slot interval) between the centers of two adjacent slots  112  in the slot antenna  200  may be set to, for example, an integer multiple of λg (typically the same value as λg), where λg is the intra-waveguide wavelength of a signal wave propagating in the waveguide (or, in the case where the operating frequency band has some expanse, a central wavelength corresponding to the center frequency). As a result of this, when standing-wave series feed is applied, an equiamplitude and equiphase state can be realized at the position of each slot. Note that the interval between the centers of two adjacent slots is determined by the required directivity characteristics, and therefore may not be equal to λg in some cases. Although the number of slots  112  is six in the present embodiment, the number of slots  112  may be any number which is equal to or greater than two. 
       FIGS. 6A to 6C  are diagrams schematically showing exemplary aperture shapes of the slot  112  as viewed from the Z direction. Edges of the waveguide face  122   a  lying directly under the slot  112  are also indicated by broken lines. In these examples, each slot  112  has an H shape composed of a pair of vertical portions  113 L and a lateral portion  113 T that interconnects the pair of vertical portions  113 L. The lateral portion  113 T connects the centers of the pair of vertical portions  113 L. A sum L (corresponding to the entire length of each of the arrows shown in  FIGS. 6A to 6C ) of the distance between centers of the pair of vertical portions  113 L of each slot  112  and the length of one whole vertical portion are set to values at which higher-order mode oscillation does not occur and at which the slot impedance is not too small. For example, L may be set to a range of λo/2&lt;L&lt;λo. The width W of the lateral portion and each vertical portion may be less than λo/2. In order to actively utilize higher-order modes, L may possibly be larger than λo. 
     In the example of  FIG. 6A , the width of the waveguide face  122   a  is equal to the length of the lateral portion  113 T. However, as in the example of  FIG. 6B , the width of the waveguide face  122   a  may be narrower than the length of the lateral portion  113 T, and the slot  112  will still function as an antenna element. Similarly, as in the example of  FIG. 6C , the width of the waveguide face  122   a  may be broader than the length of the lateral portion  113 T, and the slot  112  will still function as an antenna element. However, the width of the waveguide face  122   a  must be narrower than the width of the slot  112  (i.e., its dimension taken along the width direction of the waveguide face  122   a ) inclusive of the pair of vertical portions  113 L. Moreover, as shown in  FIGS. 6A to 6C , the waveguide face  122   a  must intersect the lateral portion  113 T of the slot  112  when viewed from the Z direction, i.e., a perpendicular direction to the waveguide face  122   a . Note that, as illustrated by the example of  FIG. 6C , even if the length of the lateral portion  113 T of the slot  112  is shorter than the width of the waveguide face  122   a , the waveguide face  122   a  is still regarded as intersecting the lateral portion  113 T so long as the direction that the lateral portion  113 T extends (i.e., the lateral direction in  FIG. 6C ) intersects the direction that the waveguide face  122   a  extends. 
     Thus, according to the present embodiment, the artificial magnetic conductor includes a plurality of conductive rods  124  each having a leading end opposing the first conductive surface  110   a . The slot antenna  200  is used for at least one of transmission and reception of an electromagnetic wave of a predetermined band. The width (i.e., dimension along the X direction) of the waveguide member  122 , the width of each conductive rod  124 , the width of a space existing between two adjacent conductive rods, the distance between the first conductive surface  110   a  and the conductive surface  120   a , and the width of a space existing between each conductive rod  124  that is adjacent to the waveguide member  122  and the waveguide member  122  may be set to less than λo/2 or less than λm/2. 
     Next, other exemplary constructions for embodiments of the present disclosure will be described. 
     Horned Structure 
       FIG. 7A  is a perspective view showing an exemplary construction of a slot antenna  200  including horns.  FIG. 7B  is an upper plan view showing a first conductive member  110  and a second conductive member  120  shown in  FIG. 7A , each viewed from the +Z direction. For simplicity,  FIG. 7A  and  FIG. 7B  illustrate an example where the first conductive member  110  has two slots  112  and two horns  114  respectively surrounding them. The number of slots  112  and the number of horns  114  may be one, or may be three or more. 
     Each horn  114  four side walls (i.e., two pairs of electrically conductive walls) at least the surface of which is composed of an electrically-conductive material. Each side wall is inclined with respect to direction that is perpendicular to the surface of the first conductive member  110 . By providing the horns  114 , the directivity of an electromagnetic wave to be radiated from each slot  112  can be improved. The shape of the horn  114  is not limited to what is shown in the figure. For example, each side wall may have a portion that is perpendicular to the surface of the first conductive member  110 . 
     Variants of Waveguide Member, Conductive Members, and Conductive Rods 
     Next, variants of the waveguide member  122 , the conductive members  110  and  120 , and the conductive rods  124  will be described. 
       FIG. 8A  is a cross-sectional view showing an exemplary structure in which only a waveguide face  122   a , defining an upper face of the waveguide member  122 , is electrically conductive, while any portion of the waveguide member  122  other than the waveguide face  122   a  is not electrically conductive. Both of the first conductive member  110  and the second conductive member  120  alike are only electrically conductive at their surface that has the waveguide member  122  provided thereon (i.e., the conductive surface  110   a ,  120   a ), while not being electrically conductive in any other portions. Thus, each of the waveguide member  122 , the first conductive member  110 , and the second conductive member  120  does not need to be entirely electrically conductive. 
       FIG. 8B  is a diagram showing a variant in which the waveguide member  122  is not formed on the second conductive member  120 . In this example, the waveguide member  122  is fixed to a supporting member (e.g., an inner wall of the housing) that supports the first conductive member  110  and the second conductive member  120 . A gap exists between the waveguide member  122  and the second conductive member  120 . Thus, the waveguide member  122  does not need to be connected to the second conductive member  120 . 
       FIG. 8C  is a diagram showing an exemplary structure where the second conductive member  120 , the waveguide member  122 , and each of the plurality of conductive rods  124  are composed of a dielectric surface that is coated with an electrically conductive material such as a metal. The second conductive member  120 , the waveguide member  122 , and the plurality of conductive rods  124  are connected to one another via the electrical conductor. On the other hand, the first conductive member  110  is made of an electrically conductive material such as a metal. 
       FIG. 8D  and  FIG. 8E  are diagrams each showing an exemplary structure in which dielectric layers  110   b  and  120   b  are respectively provided on the outermost surfaces of conductive members  110  and  120 , a waveguide member  122 , and conductive rods  124 .  FIG. 8D  shows an exemplary structure in which the surface of metal conductive members, which are conductors, are covered with a dielectric layer.  FIG. 8E  shows an example where the conductive member  120  is structured so that the surface of members which are composed of a dielectric, e.g., resin, is covered with a conductor such as a metal, this metal layer being further coated with a dielectric layer. The dielectric layer that covers the metal surface may be a coating of resin or the like, or an oxide film of passivation coating or the like which is generated as the metal becomes oxidized. 
     The dielectric layer on the outermost surface will allow losses to be increased in the electromagnetic wave propagating through the WRG waveguide, but is able to protect the conductive surfaces  110   a  and  120   a  (which are electrically conductive) from corrosion. Moreover, short-circuiting can be prevented even if a conductor line to carry a DC voltage, or an AC voltage of such a low frequency that it is not capable of propagation on certain WRG waveguides, exists in places that may come in contact with the conductive rods  124 . 
       FIG. 8F  is a diagram showing an example where the height of the waveguide member  122  is lower than the height of the conductive rods  124 , and a portion of a conductive surface  110   a  of the first conductive member  110  that opposes the waveguide face  122   a  protrudes toward the waveguide member  122 . Even such a structure will operate in a similar manner to the above-described embodiment, so long as the ranges of dimensions depicted in  FIG. 5  are satisfied. 
       FIG. 8G  is a diagram showing an example where, further in the structure of  FIG. 8F , portions of the conductive surface  110   a  that oppose the conductive rods  124  protrude toward the conductive rods  124 . Even such a structure will operate in a similar manner to the above-described embodiment, so long as the ranges of dimensions depicted in  FIG. 5  are satisfied. Instead of a structure in which the conductive surface  110   a  partially protrudes, a structure in which the conductive surface  110   a  is partially dented may be adopted. 
       FIG. 9A  is a diagram showing an example where a conductive surface  110   a  of the first conductive member  110  is shaped as a curved surface.  FIG. 9B  is a diagram showing an example where also a conductive surface  120   a  of the second conductive member  120  is shaped as a curved surface. As demonstrated by these examples, the conductive surface(s)  110   a ,  120   a  may not be shaped as a plane(s), but may be shaped as a curved surface(s). 
     A plurality of waveguide members  122  may be provided on the second conductive member  120 .  FIG. 10  is a perspective view showing an implementation where two waveguide members  122  extend in parallel upon the second conductive member  120 . By providing a plurality of waveguide members  122  within a single waveguiding structure, it becomes possible to realize an array antenna in which a plurality of slots are placed in a two-dimensional array at short intervals. In the construction of  FIG. 10 , an artificial magnetic conductor that includes three rows of conductive rods  124  exists between the two waveguide members  122 . Stretches of artificial magnetic conductor also exist on both far sides of the continuous region that accommodates the plurality of waveguide members  122 . 
       FIG. 11A  is an upper plan view of an array antenna including 16 slots  112  in an array of 4 rows and 4 columns, as viewed in the Z direction.  FIG. 11B  is a cross-sectional view taken along line B-B in  FIG. 11A . The first conductive member  110  in this array antenna includes a plurality of horns  114 , which are placed so as to respectively correspond to the plurality of slots  112 . In the antenna shown in the figures, a first waveguide device  100   a  and a second waveguide device  100   b  are layered. The first waveguide device  100   a  includes waveguide members  122 U that directly couple to slots  112 . The second waveguide device  100   b  includes further waveguide members  122 L that couple to the waveguide members  122 U of the first waveguide device  100   a . The waveguide members  122 L and the conductive rods  124 L of the second waveguide device  100   b  are arranged on a third conductive member  140 . The second waveguide device  100   b  is basically similar in construction to the first waveguide device  100   a.    
     As shown in  FIG. 11A , the conductive member  110  has a plurality of slots  112  which are arrayed along the first direction (the Y direction) and a second direction (the X direction) orthogonal to the first direction. The waveguide face  122   a  of each waveguide member  122 U extends along the Y direction, and opposes four slots that are disposed along the Y direction among the plurality of slots  112 . Although the conductive member  110  has 16 slots  112  in an array of 4 rows and 4 columns in this example, the number of slots  112  is not limited to this example. Without being limited to the example where each waveguide member  122 U opposes all slots that are disposed along the Y direction among the plurality of slots  112 , each waveguide member  122 U may oppose at least two adjacent slots along the Y direction. The interval between the centers of the waveguide faces  122   a  of any two adjacent waveguide members  122 U is set to be shorter than the wavelength λo, for example. 
       FIG. 12A  is a diagram showing a planar layout of waveguide members  122 U in the first waveguide device  100   a .  FIG. 13  is a diagram showing a planar layout of a waveguide member  122 L in the second waveguide device  100   b . As is clear from these figures, the waveguide members  122 U of the first waveguide device  100   a  extend linearly, and include no branching portions or bends; on the other hand, the waveguide members  122 L of the second waveguide device  100   b  include both branching portions and bends. The combination of the “second conductive member  120 ” and the “third conductive member  140 ” in the second waveguide device  100   b  corresponds to the combination in the first waveguide device  100   a  of the “first conductive member  110 ” and the “second conductive member  120 ”. 
     The waveguide members  122 U of the first waveguide device  100   a  couple to the waveguide member  122 L of the second waveguide device  100   b , through ports (openings)  145 U that are provided in the second conductive member  120 . Stated otherwise, an electromagnetic wave which has propagated through the waveguide member  122 L of the second waveguide device  100   b  passes through a port  145 U to reach a waveguide member  122 U of the first waveguide device  100   a , and propagates through the waveguide member  122 U of the first waveguide device  100   a . In this case, each slot  112  functions as an antenna element to allow an electromagnetic wave which has propagated through the waveguide to be radiated into space. Conversely, when an electromagnetic wave which has propagated in space impinges on a slot  112 , the electromagnetic wave couples to the waveguide member  122 U of the first waveguide device  100   a  that lies directly under that slot  112 , and propagates through the waveguide member  122 U of the first waveguide device  100   a . An electromagnetic wave which has propagated through a waveguide member  122 U of the first waveguide device  100   a  may also pass through a port  145 U to reach the waveguide member  122 L of the second waveguide device  100   b , and propagates through the waveguide member  122 L of the second waveguide device  100   b . Via a port  145 L of the third conductive member  140 , the waveguide member  122 L of the second waveguide device  100   b  may couple to an external waveguide device or radio frequency circuit (electronic circuit). As one example,  FIG. 13  illustrates an electronic circuit  190  which is connected to the port  145 L. Without being limited to a specific position, the electronic circuit  190  may be provided at any arbitrary position. The electronic circuit  190  may be provided on a circuit board which is on the rear surface side (i.e., the lower side in  FIG. 11B ) of the third conductive member  140 , for example. Such an electronic circuit is a microwave integrated circuit, and may be an MMIC (Monolithic Microwave Integrated Circuit) that generates or receives millimeter waves, for example. 
     The first conductive member  110  shown in  FIG. 11A  may be called a “radiation layer”. Moreover, the entirety of the second conductive member  120 , the waveguide members  122 U, and the conductive rods  124 U shown in  FIG. 12A  may be called an “excitation layer”, whereas the entirety of the third conductive member  140 , the waveguide member  122 L, and the conductive rods  124 L shown in  FIG. 13  may be called a “distribution layer”. Moreover, the “excitation layer” and the “distribution layer” may be collectively called a “feeding layer”. Each of the “radiation layer”, the “excitation layer”, and the “distribution layer” can be mass-produced by processing a single metal plate. The radiation layer, the excitation layer, the distribution layer, and any electronic circuitry to be provided on the rear face side of the distribution layer may be produced as a single-module product. 
     In the array antenna of this example, as can be seen from  FIG. 11B , a radiation layer, an excitation layer, and a distribution layer are layered, which are in plate form; therefore, a flat and low-profile flat panel antenna is realized as a whole. For example, the height (thickness) of a multilayer structure having a cross-sectional construction as shown in  FIG. 11B  can be 10 mm or less. 
     With the waveguide member  122 L shown in  FIG. 13 , the distances from the port  145 L of the third conductive member  140  to the respective ports  145 U (see  FIG. 12A ) of the second conductive member  120  measured along the waveguide member  122 L are all set to an identical value. Therefore, a signal wave which is input to the waveguide member  122 L reaches the four ports  145 U of the second conductive member  120  all in the same phase, from the port  145 L of the third conductive member  140 . As a result, the four waveguide members  122 U on the second conductive member  120  can be excited in the same phase. 
     It is not necessary for all slots  112  functioning as antenna elements to radiate electromagnetic waves in the same phase. The network patterns of the waveguide members  122 U and  122 L in the excitation layer and the distribution layer may be arbitrary, and they may be arranged so that the respective waveguide members  122 U and  122 L independently propagate different signals. 
     In the construction of  FIG. 12A , a stretch of artificial magnetic conductor including the plurality of conductive rods  124  is provided between two adjacent waveguide members  122 . However, this artificial magnetic conductor does not need to be provided. 
       FIG. 12B  is a diagram showing an example where no artificial magnetic conductor is provided between two adjacent waveguide members  122  among the plurality of waveguide members  122 . In the case where the plurality of slots  112  are to be excited in the same phase, it is not problematic if electromagnetic waves propagating along two adjacent waveguide members  122  become mixed with each other. Therefore, no artificial magnetic conductor such as conductive rods  124  need to be provided between two adjacent waveguide members  122 . In that case, too, stretches of artificial magnetic conductor are provided on both far sides of the continuous region that accommodates the plurality of waveguide members  122 . In the present disclosure, any structure where stretches of artificial magnetic conductor are provided on both far sides of the continuous region that accommodates the plurality of waveguide members  122 , as exemplified by  FIG. 12B , is still regarded as each waveguide member  122  separating between the stretches of artificial magnetic conductor that are on both its sides. In such an example, the length of the gap between two adjacent waveguide members  122 U along the X direction is set to less than λm/2. 
     The present specification employs the term “artificial magnetic conductor” in describing the technique according to the present disclosure, this being in line with what is set forth in a paper by one of the inventors Kirino (Non-Patent Document 1) as well as a paper by Kildal et al., who published a study directed to related subject matter around the same time. However, it has been found through a study by the inventors that the invention according to the present disclosure does not necessarily require an “artificial magnetic conductor” under its conventional definition. That is, while a periodic structure has been believed to be a requirement for an artificial magnetic conductor, the invention according to the present disclosure does not necessary require a periodic structure in order to be practiced. 
     The artificial magnetic conductor that is described in the present disclosure consists of rows of conductive rods. Therefore, in order to prevent electromagnetic waves from leaking away from the waveguide face, it has been believed essential that there exist at least two rows of conductive rods on one side of the waveguide member(s), such rows of conductive rods extending along the waveguide member(s) (ridge(s)). The reason is that it takes at least two rows of conductive rods for them to have a “period”. However, according to a study by the inventors, even when only one row of conductive rods exists between two waveguide members that extend in parallel to each other, the intensity of a signal that leaks from one waveguide member to the other waveguide member can be suppressed to −10 dB or less, which is a practically sufficient value in many applications. The reason why such a sufficient level of separation is achieved with only an imperfect periodic structure is so far unclear. However, in view of this fact, in the present disclosure, the notion of “artificial magnetic conductor” is extended so that the term also encompasses a structure including only one row of conductive rods. 
     In the example shown in  FIG. 11A , all of the slots  112  are H-shaped slots having an H shape; however, this is not a limitation. For example, in order for radiant intensity to be distributed, within a slot array constituting one vertical row, any slot(s) near the center may have an H shape, while the vertical portion of each H shape may become increasingly shorter toward the periphery, thus resulting in an I shape with no lateral portion at both ends. Thus, among the plurality of slots in the slot array antenna, at least two slots may be H-shaped slots. Instead of H-shaped slots, any other complex slot described that is below may be adopted in realizing a similar construction. In other words, the plurality of slots in the slot array antenna according to the present disclosure may include at least two complex slots. 
     Slot Variants 
     Next, variant shapes for the slots  112  will be described. Although the above examples illustrate that each slot  112  has an H shape with right-angled corners, the slots  112  may also have other shapes. Hereinafter, examples of other slot shapes will be described with reference to  FIGS. 14A through 14F . 
       FIG. 14A  shows another example of a slot  112   b  having an H shape including a pair of vertical portions  113 L and a lateral portion  113 T interconnecting the pair of vertical portions  113 L. In this example, each vertical portion  113 L has rounded corners at both ends. The lateral portion  113 T is substantially perpendicular to the pair of vertical portions  113 L, connecting substantially central portions of the pair of vertical portions  113 L together. With such an H-shaped slot  112   b , too, its shape and size are to be determined so that higher-order resonance will not occur and that the slot impedance will not be too small. In order to satisfy these conditions, a dimension L is defined which is twice the length along the lateral portion  113 T and two halves of the vertical portions  113 L that extends from the center point (i.e., the center point of the lateral portion  113 T) to an end (i.e., either end of a vertical portion  113 L) of the H shape, such that λo/2&lt;L&lt;λo, (for example, L=about λo/2). On this basis, the length (the length indicated by arrowheads in the figure) of the lateral portion  113 T can be made e.g. less than λo/2, thus reducing the slot interval along the length direction of the lateral portion  113 T. 
       FIG. 14B  shows an example of a slot  112   c  which includes a lateral portion  113 T and a pair of vertical portions  113 L extending from both ends of the lateral portion  113 T. The directions that the pair of vertical portions  113 L extend from the lateral portion  113 T, which are opposite to each other, are substantially perpendicular to the lateral portion  113 T. One end of the lateral portion  113 T is connected to one end of one of the vertical portions  113 L, whereas the other end of the lateral portion  113 T is connected to one end of the other vertical portion  113 L. Since such a shape resembles the alphabetical letter “Z” or the inverted-Z shape, it may be referred to as a “Z shape”. In this example, too, the length (the length indicated by arrowheads in the figure) of the lateral portion  113 T can be made e.g. less than λo/2, whereby the slot interval along the length direction of the lateral portion  113 T can be reduced. 
       FIG. 14C  shows an example of a slot  112   d  which includes a lateral portion  113 T and a pair of vertical portions  113 L extending from both ends of the lateral portion  113 T in the same direction perpendicular to the lateral portion  113 T. In this example, too, one end of the lateral portion  113 T is connected to one end of one of the vertical portions  113 L, whereas the other end of the lateral portion  113 T is connected to one end of the other vertical portion  113 L. Since such a shape resembles the alphabetical letter “U”, it may be referred to as a “U shape”. In this example, too, the length (the length indicated by arrowheads in the figure) of the lateral portion  113 T can be made e.g. less than λo/2, whereby the slot interval along the length direction of the lateral portion  113 T can be reduced. 
       FIG. 14D ,  FIG. 14E , and  FIG. 14F  illustrate examples of slots  112   e ,  112   f  and  112   g , where bulges  113 D are added respectively to the slots  112   b , the slot  112   c , and the slot  112   d . Slots of such shapes can also function as slot antennas according to the present disclosure. 
     In each of the aforementioned H-shaped, Z-shaped, and U-shaped slot, the lateral portion does not need to be orthogonal to the vertical portions; it suffices if they are in intersecting relationship. Moreover, the pair of vertical portions do not need to be parallel to each other. Although the terms “lateral portion” and “vertical portion” are conveniently employed herein, these terms should not limit the positioning or orientation of the slots in the slot antenna as it is actually used. Stated otherwise, the planar shape of any complex slot according to an embodiment of the present disclosure may be said to be a shape that includes a first linear portion (corresponding to the lateral portion) which extends in a second direction that intersects a first direction (i.e., the direction that the waveguide face extends), and a pair of second linear portions (corresponding to the vertical portions) which extend in a third direction that intersects the second direction, the second linear portions being respectively connected to or near both ends of the first linear portion. The length of the first linear portion and each second linear portion (i.e., dimension along the direction that they extend) may be longer or shorter than their width (i.e., dimension along a perpendicular direction to the direction that they extend). 
       FIG. 15  is a diagram showing a planar layout where the three kinds of slots  112   b  through  112   d  shown in  FIGS. 14A through 14C , as well as an I-shaped slot  112   a  as Comparative Example, are disposed on a waveguide member  122 . As shown in the figure, using the slots  112   b  through  112   d  allows the size of the lateral portion  113 T along its length direction (referred to as the “lateral direction”) to be reduced as compared to the case of using the slot  112   a . Therefore, especially in a structure where a plurality of waveguide members  122  are arranged in parallel, the interval of slots along the lateral direction can be reduced. 
     The above example illustrates that the direction that the lateral portion (or the first linear portion) of a slot extends, coincides with the width direction of the waveguide member  122 ; however, these two directions may intersect each other. In such constructions, the plane of polarization of the electromagnetic wave to be radiated can be tilted. As a result, when used for an onboard radar, for example, an electromagnetic wave which has been radiated from the driver&#39;s vehicle can be distinguished from an electromagnetic wave which has been radiated from an oncoming car. 
     Thus, in accordance with an embodiment of the present disclosure, for example, the interval between a plurality of slots on a conductive member can be narrowed, while also achieving excitation with an equiamplitude and equiphase. As a result, a small-sized and high-gain radar device, radar system, wireless communication system, or the like can be realized. Embodiments of the present disclosure are not limited to implementations where excitation with an equiamplitude and equiphase is to be achieved. For example, other purposes, such as reducing side lobes while sacrificing the output efficiency of a radar, can also be attained. By appropriately providing additional elements such as bumps, dents, etc., on the waveguide face  122   a  or the conductive surface  110   a , the amplitude and phase at each slot position can be individually adjusted. This makes it possible to radiate electromagnetic wave with an arbitrary radiation pattern. Without being limited to standing-wave feed, traveling-wave feed may also be applied. Thus, the technique of the present disclosure is applicable to a broad range of purposes and applications. 
     The waveguide device and slot antenna (antenna device) according to the present disclosure can be suitably used in a radar device or a radar system to be incorporated in moving entities such as vehicles, marine vessels, aircraft, robots, or the like, for example. A radar device would include a slot antenna according to any of the above-described embodiments and a microwave integrated circuit that is connected to the slot antenna. A radar system would include the radar device and a signal processing circuit that is connected to the microwave integrated circuit of the radar device. A slot antenna according to an embodiment of the present disclosure includes a WRG structure which permits downsizing, and thus allows the area of the face on which antenna elements are arrayed to be remarkably reduced, as compared to a construction in which a conventional hollow waveguide is used. Therefore, a radar system incorporating the antenna device can be easily mounted in a narrow place such as a face of a rearview mirror in a vehicle that is opposite to its specular surface, or a small-sized moving entity such as a UAV (an Unmanned Aerial Vehicle, a so-called drone). Note that, without being limited to the implementation where it is mounted in a vehicle, a radar system may be used while being fixed on the road or a building, for example. 
     A slot antenna according to an embodiment of the present disclosure can also be used in a wireless communication system. Such a wireless communication system would include a slot antenna according to any of the above embodiments and a communication circuit (a transmission circuit or a reception circuit). Details of exemplary applications to wireless communication systems will be described later. 
     A slot antenna according to an embodiment of the present disclosure can further be used as an antenna in an indoor positioning system (IPS). An indoor positioning system is able to identify the position of a moving entity, such as a person or an automated guided vehicle (AGV), that is in a building. A slot antenna can also be used as a radio wave transmitter (beacon) for use in a system which provides information to an information terminal device (e.g., a smartphone) that is carried by a person who has visited a store or any other facility. In such a system, once every several seconds, a beacon may radiate an electromagnetic wave carrying an ID or other information superposed thereon, for example. When the information terminal device receives this electromagnetic wave, the information terminal device transmits the received information to a remote server computer via telecommunication lines. Based on the information that has been received from the information terminal device, the server computer identifies the position of that information terminal device, and provides information which is associated with that position (e.g., product information or a coupon) to the information terminal device. 
     Application Example 1: Onboard Radar System 
     Next, as an Application Example of utilizing the above-described slot array antenna, an instance of an onboard radar system including a slot array antenna will be described. A transmission wave used in an onboard radar system may have a frequency of e.g. 76 gigahertz (GHz) band, which will have a wavelength λo of about 4 mm in free space. 
     In safety technology of automobiles, e.g., collision avoidance systems or automated driving, it is particularly essential to identify one or more vehicles (targets) that are traveling ahead of the driver&#39;s vehicle. As a method of identifying vehicles, techniques of estimating the directions of arriving waves by using a radar system have been under development. 
       FIG. 16  shows a driver&#39;s vehicle  500 , and a preceding vehicle  502  that is traveling in the same lane as the driver&#39;s vehicle  500 . The driver&#39;s vehicle  500  includes an onboard radar system which incorporates a slot array antenna according to any of the above-described embodiments. When the onboard radar system of the driver&#39;s vehicle  500  radiates a radio frequency transmission signal, the transmission signal reaches the preceding vehicle  502  and is reflected therefrom, so that a part of the signal returns to the driver&#39;s vehicle  500 . The onboard radar system receives this signal to calculate a position of the preceding vehicle  502 , a distance (“range”) to the preceding vehicle  502 , velocity, etc. 
       FIG. 17  shows the onboard radar system  510  of the driver&#39;s vehicle  500 . The onboard radar system  510  is provided within the vehicle. More specifically, the onboard radar system  510  is disposed on a face of the rearview mirror that is opposite to its specular surface. From within the vehicle, the onboard radar system  510  radiates a radio frequency transmission signal in the direction of travel of the vehicle  500 , and receives a signal(s) which arrives from the direction of travel. 
     The onboard radar system  510  of this Application Example includes a slot array antenna according to the above embodiment of the present disclosure. The slot array antenna may include a plurality of waveguide members which are parallel to one another. It is arranged so that the direction that each of the plurality of waveguide members extends coincides with the vertical direction, and that the direction in which the plurality of waveguide members are arrayed coincides with the horizontal direction. As a result, the lateral dimension and the vertical dimension of the plurality of slots as viewed from the front can be reduced. 
     Exemplary dimensions of an antenna device including the above array antenna may be 60 mm (wide)×30 mm (long)×10 mm (deep). It will be appreciated that this is a very small size for a millimeter wave radar system of the 76 GHz band. 
     Note that many a conventional onboard radar system is provided outside the vehicle, e.g., at the tip of the front nose. The reason is that the onboard radar system is relatively large in size, and thus is difficult to be provided within the vehicle as in the present disclosure. The onboard radar system  510  of this Application Example may be installed within the vehicle as described above, but may instead be mounted at the tip of the front nose. Since the footprint of the onboard radar system on the front nose is reduced, other parts can be more easily placed. 
     The Application Example allows the interval between a plurality of waveguide members (ridges) that are used in the transmission antenna to be narrow, which also narrows the interval between a plurality of slots to be provided opposite from a number of adjacent waveguide members. This reduces the influences of grating lobes. For example, when the interval between the centers of two laterally adjacent slots is shorter than the free-space wavelength λo of the transmission wave (i.e., less than about 4 mm), no grating lobes will occur frontward. As a result, influences of grating lobes are reduced. Note that grating lobes will occur when the interval at which the antenna elements are arrayed is greater than a half of the wavelength of an electromagnetic wave. If the interval at which the antenna elements are arrayed is less than the wavelength, no grating lobes will occur frontward. Therefore, in the case where each antenna element composing an array antenna is only frontward-sensitive, as in the Application Example, grating lobes will exert substantially no influences so long as the interval at which the antenna elements are arrayed is smaller than the wavelength. By adjusting the array factor of the transmission antenna, the directivity of the transmission antenna can be adjusted. A phase shifter may be provided so as to be able to individually adjust the phases of electromagnetic waves that are transmitted on plural waveguide members. By providing a phase shifter, the directivity of the transmission antenna can be changed in any desired direction. Since the construction of a phase shifter is well-known, description thereof will be omitted. 
     A reception antenna according to the Application Example is able to reduce reception of reflected waves associated with grating lobes, thereby being able to improve the precision of the below-described processing. Hereinafter, an example of a reception process will be described. 
       FIG. 18A  shows a relationship between an array antenna AA of the onboard radar system  510  and plural arriving waves k (k: an integer from 1 to K; the same will always apply below. K is the number of targets that are present in different azimuths). The array antenna AA includes M antenna elements in a linear array. Principlewise, an antenna can be used for both transmission and reception, and therefore the array antenna AA can be used for both a transmission antenna and a reception antenna. Hereinafter, an example method of processing an arriving wave which is received by the reception antenna will be described. 
     The array antenna AA receives plural arriving waves that simultaneously impinge at various angles. Some of the plural arriving waves may be arriving waves which have been radiated from the transmission antenna of the same onboard radar system  510  and reflected by a target(s). Furthermore, some of the plural arriving waves may be direct or indirect arriving waves that have been radiated from other vehicles. 
     The incident angle of each arriving wave (i.e., an angle representing its direction of arrival) is an angle with respect to the broadside B of the array antenna AA. The incident angle of an arriving wave represents an angle with respect to a direction which is perpendicular to the direction of the line along which antenna elements are arrayed. 
     Now, consider a k th  arriving wave. Where K arriving waves are impinging on the array antenna from K targets existing at different azimuths, a “k th  arriving wave” means an arriving wave which is identified by an incident angle θ k . 
       FIG. 18B  shows the array antenna AA receiving the k th  arriving wave. The signals received by the array antenna AA can be expressed as a “vector” having M elements, by Math. 1.
 
 S =[ s   1   ,s   2   , . . . ,s   M ] T   (Math. 1)
 
     In the above, s m  (where m is an integer from 1 to M; the same will also be true hereinbelow) is the value of a signal which is received by an m th  antenna element. The superscript  T  means transposition. S is a column vector. The column vector S is defined by a product of multiplication between a direction vector (referred to as a steering vector or a mode vector) as determined by the construction of the array antenna and a complex vector representing a signal from each target (also referred to as a wave source or a signal source). When the number of wave sources is K, the waves of signals arriving at each individual antenna element from the respective K wave sources are linearly superposed. In this state, s m  can be expressed by Math. 2. 
     
       
         
           
             
               
                 
                   
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     In Math.  2 , a k , θk and φ k  respectively denote the amplitude, incident angle, and initial phase of the k th  arriving wave. Moreover, A denotes the wavelength of an arriving wave, and j is an imaginary unit. 
     As will be understood from Math.  2 , s m  is expressed as a complex number consisting of a real part (Re) and an imaginary part (Im). 
     When this is further generalized by taking noise (internal noise or thermal noise) into consideration, the array reception signal X can be expressed as Math. 3.
 
 X=S+N   (Math. 3)
 
     N is a vector expression of noise. 
     The signal processing circuit generates a spatial covariance matrix Rxx (Math. 4) of arriving waves by using the array reception signal X expressed by Math.  3 , and further determines eigenvalues of the spatial covariance matrix Rxx. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     In the above, the superscript  H  means complex conjugate transposition (Hermitian conjugate). 
     Among the eigenvalues, the number of eigenvalues which have values equal to or greater than a predetermined value that is defined based on thermal noise (signal space eigenvalues) corresponds to the number of arriving waves. Then, angles that produce the highest likelihood as to the directions of arrival of reflected waves (i.e. maximum likelihood) are calculated, whereby the number of targets and the angles at which the respective targets are present can be identified. This process is known as a maximum likelihood estimation technique. 
     Next, see  FIG. 19 .  FIG. 19  is a block diagram showing an exemplary fundamental construction of a vehicle travel controlling apparatus  600  according to the present disclosure. The vehicle travel controlling apparatus  600  shown in  FIG. 19  includes a radar system  510  which is mounted in a vehicle, and a travel assistance electronic control apparatus  520  which is connected to the radar system  510 . The radar system  510  includes an array antenna AA and a radar signal processing apparatus  530 . 
     The array antenna AA includes a plurality of antenna elements, each of which outputs a reception signal in response to one or plural arriving waves. As mentioned earlier, the array antenna AA is capable of radiating a millimeter wave of a high frequency. 
     In the radar system  510 , the array antenna AA needs to be attached to the vehicle, while at least some of the functions of the radar signal processing apparatus  530  may be implemented by a computer  550  and a database  552  which are provided externally to the vehicle travel controlling apparatus  600  (e.g., outside of the driver&#39;s vehicle). In that case, the portions of the radar signal processing apparatus  530  that are located within the vehicle may be perpetually or occasionally connected to the computer  550  and database  552  external to the vehicle so that bidirectional communications of signal or data are possible. The communications are to be performed via a communication device  540  of the vehicle and a commonly-available communications network. 
     The database  552  may store a program which defines various signal processing algorithms. The content of the data and program needed for the operation of the radar system  510  may be externally updated via the communication device  540 . Thus, at least some of the functions of the radar system  510  can be realized externally to the driver&#39;s vehicle (which is inclusive of the interior of another vehicle), by a cloud computing technique. Therefore, an “onboard” radar system in the meaning of the present disclosure does not require that all of its constituent elements be mounted within the (driver&#39;s) vehicle. However, for simplicity, the present application will describe an implementation in which all constituent elements according to the present disclosure are mounted in a single vehicle (i.e., the driver&#39;s vehicle), unless otherwise specified. 
     The radar signal processing apparatus  530  includes a signal processing circuit  560 . The signal processing circuit  560  directly or indirectly receives reception signals from the array antenna AA, and inputs the reception signals, or a secondary signal(s) which has been generated from the reception signals, to an arriving wave estimation unit AU. A part or a whole of the circuit (not shown) which generates a secondary signal(s) from the reception signals does not need to be provided inside of the signal processing circuit  560 . A part or a whole of such a circuit (preprocessing circuit) may be provided between the array antenna AA and the radar signal processing apparatus  530 . 
     The signal processing circuit  560  is configured to perform computation by using the reception signals or secondary signal(s), and output a signal indicating the number of arriving waves. As used herein, a “signal indicating the number of arriving waves” can be said to be a signal indicating the number of preceding vehicles (which may be one preceding vehicle or plural preceding vehicles) ahead of the driver&#39;s vehicle. 
     The signal processing circuit  560  may be configured to execute various signal processing which is executable by known radar signal processing apparatuses. For example, the signal processing circuit  560  may be configured to execute “super-resolution algorithms” such as the MUSIC method, the ESPRIT method, or the SAGE method, or other algorithms for direction-of-arrival estimation of relatively low resolution. 
     The arriving wave estimation unit AU shown in FIG. estimates an angle representing the azimuth of each arriving wave by an arbitrary algorithm for direction-of-arrival estimation, and outputs a signal indicating the estimation result. The signal processing circuit  560  estimates the distance to each target as a wave source of an arriving wave, the relative velocity of the target, and the azimuth of the target by using a known algorithm which is executed by the arriving wave estimation unit AU, and output a signal indicating the estimation result. 
     In the present disclosure, the term “signal processing circuit” is not limited to a single circuit, but encompasses any implementation in which a combination of plural circuits is conceptually regarded as a single functional part. The signal processing circuit  560  may be realized by one or more System-on-Chips (SoCs). For example, a part or a whole of the signal processing circuit  560  may be an FPGA (Field-Programmable Gate Array), which is a programmable logic device (PLD). In that case, the signal processing circuit  560  includes a plurality of computation elements (e.g., general-purpose logics and multipliers) and a plurality of memory elements (e.g., look-up tables or memory blocks). Alternatively, the signal processing circuit  560  may be a set of a general-purpose processor(s) and a main memory device(s). The signal processing circuit  560  may be a circuit which includes a processor core(s) and a memory device(s). These may function as the signal processing circuit  560 . 
     The travel assistance electronic control apparatus  520  is configured to provide travel assistance for the vehicle based on various signals which are output from the radar signal processing apparatus  530 . The travel assistance electronic control apparatus  520  instructs various electronic control units to fulfill predetermined functions, e.g., a function of issuing an alarm to prompt the driver to make a braking operation when the distance to a preceding vehicle (vehicular gap) has become shorter than a predefined value; a function of controlling the brakes; and a function of controlling the accelerator. For example, in the case of an operation mode which performs adaptive cruise control of the driver&#39;s vehicle, the travel assistance electronic control apparatus  520  sends predetermined signals to various electronic control units (not shown) and actuators, to maintain the distance of the driver&#39;s vehicle to a preceding vehicle at a predefined value, or maintain the traveling velocity of the driver&#39;s vehicle at a predefined value. 
     In the case of the MUSIC method, the signal processing circuit  560  determines eigenvalues of the spatial covariance matrix, and, as a signal indicating the number of arriving waves, outputs a signal indicating the number of those eigenvalues (“signal space eigenvalues”) which are greater than a predetermined value (thermal noise power) that is defined based on thermal noise. 
     Next, see  FIG. 20 .  FIG. 20  is a block diagram showing another exemplary construction for the vehicle travel controlling apparatus  600 . The radar system  510  in the vehicle travel controlling apparatus  600  of  FIG. 20  includes an array antenna AA, which includes an array antenna that is dedicated to reception only (also referred to as a reception antenna) Rx and an array antenna that is dedicated to transmission only (also referred to as a transmission antenna) Tx; and an object detection apparatus  570 . 
     At least one of the transmission antenna Tx and the reception antenna Rx has the aforementioned waveguide structure. The transmission antenna Tx radiates a transmission wave, which may be a millimeter wave, for example. The reception antenna Rx that is dedicated to reception only outputs a reception signal in response to one or plural arriving waves (e.g., a millimeter wave(s)). 
     A transmission/reception circuit  580  sends a transmission signal for a transmission wave to the transmission antenna Tx, and performs “preprocessing” for reception signals of reception waves received at the reception antenna Rx. A part or a whole of the preprocessing may be performed by the signal processing circuit  560  in the radar signal processing apparatus  530 . A typical example of preprocessing to be performed by the transmission/reception circuit  580  may be generating a beat signal from a reception signal, and converting a reception signal of analog format into a reception signal of digital format. 
     Note that the radar system according to the present disclosure may, without being limited to the implementation where it is mounted in the driver&#39;s vehicle, be used while being fixed on the road or a building. 
     Next, an example of a more specific construction of the vehicle travel controlling apparatus  600  will be described. 
       FIG. 21  is a block diagram showing an example of a more specific construction of the vehicle travel controlling apparatus  600 . The vehicle travel controlling apparatus  600  shown in  FIG. 21  includes a radar system  510  and an onboard camera system  700 . The radar system  510  includes an array antenna AA, a transmission/reception circuit  580  which is connected to the array antenna AA, and a signal processing circuit  560 . 
     The onboard camera system  700  includes an onboard camera  710  which is mounted in a vehicle, and an image processing circuit  720  which processes an image or video that is acquired by the onboard camera  710 . 
     The vehicle travel controlling apparatus  600  of this Application Example includes an object detection apparatus  570  which is connected to the array antenna AA and the onboard camera  710 , and a travel assistance electronic control apparatus  520  which is connected to the object detection apparatus  570 . The object detection apparatus  570  includes a transmission/reception circuit  580  and an image processing circuit  720 , in addition to the above-described radar signal processing apparatus  530  (including the signal processing circuit  560 ). The object detection apparatus  570  detects a target on the road or near the road, by using not only the information which is obtained by the radar system  510  but also the information which is obtained by the image processing circuit  720 . For example, while the driver&#39;s vehicle is traveling in one of two or more lanes of the same direction, the image processing circuit  720  can distinguish which lane the driver&#39;s vehicle is traveling in, and supply that result of distinction to the signal processing circuit  560 . When the number and azimuth(s) of preceding vehicles are to be recognized by using a predetermined algorithm for direction-of-arrival estimation (e.g., the MUSIC method), the signal processing circuit  560  is able to provide more reliable information concerning a spatial distribution of preceding vehicles by referring to the information from the image processing circuit  720 . 
     Note that the onboard camera system  700  is an example of a means for identifying which lane the driver&#39;s vehicle is traveling in. The lane position of the driver&#39;s vehicle may be identified by any other means. For example, by utilizing an ultra-wide band (UWB) technique, it is possible to identify which one of a plurality of lanes the driver&#39;s vehicle is traveling in. It is widely known that the ultra-wide band technique is applicable to position measurement and/or radar. Using the ultra-wide band technique enhances the range resolution of the radar, so that, even when a large number of vehicles exist ahead, each individual target can be detected with distinction, based on differences in distance. This makes it possible to identify distance from a guardrail on the road shoulder, or from the median strip, with good precision. The width of each lane is predefined based on each country&#39;s law or the like. By using such information, it becomes possible to identify where the lane in which the driver&#39;s vehicle is currently traveling is. Note that the ultra-wide band technique is an example. A radio wave based on any other wireless technique may be used. Moreover, LIDAR (Light Detection and Ranging) may be used together with a radar. LIDAR is sometimes called “laser radar”. 
     The array antenna AA may be a generic millimeter wave array antenna for onboard use. The transmission antenna Tx in this Application Example radiates a millimeter wave as a transmission wave ahead of the vehicle. A portion of the transmission wave is reflected off a target which is typically a preceding vehicle, whereby a reflected wave occurs from the target being a wave source. A portion of the reflected wave reaches the array antenna (reception antenna) AA as an arriving wave. Each of the plurality of antenna elements of the array antenna AA outputs a reception signal in response to one or plural arriving waves. In the case where the number of targets functioning as wave sources of reflected waves is K (where K is an integer of one or more), the number of arriving waves is K, but this number K of arriving waves is not known beforehand. 
     The example of  FIG. 19  assumes that the radar system  510  is provided as an integral piece, including the array antenna AA, on the rearview mirror. However, the number and positions of array antennas AA are not limited to any specific number or specific positions. An array antenna AA may be disposed on the rear surface of the vehicle so as to be able to detect targets that are behind the vehicle. Moreover, a plurality of array antennas AA may be disposed on the front surface and the rear surface of the vehicle. The array antenna(s) AA may be disposed inside the vehicle. Even in the case where a horn antenna whose respective antenna elements include horns as mentioned above is to be adopted as the array antenna(s) AA, the array antenna(s) with such antenna elements may be situated inside the vehicle. 
     The signal processing circuit  560  receives and processes the reception signals which have been received by the reception antenna Rx and subjected to preprocessing by the transmission/reception circuit  580 . This process encompasses inputting the reception signals to the arriving wave estimation unit AU, or alternatively, generating a secondary signal(s) from the reception signals and inputting the secondary signal(s) to the arriving wave estimation unit AU. 
     In the example of  FIG. 21 , a selection circuit  596  which receives the signal being output from the signal processing circuit  560  and the signal being output from the image processing circuit  720  is provided in the object detection apparatus  570 . The selection circuit  596  allows one or both of the signal being output from the signal processing circuit  560  and the signal being output from the image processing circuit  720  to be fed to the travel assistance electronic control apparatus  520 . 
       FIG. 22  is a block diagram showing a more detailed exemplary construction of the radar system  510  according to this Application Example. 
     As shown in  FIG. 22 , the array antenna AA includes a transmission antenna Tx which transmits a millimeter wave and reception antennas Rx which receive arriving waves reflected from targets. Although only one transmission antenna Tx is illustrated in the figure, two or more kinds of transmission antennas with different characteristics may be provided. The array antenna AA includes M antenna elements  11   1 ,  11   2 , . . . ,  11   M  (where M is an integer of 3 or more). In response to the arriving waves, the plurality of antenna elements  11   1 ,  11   2 , . . . ,  11   M  respectively output reception signals s 1 , s 2 , . . . , s M  ( FIG. 18B ). 
     In the array antenna AA, the antenna elements  11   1  to  11   M  are arranged in a linear array or a two-dimensional array at fixed intervals, for example. Each arriving wave will impinge on the array antenna AA from a direction at an angle θ with respect to the normal of the plane in which the antenna elements  11   1  to  11   M  are arrayed. Thus, the direction of arrival of an arriving wave is defined by this angle θ. 
     When an arriving wave from one target impinges on the array antenna AA, this approximates to a plane wave impinging on the antenna elements  11   1  to  11   M  from azimuths of the same angle θ. When K arriving waves impinge on the array antenna AA from K targets with different azimuths, the individual arriving waves can be identified in terms of respectively different angles θ 1  to θ K . 
     As shown in  FIG. 22 , the object detection apparatus  570  includes the transmission/reception circuit  580  and the signal processing circuit  560 . 
     The transmission/reception circuit  580  includes a triangular wave generation circuit  581 , a VCO (voltage controlled oscillator)  582 , a distributor  583 , mixers  584 , filters  585 , a switch  586 , an A/D converter  587 , and a controller  588 . Although the radar system in this Application Example is configured to perform transmission and reception of millimeter waves by the FMCW method, the radar system of the present disclosure is not limited to this method. The transmission/reception circuit  580  is configured to generate a beat signal based on a reception signal from the array antenna AA and a transmission signal from the transmission antenna Tx. 
     The signal processing circuit  560  includes a distance detection section  533 , a velocity detection section  534 , and an azimuth detection section  536 . The signal processing circuit  560  is configured to process a signal from the A/D converter  587  in the transmission/reception circuit  580 , and output signals respectively indicating the detected distance to the target, the relative velocity of the target, and the azimuth of the target. 
     First, the construction and operation of the transmission/reception circuit  580  will be described in detail. 
     The triangular wave generation circuit  581  generates a triangular wave signal, and supplies it to the VCO  582 . The VCO  582  outputs a transmission signal having a frequency as modulated based on the triangular wave signal.  FIG. 23  is a diagram showing change in frequency of a transmission signal which is modulated based on the signal that is generated by the triangular wave generation circuit  581 . This waveform has a modulation width Δf and a center frequency of f 0 . The transmission signal having a thus modulated frequency is supplied to the distributor  583 . The distributor  583  allows the transmission signal obtained from the VCO  582  to be distributed among the mixers  584  and the transmission antenna Tx. Thus, the transmission antenna radiates a millimeter wave having a frequency which is modulated in triangular waves, as shown in  FIG. 23 . 
     In addition to the transmission signal,  FIG. 23  also shows an example of a reception signal from an arriving wave which is reflected from a single preceding vehicle. The reception signal is delayed from the transmission signal. This delay is in proportion to the distance between the driver&#39;s vehicle and the preceding vehicle. Moreover, the frequency of the reception signal increases or decreases in accordance with the relative velocity of the preceding vehicle, due to the Doppler effect. 
     When the reception signal and the transmission signal are mixed, a beat signal is generated based on their frequency difference. The frequency of this beat signal (beat frequency) differs between a period in which the transmission signal increases in frequency (ascent) and a period in which the transmission signal decreases in frequency (descent). Once a beat frequency for each period is determined, based on such beat frequencies, the distance to the target and the relative velocity of the target are calculated. 
       FIG. 24  shows a beat frequency fu in an “ascent” period and a beat frequency fd in a “descent” period. In the graph of  FIG. 24 , the horizontal axis represents frequency, and the vertical axis represents signal intensity. This graph is obtained by subjecting the beat signal to time-frequency conversion. Once the beat frequencies fu and fd are obtained, based on a known equation, the distance to the target and the relative velocity of the target are calculated. In this Application Example, with the construction and operation described below, beat frequencies corresponding to each antenna element of the array antenna AA are obtained, thus enabling estimation of the position information of a target. 
     In the example shown in  FIG. 22 , reception signals from channels Ch 1  to Ch M  corresponding to the respective antenna elements  11   1  to  11   M  are each amplified by an amplifier, and input to the corresponding mixers  584 . Each mixer  584  mixes the transmission signal into the amplified reception signal. Through this mixing, a beat signal is generated corresponding to the frequency difference between the reception signal and the transmission signal. The generated beat signal is fed to the corresponding filter  585 . The filters  585  apply bandwidth control to the beat signals on the channels Ch 1  to Ch M , and supply bandwidth-controlled beat signals to the switch  586 . 
     The switch  586  performs switching in response to a sampling signal which is input from the controller  588 . The controller  588  may be composed of a microcomputer, for example. Based on a computer program which is stored in a memory such as a ROM, the controller  588  controls the entire transmission/reception circuit  580 . The controller  588  does not need to be provided inside the transmission/reception circuit  580 , but may be provided inside the signal processing circuit  560 . In other words, the transmission/reception circuit  580  may operate in accordance with a control signal from the signal processing circuit  560 . Alternatively, some or all of the functions of the controller  588  may be realized by a central processing unit which controls the entire transmission/reception circuit  580  and signal processing circuit  560 . 
     The beat signals on the channels Ch 1  to Ch M  having passed through the respective filters  585  are consecutively supplied to the A/D converter  587  via the switch  586 . In synchronization with the sampling signal, the A/D converter  587  converts the beat signals on the channels Ch 1  to Ch M , which are input from the switch  586 , into digital signals. 
     Hereinafter, the construction and operation of the signal processing circuit  560  will be described in detail. In this Application Example, the distance to the target and the relative velocity of the target are estimated by the FMCW method. Without being limited to the FMCW method as described below, the radar system can also be implemented by using other methods, e.g., 2 frequency CW and spread spectrum methods. 
     In the example shown in  FIG. 22 , the signal processing circuit  560  includes a memory  531 , a reception intensity calculation section  532 , a distance detection section  533 , a velocity detection section  534 , a DBF (digital beam forming) processing section  535 , an azimuth detection section  536 , a target link processing section  537 , a matrix generation section  538 , a target output processing section  539 , and an arriving wave estimation unit AU. As mentioned earlier, a part or a whole of the signal processing circuit  560  may be implemented by FPGA, or by a set of a general-purpose processor(s) and a main memory device(s). The memory  531 , the reception intensity calculation section  532 , the DBF processing section  535 , the distance detection section  533 , the velocity detection section  534 , the azimuth detection section  536 , the target link processing section  537 , and the arriving wave estimation unit AU may be individual parts that are implemented in distinct pieces of hardware, or functional blocks of a single signal processing circuit. 
       FIG. 25  shows an exemplary implementation in which the signal processing circuit  560  is implemented in hardware including a processor PR and a memory device MD. In the signal processing circuit  560  with this construction, too, a computer program that is stored in the memory device MD may fulfill the functions of the reception intensity calculation section  532 , the DBF processing section  535 , the distance detection section  533 , the velocity detection section  534 , the azimuth detection section  536 , the target link processing section  537 , the matrix generation section  538 , and the arriving wave estimation unit AU shown in  FIG. 22 . 
     The signal processing circuit  560  in this Application Example is configured to estimate the position information of a preceding vehicle by using each beat signal converted into a digital signal as a secondary signal of the reception signal, and output a signal indicating the estimation result. Hereinafter, the construction and operation of the signal processing circuit  560  in this Application Example will be described in detail. 
     For each of the channels Ch 1  to Ch M , the memory  531  in the signal processing circuit  560  stores a digital signal which is output from the A/D converter  587 . The memory  531  may be composed of a generic storage medium such as a semiconductor memory or a hard disk and/or an optical disk. 
     The reception intensity calculation section  532  applies Fourier transform to the respective beat signals for the channels Ch 1  to Ch M  (shown in the lower graph of  FIG. 23 ) that are stored in the memory  531 . In the present specification, the amplitude of a piece of complex number data after the Fourier transform is referred to as “signal intensity”. The reception intensity calculation section  532  converts the complex number data of a reception signal from one of the plurality of antenna elements, or a sum of the complex number data of all reception signals from the plurality of antenna elements, into a frequency spectrum. In the resultant spectrum, beat frequencies corresponding to respective peak values, which are indicative of presence and distance of targets (preceding vehicles), can be detected. Taking a sum of the complex number data of the reception signals from all antenna elements will allow the noise components to average out, whereby the S/N ratio is improved. 
     In the case where there is one target, i.e., one preceding vehicle, as shown in  FIG. 24 , the Fourier transform will produce a spectrum having one peak value in a period of increasing frequency (the “ascent” period) and one peak value in a period of decreasing frequency (“the descent” period). The beat frequency of the peak value in the “ascent” period is denoted by “fu”, whereas the beat frequency of the peak value in the “descent” period is denoted by “fd”. 
     From the signal intensities of beat frequencies, the reception intensity calculation section  532  detects any signal intensity that exceeds a predefined value (threshold value), thus determining the presence of a target. Upon detecting a signal intensity peak, the reception intensity calculation section  532  outputs the beat frequencies (fu, fd) of the peak values to the distance detection section  533  and the velocity detection section  534  as the frequencies of the object of interest. The reception intensity calculation section  532  outputs information indicating the frequency modulation width Δf to the distance detection section  533 , and outputs information indicating the center frequency f 0  to the velocity detection section  534 . 
     In the case where signal intensity peaks corresponding to plural targets are detected, the reception intensity calculation section  532  find associations between the ascents peak values and the descent peak values based on predefined conditions. Peaks which are determined as belonging to signals from the same target are given the same number, and thus are fed to the distance detection section  533  and the velocity detection section  534 . 
     When there are plural targets, after the Fourier transform, as many peaks as there are targets will appear in the ascent portions and the descent portions of the beat signal. In proportion to the distance between the radar and a target, the reception signal will become more delayed and the reception signal in  FIG. 23  will shift more toward the right. Therefore, a beat signal will have a greater frequency as the distant between the target and the radar increases. 
     Based on the beat frequencies fu and fd which are input from the reception intensity calculation section  532 , the distance detection section  533  calculates a distance R through the equation below, and supplies it to the target link processing section  537 .
 
 R={C·T /(2·Δ f )}·{( fu+fd )/2}
 
     Moreover, based on the beat frequencies fu and fd being input from the reception intensity calculation section  532 , the velocity detection section  534  calculates a relative velocity V through the equation below, and supplies it to the target link processing section  537 .
 
 V={C /(2· f 0)}·{( fu−fd )/2}
 
     In the equation which calculates the distance R and the relative velocity V, C is velocity of light, and T is the modulation period. 
     Note that the lower limit resolution of distance R is expressed as C/(2Δf). Therefore, as Δf increases, the resolution of distance R increases. In the case where the frequency f 0  is in the 76 GHz band, when Δf is set on the order of 660 megahertz (MHz), the resolution of distance R will be on the order of 0.23 meters (m), for example. Therefore, if two preceding vehicles are traveling abreast of each other, it may be difficult with the FMCW method to identify whether there is one vehicle or two vehicles. In such a case, it might be possible to run an algorithm for direction-of-arrival estimation that has an extremely high angular resolution to separate between the azimuths of the two preceding vehicles and enable detection. 
     By utilizing phase differences between signals from the antenna elements  11   1 ,  11   2 , . . . ,  11   M , the DBF processing section  535  allows the incoming complex data corresponding to the respective antenna elements, which has been Fourier transformed with respect to the time axis, to be Fourier transformed with respect to the direction in which the antenna elements are arrayed. Then, the DBF processing section  535  calculates spatial complex number data indicating the spectrum intensity for each angular channel as determined by the angular resolution, and outputs it to the azimuth detection section  536  for the respective beat frequencies. 
     The azimuth detection section  536  is provided for the purpose of estimating the azimuth of a preceding vehicle. Among the values of spatial complex number data that has been calculated for the respective beat frequencies, the azimuth detection section  536  chooses an angle θ that takes the largest value, and outputs it to the target link processing section  537  as the azimuth at which an object of interest exists. 
     Note that the method of estimating the angle θ indicating the direction of arrival of an arriving wave is not limited to this example. Various algorithms for direction-of-arrival estimation that have been mentioned earlier can be employed. 
     The target link processing section  537  calculates absolute values of the differences between the respective values of distance, relative velocity, and azimuth of the object of interest as calculated in the current cycle and the respective values of distance, relative velocity, and azimuth of the object of interest as calculated 1 cycle before, which are read from the memory  531 . Then, if the absolute value of each difference is smaller than a value which is defined for the respective value, the target link processing section  537  determines that the target that was detected 1 cycle before and the target detected in the current cycle are an identical target. In that case, the target link processing section  537  increments the count of target link processes, which is read from the memory  531 , by one. 
     If the absolute value of a difference is greater than predetermined, the target link processing section  537  determines that a new object of interest has been detected. The target link processing section  537  stores the respective values of distance, relative velocity, and azimuth of the object of interest as calculated in the current cycle and also the count of target link processes for that object of interest to the memory  531 . 
     In the signal processing circuit  560 , the distance to the object of interest and its relative velocity can be detected by using a spectrum which is obtained through a frequency analysis of beat signals, which are signals generated based on received reflected waves. 
     The matrix generation section  538  generates a spatial covariance matrix by using the respective beat signals for the channels Ch 1  to Ch M  (lower graph in  FIG. 23 ) stored in the memory  531 . In the spatial covariance matrix of Math.  4 , each component is the value of a beat signal which is expressed in terms of real and imaginary parts. The matrix generation section  538  further determines eigenvalues of the spatial covariance matrix Rxx, and inputs the resultant eigenvalue information to the arriving wave estimation unit AU. 
     When a plurality of signal intensity peaks corresponding to plural objects of interest have been detected, the reception intensity calculation section  532  numbers the peak values respectively in the ascent portion and in the descent portion, beginning from those with smaller frequencies first, and output them to the target output processing section  539 . In the ascent and descent portions, peaks of any identical number correspond to the same object of interest. The identification numbers are to be regarded as the numbers assigned to the objects of interest. For simplicity of illustration, a leader line from the reception intensity calculation section  532  to the target output processing section  539  is conveniently omitted from  FIG. 22 . 
     When the object of interest is a structure ahead, the target output processing section  539  outputs the identification number of that object of interest as indicating a target. When receiving results of determination concerning plural objects of interest, such that all of them are structures ahead, the target output processing section  539  outputs the identification number of an object of interest that is in the lane of the driver&#39;s vehicle as the object position information indicating where a target is. Moreover, When receiving results of determination concerning plural objects of interest, such that all of them are structures ahead and that two or more objects of interest are in the lane of the driver&#39;s vehicle, the target output processing section  539  outputs the identification number of an object of interest that is associated with the largest count of target being read from the link processes memory  531  as the object position information indicating where a target is. 
     Referring back to  FIG. 21 , an example where the onboard radar system  510  is incorporated in the exemplary construction shown in  FIG. 21  will be described. The image processing circuit  720  acquires information of an object from the video, and detects target position information from the object information. For example, the image processing circuit  720  is configured to estimate distance information of an object by detecting the depth value of an object within an acquired video, or detect size information and the like of an object from characteristic amounts in the video, thus detecting position information of the object. 
     The selection circuit  596  selectively feeds position information which is received from the signal processing circuit  560  or the image processing circuit  720  to the travel assistance electronic control apparatus  520 . For example, the selection circuit  596  compares a first distance, i.e., the distance from the driver&#39;s vehicle to a detected object as contained in the object position information from the signal processing circuit  560 , against a second distance, i.e., the distance from the driver&#39;s vehicle to the detected object as contained in the object position information from the image processing circuit  720 , and determines which is closer to the driver&#39;s vehicle. For example, based on the result of determination, the selection circuit  596  may select the object position information which indicates a closer distance to the driver&#39;s vehicle, and output it to the travel assistance electronic control apparatus  520 . If the result of determination indicates the first distance and the second distance to be of the same value, the selection circuit  596  may output either one, or both of them, to the travel assistance electronic control apparatus  520 . 
     If information indicating that there is no prospective target is input from the reception intensity calculation section  532 , the target output processing section  539  ( FIG. 22 ) outputs zero, indicating that there is no target, as the object position information. Then, on the basis of the object position information from the target output processing section  539 , through comparison against a predefined threshold value, the selection circuit  596  chooses either the object position information from the signal processing circuit  560  or the object position information from the image processing circuit  720  to be used. 
     Based on predefined conditions, the travel assistance electronic control apparatus  520  having received the position information of a preceding object from the object detection apparatus  570  performs control to make the operation safer or easier for the driver who is driving the driver&#39;s vehicle, in accordance with the distance and size indicated by the object position information, the velocity of the driver&#39;s vehicle, road surface conditions such as rainfall, snowfall or clear weather, or other conditions. For example, if the object position information indicates that no object has been detected, the travel assistance electronic control apparatus  520  may send a control signal to an accelerator control circuit  526  to increase speed up to a predefined velocity, thereby controlling the accelerator control circuit  526  to make an operation that is equivalent to stepping on the accelerator pedal. 
     In the case where the object position information indicates that an object has been detected, if it is found to be at a predetermined distance from the driver&#39;s vehicle, the travel assistance electronic control apparatus  520  controls the brakes via a brake control circuit  524  through a brake-by-wire construction or the like. In other words, it makes an operation of decreasing the velocity to maintain a constant vehicular gap. Upon receiving the object position information, the travel assistance electronic control apparatus  520  sends a control signal to an alarm control circuit  522  so as to control lamp illumination or control audio through a loudspeaker which is provided within the vehicle, so that the driver is informed of the nearing of a preceding object. Upon receiving object position information including a spatial distribution of preceding vehicles, the travel assistance electronic control apparatus  520  may, if the traveling velocity is within a predefined range, automatically make the steering wheel easier to operate to the right or left, or control the hydraulic pressure on the steering wheel side so as to force a change in the direction of the wheels, thereby providing assistance in collision avoidance with respect to the preceding object. 
     The object detection apparatus  570  may be arranged so that, if a piece of object position information which was being continuously detected by the selection circuit  596  for a while in the previous detection cycle but which is not detected in the current detection cycle becomes associated with a piece of object position information from a camera-detected video indicating a preceding object, then continued tracking is chosen, and object position information from the signal processing circuit  560  is output with priority. 
     An exemplary specific construction and an exemplary operation for the selection circuit  596  to make a selection between the outputs from the signal processing circuit  560  and the image processing circuit  720  are disclosed in the specification of USP U.S. Pat. No. 8,446,312, the specification of USP U.S. Pat. No. 8,730,096, and the specification of USP U.S. Pat. No. 8,730,099. The entire disclosure thereof is incorporated herein by reference. 
     [First Variant] 
     In the radar system for onboard use of the above Application Example, the (sweep) condition for a single instance of FMCW (Frequency Modulated Continuous Wave) frequency modulation, i.e., a time span required for such a modulation (sweep time), is e.g. 1 millisecond, although the sweep time could be shortened to about 100 microseconds. 
     However, in order to realize such a rapid sweep condition, not only the constituent elements involved in the radiation of a transmission wave, but also the constituent elements involved in the reception under that sweep condition must also be able to rapidly operate. For example, an A/D converter  587  ( FIG. 22 ) which rapidly operates under that sweep condition will be needed. The sampling frequency of the A/D converter  587  may be 10 MHz, for example. The sampling frequency may be faster than 10 MHz. 
     In the present variant, a relative velocity with respect to a target is calculated without utilizing any Doppler shift-based frequency component. In the present embodiment, the sweep time is Tm=100 microseconds, which is very short. The lowest frequency of a detectable beat signal, which is 1/Tm, equals 10 kHz in this case. This would correspond to a Doppler shift of a reflected wave from a target which has a relative velocity of approximately 20 m/second. In other words, so long as one relies on a Doppler shift, it would be impossible to detect relative velocities that are equal to or smaller than this. Thus, a method of calculation which is different from a Doppler shift-based method of calculation is preferably adopted. 
     As an example, this variant illustrates a process that utilizes a signal (upbeat signal) representing a difference between a transmission wave and a reception wave which is obtained in an upbeat (ascent) portion where the transmission wave increases in frequency. A single sweep time of FMCW is 100 microseconds, and its waveform is a sawtooth shape which is composed only of an upbeat portion. In other words, in the present embodiment, the signal wave which is generated by the triangular wave/CW wave generation circuit  581  has a sawtooth shape. The sweep width in frequency is 500 MHz. Since no peaks are to be utilized that are associated with Doppler shifts, the process is not one that generates an upbeat signal and a downbeat signal to utilize the peaks of both, but will rely on only one of such signals. Although a case of utilizing an upbeat signal will be illustrated herein, a similar process can also be performed by using a downbeat signal. 
     The A/D converter  587  ( FIG. 22 ) samples each upbeat signal at a sampling frequency of 10 MHz, and outputs several hundred pieces of digital data (hereinafter referred to as “sampling data”). The sampling data is generated based on upbeat signals after a point in time where a reception wave is obtained and until a point in time at which a transmission wave completes transmission, for example. Note that the process may be ended as soon as a certain number of pieces of sampling data are obtained. 
     In this variant, 128 upbeat signals are transmitted/received in series, for each of which some several hundred pieces of sampling data are obtained. The number of upbeat signals is not limited to 128. It may be 256, or 8. An arbitrary number may be selected depending on the purpose. 
     The resultant sampling data is stored to the memory  531 . The reception intensity calculation section  532  applies a two-dimensional fast Fourier transform (FFT) to the sampling data. Specifically, first, for each of the sampling data pieces that have been obtained through a single sweep, a first FFT process (frequency analysis process) is performed to generate a power spectrum. Next, the velocity detection section  534  performs a second FFT process for the processing results that have been collected from all sweeps. 
     When the reflected waves are from the same target, peak components in the power spectrum to be detected in each sweep period will be of the same frequency. On the other hand, for different targets, the peak components will differ in frequency. Through the first FFT process, plural targets that are located at different distances can be separated. 
     In the case where a relative velocity with respect to a target is non-zero, the phase of the upbeat signal changes slightly from sweep to sweep. In other words, through the second FFT process, a power spectrum whose elements are the data of frequency components that are associated with such phase changes will be obtained for the respective results of the first FFT process. 
     The reception intensity calculation section  532  extracts peak values in the second power spectrum above, and sends them to the velocity detection section  534 . 
     The velocity detection section  534  determines a relative velocity from the phase changes. For example, suppose that a series of obtained upbeat signals undergo phase changes by every phase θ[R×d]. Assuming that the transmission wave has an average wavelength A, this means there is a λ/(4Π/θ) change in distance every time an upbeat signal is obtained. Since this change has occurred over an interval of upbeat signal transmission Tm (=100 microseconds), the relative velocity is determined to be {A/(4Π/θ)}/Tm. 
     Through the above processes, a relative velocity with respect to a target as well as a distance from the target can be obtained. 
     [Second Variant] 
     The radar system  510  is able to detect a target by using a continuous wave(s) CW of one or plural frequencies. This method is especially useful in an environment where a multitude of reflected waves impinge on the radar system  510  from still objects in the surroundings, e.g., when the vehicle is in a tunnel. 
     The radar system  510  has an antenna array for reception purposes, including five channels of independent reception elements. In such a radar system, the azimuth-of-arrival estimation for incident reflected waves is only possible if there are four or fewer reflected waves that are simultaneously incident. In an FMCW-type radar, the number of reflected waves to be simultaneously subjected to an azimuth-of-arrival estimation can be reduced by exclusively selecting reflected waves from a specific distance. However, in an environment where a large number of still objects exist in the surroundings, e.g., in a tunnel, it is as if there were a continuum of objects to reflect radio waves; therefore, even if one narrows down on the reflected waves based on distance, the number of reflected waves may still not be equal to or smaller than four. However, any such still object in the surroundings will have an identical relative velocity with respect to the driver&#39;s vehicle, and the relative velocity will be greater than that associated with any other vehicle that is traveling ahead. On this basis, such still objects can be distinguished from any other vehicle based on the magnitudes of Doppler shifts. 
     Therefore, the radar system  510  performs a process of: radiating continuous waves CW of plural frequencies; and, while ignoring Doppler shift peaks that correspond to still objects in the reception signals, detecting a distance by using a Doppler shift peak(s) of any smaller shift amount(s). Unlike in the FMCW method, in the CW method, a frequency difference between a transmission wave and a reception wave is ascribable only to a Doppler shift. In other words, any peak frequency that appears in a beat signal is ascribable only to a Doppler shift. 
     In the description of this variant, too, a continuous wave to be used in the CW method will be referred to as a “continuous wave CW”. As described above, a continuous wave CW has a constant frequency; that is, it is unmodulated. 
     Suppose that the radar system  510  has radiated a continuous wave CW of a frequency fp, and detected a reflected wave of a frequency fq that has been reflected off a target. The difference between the transmission frequency fp and the reception frequency fq is called a Doppler frequency, which approximates to fp−fq=2·Vr·fp/c. Herein, Vr is a relative velocity between the radar system and the target, and c is the velocity of light. The transmission frequency fp, the Doppler frequency (fp−fq), and the velocity of light c are known. Therefore, from this equation, the relative velocity Vr=(fp−fq)·c/2fp can be determined. The distance to the target is calculated by utilizing phase information as will be described later. 
     In order to detect a distance to a target by using continuous waves CW, a 2 frequency CW method is adopted. In the 2 frequency CW method, continuous waves CW of two frequencies which are slightly apart are radiated each for a certain period, and their respective reflected waves are acquired. For example, in the case of using frequencies in the 76 GHz band, the difference between the two frequencies would be several hundred kHz. As will be described later, it is more preferable to determine the difference between the two frequencies while taking into account the minimum distance at which the radar used is able to detect a target. 
     Suppose that the radar system  510  has sequentially radiated continuous waves CW of frequencies fp1 and fp2 (fp1&lt;fp2), and that the two continuous waves CW have been reflected off a single target, resulting in reflected waves of frequencies fq1 and fq2 being received by the radar system  510 . 
     Based on the continuous wave CW of the frequency fp1 and the reflected wave (frequency fq1) thereof, a first Doppler frequency is obtained. Based on the continuous wave CW of the frequency fp2 and the reflected wave (frequency fq2) thereof, a second Doppler frequency is obtained. The two Doppler frequencies have substantially the same value. However, due to the difference between the frequencies fp1 and fp2, the complex signals of the respective reception waves differ in phase. By utilizing this phase information, a distance (range) to the target can be calculated. 
     Specifically, the radar system  10  is able to determine the distance R as R=c·Δφ/4Π(fp2−fp1). Herein, Δφ denotes the phase difference between two beat signals, i.e., a beat signal fb 1  which is obtained as a difference between the continuous wave CW of the frequency fp1 and the reflected wave (frequency fq1) thereof and a beat signal fb 2  which is obtained as a difference between the continuous wave CW of the frequency fp2 and the reflected wave (frequency fq2) thereof. The method of identifying the frequencies fb 1  and fb 2  of the respective beat signals is identical to that in the aforementioned instance of a beat signal from a continuous wave CW of a single frequency. 
     Note that a relative velocity Vr under the 2 frequency CW method is determined as follows.
 
 Vr=fb 1· c/ 2· fp 1 or  Vr=fb 2· c/ 2· fp 2
 
     Moreover, the range in which a distance to a target can be uniquely identified is limited to the range defined by Rmax&lt;c/2(fp2−fp1). The reason is that beat signals resulting from a reflected wave from any farther target would produce a Δφ which is greater than 2Π, such that they are indistinguishable from beat signals associated with targets at closer positions. Therefore, it is more preferable to adjust the difference between the frequencies of the two continuous waves CW so that Rmax becomes greater than the minimum detectable distance of the radar. In the case of a radar whose minimum detectable distance is 100 m, fp2−fp1 may be made e.g. 1.0 MHz. In this case, Rmax=150 m, so that a signal from any target from a position beyond Rmax is not detected. In the case of mounting a radar which is capable of detection up to 250 m, fp2−fp1 may be made e.g. 500 kHz. In this case, Rmax=300 m, so that a signal from any target from a position beyond Rmax is not detected, either. In the case where the radar has both of an operation mode in which the minimum detectable distance is 100 m and the horizontal viewing angle is 120 degrees and an operation mode in which the minimum detectable distance is 250 m and the horizontal viewing angle is 5 degrees, it is preferable to switch the fp2-fp1 value be 1.0 MHz and 500 kHz for operation in the respective operation modes. 
     A detection approach is known which, by transmitting continuous waves CW at N different frequencies (where N is an integer of 3 or more), and utilizing phase information of the respective reflected waves, detects a distance to each target. Under this detection approach, distance can be properly recognized up to N−1 targets. As the processing to enable this, a fast Fourier transform (FFT) is used, for example. Given N=64 or 128, an FFT is performed for sampling data of a beat signal as a difference between a transmission signal and a reception signal for each frequency, thus obtaining a frequency spectrum (relative velocity). Thereafter, at the frequency of the CW wave, a further FFT is performed for peaks of the same frequency, thus to derive distance information. 
     Hereinafter, this will be described more specifically. 
     For ease of explanation, first, an instance will be described where signals of three frequencies f 1 , f 2  and f 3  are transmitted while being switched over time. It is assumed that f 1 &gt;f 2 &gt;f 3 , and f 1 −f 2 =f 2 −f 3 =Δf. A transmission time Δt is assumed for the signal wave for each frequency.  FIG. 26  shows a relationship between three frequencies f 1 , f 2  and f 3 . 
     Via the transmission antenna Tx, the triangular wave/CW wave generation circuit  581  ( FIG. 22 ) transmits continuous waves CW of frequencies f 1 , f 2  and f 3 , each lasting for the time Δt. The reception antennas Rx receive reflected waves resulting by the respective continuous waves CW being reflected off one or plural targets. 
     Each mixer  584  mixes a transmission wave and a reception wave to generate a beat signal. The A/D converter  587  converts the beat signal, which is an analog signal, into several hundred pieces of digital data (sampling data), for example. 
     Using the sampling data, the reception intensity calculation section  532  performs FFT computation. Through the FFT computation, frequency spectrum information of reception signals is obtained for the respective transmission frequencies f 1 , f 2  and f 3 . 
     Thereafter, the reception intensity calculation section  532  separates peak values from the frequency spectrum information of the reception signals. The frequency of any peak value which is predetermined or greater is in proportion to a relative velocity with respect to a target. Separating a peak value(s) from the frequency spectrum information of reception signals is synonymous with separating one or plural targets with different relative velocities. 
     Next, with respect to each of the transmission frequencies f 1  to f 3 , the reception intensity calculation section  532  measures spectrum information of peak values of the same relative velocity or relative velocities within a predefined range. 
     Now, consider a scenario where two targets A and B exist which have about the same relative velocity but are at respectively different distances. A transmission signal of the frequency f 1  will be reflected from both of targets A and B to result in reception signals being obtained. The reflected waves from targets A and B will result in substantially the same beat signal frequency. Therefore, the power spectra at the Doppler frequencies of the reception signals, corresponding to their relative velocities, are obtained as a synthetic spectrum F 1  into which the power spectra of two targets A and B have been merged. 
     Similarly, for each of the frequencies f 2  and f 3 , the power spectra at the Doppler frequencies of the reception signals, corresponding to their relative velocities, are obtained as a synthetic spectrum F 1  into which the power spectra of two targets A and B have been merged. 
       FIG. 27  shows a relationship between synthetic spectra F 1  to F 3  on a complex plane. In the directions of the two vectors composing each of the synthetic spectra F 1  to F 3 , the right vector corresponds to the power spectrum of a reflected wave from target A; i.e., vectors f 1 A, f 2 A and f 3 A, in  FIG. 27 . On the other hand, in the directions of the two vectors composing each of the synthetic spectra F 1  to F 3 , the left vector corresponds to the power spectrum of a reflected wave from target B; i.e., vectors f 1 B, f 2 B and f 3 B in  FIG. 27 . 
     Under a constant difference Δf between the transmission frequencies, the phase difference between the reception signals corresponding to the respective transmission signals of the frequencies f 1  and f 2  is in proportion to the distance to a target. Therefore, the phase difference between the vectors f 1 A and f 2 A and the phase difference between the vectors f 2 A and f 3 A are of the same value θA, this phase difference θA being in proportion to the distance to target A. Similarly, the phase difference between the vectors f 1 B and f 2 B and the phase difference between the vectors f 2 B and f 3 B are of the same value θB, this phase difference θB being in proportion to the distance to target B. 
     By using a well-known method, the respective distances to targets A and B can be determined from the synthetic spectra F 1  to F 3  and the difference Δf between the transmission frequencies. This technique is disclosed in U.S. Pat. No. 6,703,967, for example. The entire disclosure of this publication is incorporated herein by reference. 
     Similar processing is also applicable when the transmitted signals have four or more frequencies. 
     Note that, before transmitting continuous wave CWs at N different frequencies, a process of determining the distance to and relative velocity of each target may be performed by the 2 frequency CW method. Then, under predetermined conditions, this process may be switched to a process of transmitting continuous waves CW at N different frequencies. For example, FFT computation may be performed by using the respective beat signals at the two frequencies, and if the power spectrum of each transmission frequency undergoes a change over time of 30% or more, the process may be switched. The amplitude of a reflected wave from each target undergoes a large change over time due to multipath influences and the like. When there exists a change of a predetermined magnitude or greater, it may be considered that plural targets may exist. 
     Moreover, the CW method is known to be unable to detect a target when the relative velocity between the radar system and the target is zero, i.e., when the Doppler frequency is zero. However, when a pseudo Doppler signal is determined by the following methods, for example, it is possible to detect a target by using that frequency. 
     (Method 1) A mixer that causes a certain frequency shift in the output of a receiving antenna is added. By using a transmission signal and a reception signal with a shifted frequency, a pseudo Doppler signal can be obtained. 
     (Method 2) A variable phase shifter to introduce phase changes continuously over time is inserted between the output of a receiving antenna and a mixer, thus adding a pseudo phase difference to the reception signal. By using a transmission signal and a reception signal with an added phase difference, a pseudo Doppler signal can be obtained. 
     An example of specific construction and operation of inserting a variable phase shifter to generate a pseudo Doppler signal under Method 2 is disclosed in Japanese Laid-Open Patent Publication No. 2004-257848. The entire disclosure of this publication is incorporated herein by reference. 
     When targets with zero or very little relative velocity need to be detected, the aforementioned processes of generating a pseudo Doppler signal may be adopted, or the process may be switched to a target detection process under the FMCW method. 
     Next, with reference to  FIG. 28 , a procedure of processing to be performed by the object detection apparatus  570  of the onboard radar system  510  will be described. 
     The example below will illustrate a case where continuous waves CW are transmitted at two different frequencies fp1 and fp2 (fp1&lt;fp2), and the phase information of each reflected wave is utilized to respectively detect a distance with respect to a target. 
       FIG. 28  is a flowchart showing the procedure of a process of determining relative velocity and distance according to this variant. 
     At step S 41 , the triangular wave/CW wave generation circuit  581  generates two continuous waves CW of frequencies which are slightly apart, i.e., frequencies fp1 and fp2. 
     At step S 42 , the transmission antenna Tx and the reception antennas Rx perform transmission/reception of the generated series of continuous waves CW. Note that the process of step S 41  and the process of step S 42  are to be performed in parallel fashion by the triangular wave/CW wave generation circuit  581  and the antenna elements Tx/Rx, rather than step S 42  following only after completion of step S 41 . 
     At step S 43 , each mixer  584  generates a difference signal by utilizing each transmission wave and each reception wave, whereby two difference signals are obtained. Each reception wave is inclusive of a reception wave emanating from a still object and a reception wave emanating from a target. Therefore, next, a process of identifying frequencies to be utilized as the beat signals is performed. Note that the process of step S 41 , the process of step S 42 , and the process of step  43  are to be performed in parallel fashion by the triangular wave/CW wave generation circuit  581 , the antenna elements Tx/Rx, and the mixers  584 , rather than step S 42  following only after completion of step S 41 , or step  43  following only after completion of step  42 . 
     At step S 44 , for each of the two difference signals, the object detection apparatus  570  identifies certain peak frequencies to be frequencies fb 1  and fb 2  of beat signals, such that these frequencies are equal to or smaller than a frequency which is predefined as a threshold value and yet they have amplitude values which are equal to or greater than a predetermined amplitude value, and that the difference between the two frequencies is equal to or smaller than a predetermined value. 
     At step S 45 , based on one of the two beat signal frequencies identified, the reception intensity calculation section  532  detects a relative velocity. The reception intensity calculation section  532  calculates the relative velocity according to Vr=fb 1 ·c/2·fp1, for example. Note that a relative velocity may be calculated by utilizing each of the two beat signal frequencies, which will allow the reception intensity calculation section  532  to verify whether they match or not, thus enhancing the precision of relative velocity calculation. 
     At step S 46 , the reception intensity calculation section  532  determines a phase difference Δφ between the two beat signals fb 1  and fb 2 , and determines a distance R=c·Δφ/4Π(fp2−fp1) to the target. 
     Through the above processes, the relative velocity and distance to a target can be detected. 
     Note that continuous waves CW may be transmitted at N different frequencies (where N is 3 or more), and by utilizing phase information of the respective reflected wave, distances to plural targets which are of the same relative velocity but at different positions may be detected. 
     In addition to the radar system  510 , the vehicle  500  described above may further include another radar system. For example, the vehicle  500  may further include a radar system having a detection range toward the rear or the sides of the vehicle body. In the case of incorporating a radar system having a detection range toward the rear of the vehicle body, the radar system may monitor the rear, and if there is any danger of having another vehicle bump into the rear, make a response by issuing an alarm, for example. In the case of incorporating a radar system having a detection range toward the sides of the vehicle body, the radar system may monitor an adjacent lane when the driver&#39;s vehicle changes its lane, etc., and make a response by issuing an alarm or the like as necessary. 
     The applications of the above-described radar system  510  are not limited to onboard use only. Rather, the radar system  510  may be used as sensors for various purposes. For example, it may be used as a radar for monitoring the surroundings of a house or any other building. Alternatively, it may be used as a sensor for detecting the presence or absence of a person at a specific indoor place, or whether or not such a person is undergoing any motion, etc., without utilizing any optical images. 
     [Supplementary Details of Processing] 
     Other embodiments will be described in connection with the 2 frequency CW or FMCW techniques for array antennas as described above. As described earlier, in the example of  FIG. 22 , the reception intensity calculation section  532  applies a Fourier transform to the respective beat signals for the channels Ch 1  to Ch M  (lower graph in  FIG. 23 ) stored in the memory  531 . These beat signals are complex signals, in order that the phase of the signal of computational interest be identified. This allows the direction of an arriving wave to be accurately identified. In this case, however, the computational load for Fourier transform increases, thus calling for a larger-scaled circuit. 
     In order to solve this problem, a scalar signal may be generated as a beat signal. For each of a plurality of beat signals that have been generated, two complex Fourier transforms may be performed with respect to the spatial axis direction, which conforms to the antenna array, and to the time axis direction, which conforms to the lapse of time, thus to obtain results of frequency analysis. As a result, with only a small amount of computation, beam formation can eventually be achieved so that directions of arrival of reflected waves can be identified, whereby results of frequency analysis can be obtained for the respective beams. As a patent document related to the present disclosure, the entire disclosure of the specification of U.S. Pat. No. 6,339,395 is incorporated herein by reference. 
     [Optical Sensor, e.g., Camera, and Millimeter Wave Radar] 
     Next, a comparison between the above-described array antenna and conventional antennas, as well as an exemplary application in which both of the array antenna according to the present disclosure and an optical sensor (e.g., a camera) are utilized, will be described. Note that LIDAR or the like may be employed as the optical sensor. 
     A millimeter wave radar is able to directly detect a distance (range) to a target and a relative velocity thereof. Another characteristic is that its detection performance is not much deteriorated in the nighttime (including dusk), or in bad weather, e.g., rainfall, fog, or snowfall. On the other hand, it is believed that it is not just as easy for a millimeter wave radar to take a two-dimensional grasp of a target as it is for a camera. On the other hand, it is relatively easy for a camera to take a two-dimensional grasp of a target and recognize its shape. However, a camera may not be able to image a target in nighttime or bad weather, which presents a considerable problem. This problem is particularly outstanding when droplets of water have adhered to the portion through which to ensure lighting, or the eyesight is narrowed by a fog. This problem similarly exists for LIDAR or the like, which also pertains to the realm of optical sensors. 
     In these years, in answer to increasing demand for safer vehicle operation, driver assist systems for preventing collisions or the like are being developed. A driver assist system acquires an image in the direction of vehicle travel with a sensor such as a camera or a millimeter wave radar, and when any obstacle is recognized that is predicted to hinder vehicle travel, brakes or the like are automatically applied to prevent collisions or the like. Such a function of collision avoidance is expected to operate normally, even in nighttime or bad weather. 
     Hence, driver assist systems of a so-called fusion construction are gaining prevalence, where, in addition to a conventional optical sensor such as a camera, a millimeter wave radar is mounted as a sensor, thus realizing a recognition process that takes advantage of both. Such a driver assist system will be discussed later. 
     On the other hand, higher and higher functions are being required of the millimeter wave radar itself. A millimeter wave radar for onboard use mainly uses electromagnetic waves of the 76 GHz band. The antenna power of its antenna is restricted to below a certain level under each country&#39;s law or the like. For example, it is restricted to 0.01 W or below in Japan. Under such restrictions, a millimeter wave radar for onboard use is expected to satisfy the required performance that, for example, its detection range is 200 m or more; the antenna size is 60 mm×60 mm or less; its horizontal detection angle is 90 degrees or more; its range resolution is 20 cm or less; it is capable of short-range detection within 10 m; and so on. Conventional millimeter wave radars have used microstrip lines as waveguides, and patch antennas as antennas (hereinafter, these will both be referred to as “patch antennas”). However, with a patch antenna, it has been difficult to attain the aforementioned performance. 
     By using a slot array antenna to which the technique of the present disclosure is applied, the inventors have successfully achieved the aforementioned performance. As a result, a millimeter wave radar has been realized which is smaller in size, more efficient, and higher-performance than are conventional patch antennas and the like. In addition, by combining this millimeter wave radar and an optical sensor such as a camera, a small-sized, highly efficient, and high-performance fusion apparatus has been realized which has existed never before. This will be described in detail below. 
       FIG. 29  is a diagram concerning a fusion apparatus in a vehicle  500 , the fusion apparatus including a camera  700  and a radar system  510  (hereinafter referred to also as the millimeter wave radar  510 ) having a slot array antenna to which the technique of the present disclosure is applied. With reference to this figure, various embodiments will be described below. 
     [Installment of Millimeter Wave Radar within Vehicle Room] 
     A conventional patch antenna-based millimeter wave radar  510 ′ is placed behind and inward of a grill  512  which is at the front nose of a vehicle. An electromagnetic wave that is radiated from an antenna goes through the apertures in the grill  512 , and is radiated ahead of the vehicle  500 . In this case, no dielectric layer, e.g., glass, exists that decays or reflects electromagnetic wave energy, in the region through which the electromagnetic wave passes. As a result, an electromagnetic wave that is radiated from the patch antenna-based millimeter wave radar  510 ′ reaches over a long range, e.g., to a target which is 150 m or farther away. By receiving with the antenna the electromagnetic wave reflected therefrom, the millimeter wave radar  510 ′ is able to detect a target. In this case, however, since the antenna is placed behind and inward of the grill  512  of the vehicle, the radar may be broken when the vehicle collides into an obstacle. Moreover, it may be soiled with mud or the like in rain, etc., and the soil that has adhered to the antenna may hinder radiation and reception of electromagnetic waves. 
     Similarly to the conventional manner, the millimeter wave radar  510  incorporating a slot array antenna according to an embodiment of the present disclosure may be placed behind the grill  512 , which is located at the front nose of the vehicle (not shown). This allows the energy of the electromagnetic wave to be radiated from the antenna to be utilized by 100%, thus enabling long-range detection beyond the conventional level, e.g., detection of a target which is at a distance of 250 m or more. 
     Furthermore, the millimeter wave radar  510  according to an embodiment of the present disclosure can also be placed within the vehicle room, i.e., inside the vehicle. In that case, the millimeter wave radar  510  is placed inward of the windshield  511  of the vehicle, to fit in a space between the windshield  511  and a face of the rearview mirror (not shown) that is opposite to its specular surface. On the other hand, the conventional patch antenna-based millimeter wave radar  510 ′ cannot be placed inside the vehicle room mainly for the two following reasons. A first reason is its large size, which prevents itself from being accommodated within the space between the windshield  511  and the rearview mirror. A second reason is that an electromagnetic wave that is radiated ahead reflects off the windshield  511  and decays due to dielectric loss, thus becoming unable to travel the desired distance. As a result, if a conventional patch antenna-based millimeter wave radar is placed within the vehicle room, only targets which are 100 m ahead or less can be detected, for example. On the other hand, a millimeter wave radar according to an embodiment of the present disclosure is able to detect a target which is at a distance of 200 m or more, despite reflection or decay at the windshield  511 . This performance is equivalent to, or even greater than, the case where a conventional patch antenna-based millimeter wave radar is placed outside the vehicle room. 
     [Fusion Construction Based on Millimeter Wave Radar and Camera, Etc., being Placed within Vehicle Room] 
     Currently, an optical imaging device such as a CCD camera is used as the main sensor in many a driver assist system (Driver Assist System). Usually, a camera or the like is placed within the vehicle room, inward of the windshield  511 , in order to account for unfavorable influences of the external environment, etc. In this context, in order to minimize the optical effect of raindrops and the like, the camera or the like is placed in a region which is swept by the wipers (not shown) but is inward of the windshield  511 . 
     In recent years, due to needs for improved performance of a vehicle in terms of e.g. automatic braking, there has been a desire for automatic braking or the like that is guaranteed to work regardless of whatever external environment may exist. In this case, if the only sensor in the driver assist system is an optical device such as a camera, a problem exists in that reliable operation is not guaranteed in nighttime or bad weather. This has led to the need for a driver assist system that incorporates not only an optical sensor (such as a camera) but also a millimeter wave radar, these being used for cooperative processing, so that reliable operation is achieved even in nighttime or bad weather. 
     As described earlier, a millimeter wave radar incorporating the slot array antenna according to the present disclosure permits itself to be placed within the vehicle room, due to downsizing and remarkable enhancement in the efficiency of the radiated electromagnetic wave over that of a conventional patch antenna. By taking advantage of these properties, as shown in  FIG. 29 , the millimeter wave radar  510 , which incorporates not only an optical sensor  700  such as a camera but also a slot array antenna according to the present disclosure, allows both to be placed inward of the windshield  511  of the vehicle  500 . This has created the following novel effects. 
     (1) It is easier to install the driver assist system on the vehicle  500 . The conventional patch antenna  510 ′ has required a space behind the grill  512 , which is at the front nose, in order to accommodate the radar. Since this space may include some sites that affect the structural design of the vehicle, if the size of the radar device is changed, it may have been necessary to reconsider the structural design. This inconvenience is avoided by placing the millimeter wave radar within the vehicle room. 
     (2) Free from the influences of rain, nighttime, or other external environment factors to the vehicle, more reliable operation can be achieved. Especially, as shown in  FIG. 30 , by placing the millimeter wave radar  510  and the camera  700  at substantially the same position within the vehicle room, they can attain an identical field of view and line of sight, thus facilitating the “matching process” which will be described later, i.e., a process through which to establish that respective pieces of target information captured by them actually come from an identical object. On the other hand, if the millimeter wave radar  510 ′ were placed behind the grill  512 , which is at the front nose outside the vehicle room, its radar line of sight L would differ from a radar line of sight M of the case where it was placed within the vehicle room, thus resulting in a large offset with the image to be acquired by the camera  700 . 
     (3) Reliability of the millimeter wave radar device is improved. As described above, since the conventional patch antenna  510 ′ is placed behind the grill  512 , which is at the front nose, it is likely to gather soil, and may be broken even in a minor collision accident or the like. For these reasons, cleaning and functionality checks are always needed. Moreover, as will be described below, if the position or direction of attachment of the millimeter wave radar becomes shifted due to an accident or the like, it is necessary to reestablish alignment with respect to the camera. The chances of such occurrences are reduced by placing the millimeter wave radar within the vehicle room, whereby the aforementioned inconveniences are avoided. 
     In a driver assist system of such fusion construction, the optical sensor  700 , e.g., a camera, and the millimeter wave radar  510  incorporating the slot array antenna according to the present disclosure may have an integrated construction, i.e., being in fixed position with respect to each other. In that case, certain relative positioning should be kept between the optical axis of the optical sensor such as a camera and the directivity of the antenna of the millimeter wave radar, as will be described later. When this driver assist system having an integrated construction is fixed within the vehicle room of the vehicle  500 , the optical axis of the camera, etc., should be adjusted so as to be oriented in a certain direction ahead of the vehicle. For these matters, see US Patent Application Publication No. 2015/0264230, US Patent Application Publication No. 2016/0264065, U.S. patent application Ser. No. 15/248,141, U.S. patent application Ser. No. 15/248,149, and U.S. patent application Ser. No. 15/248,156, which are incorporated herein by reference. Related techniques concerning the camera are described in the specification of U.S. Pat. No. 7,355,524, and the specification of U.S. Pat. No. 7,420,159, the entire disclosure of each which is incorporated herein by reference. 
     Regarding placement of an optical sensor such as a camera and a millimeter wave radar within the vehicle room, see, for example, the specification of U.S. Pat. No. 8,604,968, the specification of U.S. Pat. No. 8,614,640, and the specification of U.S. Pat. No. 7,978,122, the entire disclosure of each which is incorporated herein by reference. However, at the time when these patents were filed for, only conventional antennas with patch antennas were the known millimeter wave radars, and thus observation was not possible over sufficient distances. For example, the distance that is observable with a conventional millimeter wave radar is considered to be at most 100 m to 150 m. Moreover, when a millimeter wave radar is placed inward of the windshield, the large radar size inconveniently blocks the driver&#39;s field of view, thus hindering safe driving. On the other hand, a millimeter wave radar incorporating a slot array antenna according to an embodiment of the present disclosure is capable of being placed within the vehicle room because of its small size and remarkable enhancement in the efficiency of the radiated electromagnetic wave over that of a conventional patch antenna. This enables a long-range observation over 200 m, while not blocking the driver&#39;s field of view. 
     [Adjustment of Position of Attachment Between Millimeter Wave Radar and Camera, Etc.,] 
     In the processing under fusion construction (which hereinafter may be referred to as a “fusion process”), it is desired that an image which is obtained with a camera or the like and the radar information which is obtained with the millimeter wave radar map onto the same coordinate system because, if they differ as to position and target size, cooperative processing between both will be hindered. 
     This involves adjustment from the following three standpoints. 
     (1) The optical axis of the camera or the like and the antenna directivity of the millimeter wave radar must have a certain fixed relationship. 
     It is required that the optical axis of the camera or the like and the antenna directivity of the millimeter wave radar are matched. Alternatively, a millimeter wave antenna may include two or more transmission antennas and two or more reception antennas, the directivities of these antennas being intentionally made different. Therefore, it is necessary to guarantee that at least a certain known relationship exists between the optical axis of the camera or the like and the directivities of these antennas. 
     In the case where the camera or the like and the millimeter wave radar have the aforementioned integrated construction, i.e., being in fixed position to each other, the relative positioning between the camera or the like and the millimeter wave radar stays fixed. Therefore, the aforementioned requirements are satisfied with respect to such an integrated construction. On the other hand, in a conventional patch antenna or the like, where the millimeter wave antenna is placed behind the grill  512  of the vehicle  500 , the relative positioning between them is usually to be adjusted according to (2) below. 
     (2) A certain fixed relationship exists between an image acquired with the camera or the like and radar information of the millimeter wave radar in an initial state (e.g., upon shipment) of having been attached to the vehicle. 
     The positions of attachment of the optical sensor  700  such as a camera and the millimeter wave radar  510  or  510 ′ on the vehicle  500  will finally be determined in the following manner. At a predetermined position  800  ahead of the vehicle  500 , a chart to serve as a reference or a target which is subject to observation by the radar (which will hereinafter be referred to as, respectively, a “reference chart” and a “reference target”, and collectively as the “benchmark”) is accurately positioned. This is observed with the optical sensor  700  such as a camera or with the millimeter wave radar  510 . The observation information regarding the observed benchmark is compared against previously-stored shape information or the like of the benchmark, and the current offset information is quantitated. Based on this offset information, by at least one of the following means, the positions of attachment of the optical sensor  700  such as a camera and the millimeter wave radar  510  or  510 ′ are adjusted or corrected. Any other means may also be employed that can provide similar results. 
     (i) Adjust the positions of attachment of the camera and the radar device so that the benchmark will come at a midpoint between the camera and the radar. This adjustment may be done by using a jig or tool, etc., which is separately provided. 
     (ii) Determine an offset amounts of the camera and the radar relative to the benchmark, and through image processing of the camera image and radar processing, correct for these offset amounts. 
     What is to be noted is that, in the case where the optical sensor  700  such as a camera and the millimeter wave radar  510  incorporating a slot array antenna according to an embodiment of the present disclosure have an integrated construction, i.e., being in fixed position to each other, adjusting an offset of either the camera or the radar with respect to the benchmark will make the offset amount known for the other as well, thus making it unnecessary to check for the other&#39;s offset with respect to the benchmark. 
     Specifically, with respect to the camera  700 , a reference chart may be placed at a predetermined position  750 , and an image taken by the camera  700  is compared against advance information indicating where in the field of view of the camera  700  the reference chart image is supposed to be located, thereby detecting an offset amount. Based on this, the camera  700  is adjusted by at least one of the above means (i) and (ii). Next, the offset amount which has been ascertained for the camera is translated into an offset amount of the millimeter wave radar. Thereafter, an offset amount adjustment is made with respect to the radar information, by at least one of the above means (i) and (ii). 
     Alternatively, this may be performed on the basis of the millimeter wave radar  510 . In other words, with respect to the millimeter wave radar  510 , a reference target may be placed at a predetermined position, and the radar information thereof is compared against advance information indicating where in the field of view of the millimeter wave radar  510  the reference target is supposed to be located, thereby detecting an offset amount. Based on this, the millimeter wave radar  510  is adjusted by at least one of the above means (i) and (ii). Next, the offset amount which has been ascertained for the millimeter wave radar is translated into an offset amount of the camera. Thereafter, an offset amount adjustment is made with respect to the image information obtained by the camera  700 , by at least one of the above means (i) and (ii). 
     (3) Even after an initial state of the vehicle, a certain relationship is maintained between an image acquired with the camera or the like and radar information of the millimeter wave radar. 
     Usually, an image acquired with the camera or the like and radar information of the millimeter wave radar are supposed to be fixed in the initial state, and hardly vary unless in an accident of the vehicle or the like. However, if an offset in fact occurs between these, an adjustment is possible by the following means. 
     The camera  700  is attached in such a manner that portions  513  and  514  (characteristic points) that are characteristic of the driver&#39;s vehicle fit within its field of view, for example. The positions at which these characteristic points are actually imaged by the camera  700  are compared against the information of the positions to be assumed by these characteristic points when the camera  700  is attached accurately in place, and an offset amount(s) is detected therebetween. Based on this detected offset amount(s), the position of any image that is taken thereafter may be corrected, whereby an offset of the physical position of attachment of the camera  700  can be corrected for. If this correction sufficiently embodies the performance that is required of the vehicle, then the adjustment per the above (2) may not be needed. By regularly performing this adjustment during startup or operation of the vehicle  500 , even if an offset of the camera or the like occurs anew, it is possible to correct for the offset amount, thus helping safe travel. 
     However, this means is generally considered to result in poorer accuracy of adjustment than with the above means (2). Supposedly, a reference object(s) that will provide sufficient accuracy is placed at a predetermined position(s) moderately distant from the vehicle before the adjustment, thus enabling adjustment with a predetermined accuracy. However, this means (3) involves an adjustment that is based on parts of the vehicle body, which can only provide a poorer accuracy than that will be provided by a benchmark, and thus the resultant accuracy of adjustment will be somewhat inferior. However, it may still be effective as a means of correction when the position of attachment of the camera or the like is considerably altered for reasons such as an accident or a large external force being applied to the camera or the like within the vehicle room, etc. 
     [Mapping of Target as Detected by Millimeter Wave Radar and Camera or the Like: Matching Process] 
     In a fusion process, for a given target, it needs to be established that an image thereof which is acquired with a camera or the like and radar information which is acquired with the millimeter wave radar pertain to “the same target”. For example, suppose that two obstacles (first and second obstacles), e.g., two bicycles, have appeared ahead of the vehicle  500 . These two obstacles will be captured as camera images, and detected as radar information of the millimeter wave radar. At this time, the camera image and the radar information with respect to the first obstacle need to be mapped to each other so that they are both directed to the same target. Similarly, the camera image and the radar information with respect to the second obstacle need to be mapped to each other so that they are both directed to the same target. If the camera image of the first obstacle and the radar information of the second obstacle are mistakenly recognized to pertain to an identical object, a considerable accident may occur. Hereinafter, in the present specification, such a process of determining whether a camera image and a radar target pertain to the same target may be referred to as a “matching process”. 
     This matching process may be implemented by various detection devices (or methods) described below. Hereinafter, these will be specifically described. Note that the each of the following detection devices is to be installed in the vehicle, and at least includes a millimeter wave radar detection section, an image detection section (e.g., a camera) which is oriented in a direction overlapping the direction of detection by the millimeter wave radar detection section, and a matching section. Herein, the millimeter wave radar detection section includes a slot array antenna according to any of the embodiments of the present disclosure, and at least acquires radar information in its own field of view. The image acquisition section at least acquires image information in its own field of view. The matching section includes a processing circuit which matches a result of detection by the millimeter wave radar detection section against a result of detection by the image detection section to determine whether or not the same target is being detected by the two detection sections. Herein, the image detection section may be composed of a selected one of, or selected two or more of, an optical camera, LIDAR, an infrared radar, and an ultrasonic radar. The following detection devices differ from one another in terms of the detection process at their respective matching section. 
     In a first detection device, the matching section performs two matches as follows. A first match involves, for a target of interest that has been detected by the millimeter wave radar detection section, obtaining distance information and lateral position information thereof, and also finding a target that is the closest to the target of interest among a target or two or more targets detected by the image detection section, and detecting a combination(s) thereof. A second match involves, for a target of interest that has been detected by the image detection section, obtaining distance information and lateral position information thereof, and also finding a target that is the closest to the target of interest among a target or two or more targets detected by the millimeter wave radar detection section, and detecting a combination(s) thereof. Furthermore, this matching section determines whether there is any matching combination between the combination(s) of such targets as detected by the millimeter wave radar detection section and the combination(s) of such targets as detected by the image detection section. Then, if there is any matching combination, it is determined that the same object is being detected by the two detection sections. In this manner, a match is attained between the respective targets that have been detected by the millimeter wave radar detection section and the image detection section. 
     A related technique is described in the specification of U.S. Pat. No. 7,358,889, the entire disclosure of which is incorporated herein by reference. In this publication, the image detection section is illustrated by way of a so-called stereo camera that includes two cameras. However, this technique is not limited thereto. In the case where the image detection section includes a single camera, detected targets may be subjected to an image recognition process or the like as appropriate, in order to obtain distance information and lateral position information of the targets. Similarly, a laser sensor such as a laser scanner may be used as the image detection section. 
     In a second detection device, the matching section matches a result of detection by the millimeter wave radar detection section and a result of detection by the image detection section every predetermined period of time. If the matching section determines that the same target was being detected by the two detection sections in the previous result of matching, it performs a match by using this previous result of matching. Specifically, the matching section matches a target which is currently detected by the millimeter wave radar detection section and a target which is currently detected by the image detection section, against the target which was determined in the previous result of matching to be being detected by the two detection sections. Then, based on the result of matching for the target which is currently detected by the millimeter wave radar detection section and the result of matching for the target which is currently detected by the image detection section, the matching section determines whether or not the same target is being detected by the two detection sections. Thus, rather than directly matching the results of detection by the two detection sections, this detection device performs a chronological match between the two results of detection and a previous result of matching. Therefore, the accuracy of detection is improved over the case of only performing a momentary match, whereby stable matching is realized. In particular, even if the accuracy of the detection section drops momentarily, matching is still possible because of utilizing past results of matching. Moreover, by utilizing the previous result of matching, this detection device is able to easily perform a match between the two detection sections. 
     In the current match which utilizes the previous result of matching, if the matching section of this detection device determines that the same object is being detected by the two detection sections, then the matching section of this detection device excludes this determined object in performing matching between objects which are currently detected by the millimeter wave radar detection section and objects which are currently detected by the image detection section. Then, this matching section determines whether there exists any identical object that is currently detected by the two detection sections. Thus, while taking into account the result of chronological matching, the detection device also makes a momentary match based on two results of detection that are obtained from moment to moment. As a result, the detection device is able to surely perform a match for any object that is detected during the current detection. 
     A related technique is described in the specification of U.S. Pat. No. 7,417,580, the entire disclosure of which is incorporated herein by reference. In this publication, the image detection section is illustrated by way of a so-called stereo camera that includes two cameras. However, this technique is not limited thereto. In the case where the image detection section includes a single camera, detected targets may be subjected to an image recognition process or the like as appropriate, in order to obtain distance information and lateral position information of the targets. Similarly, a laser sensor such as a laser scanner may be used as the image detection section. 
     In a third detection device, the two detection sections and matching section perform detection of targets and performs matches therebetween at predetermined time intervals, and the results of such detection and the results of such matching are chronologically stored to a storage medium, e.g., memory. Then, based on a rate of change in the size of a target in the image as detected by the image detection section, and on a distance to a target from the driver&#39;s vehicle and its rate of change (relative velocity with respect to the driver&#39;s vehicle) as detected by the millimeter wave radar detection section, the matching section determines whether the target which has been detected by the image detection section and the target which has been detected by the millimeter wave radar detection section are an identical object. 
     When determining that these targets are an identical object, based on the position of the target in the image as detected by the image detection section, and on the distance to the target from the driver&#39;s vehicle and/or its rate of change as detected by the millimeter wave radar detection section, the matching section predicts a possibility of collision with the vehicle. 
     A related technique is described in the specification of U.S. Pat. No. 6,903,677, the entire disclosure of which is incorporated herein by reference. 
     As described above, in a fusion process of a millimeter wave radar and an imaging device such as a camera, an image which is obtained with the camera or the like and radar information which is obtained with the millimeter wave radar are matched against each other. A millimeter wave radar incorporating the aforementioned array antenna according to an embodiment of the present disclosure can be constructed so as to have a small size and high performance. Therefore, high performance and downsizing, etc., can be achieved for the entire fusion process including the aforementioned matching process. This improves the accuracy of target recognition, and enables safer travel control for the vehicle. 
     [Other Fusion Processes] 
     In a fusion process, various functions are realized based on a matching process between an image which is obtained with a camera or the like and radar information which is obtained with the millimeter wave radar detection section. Examples of processing apparatuses that realize representative functions of a fusion process will be described below. 
     Each of the following processing apparatuses is to be installed in a vehicle, and at least includes: a millimeter wave radar detection section to transmit or receive electromagnetic waves in a predetermined direction; an image acquisition section, such as a monocular camera, that has a field of view overlapping the field of view of the millimeter wave radar detection section; and a processing section which obtains information therefrom to perform target detection and the like. The millimeter wave radar detection section acquires radar information in its own field of view. The image acquisition section acquires image information in its own field of view. A selected one, or selected two or more of, an optical camera, LIDAR, an infrared radar, and an ultrasonic radar may be used as the image acquisition section. The processing section can be implemented by a processing circuit which is connected to the millimeter wave radar detection section and the image acquisition section. The following processing apparatuses differ from one another with respect to the content of processing by this processing section. 
     In a first processing apparatus, the processing section extracts, from an image which is captured by the image acquisition section, a target which is recognized to be the same as the target which is detected by the millimeter wave radar detection section. In other words, a matching process according to the aforementioned detection device is performed. Then, it acquires information of a right edge and a left edge of the extracted target image, and derives locus approximation lines, which are straight lines or predetermined curved lines for approximating loci of the acquired right edge and the left edge, are derived for both edges. The edge which has a larger number of edges existing on the locus approximation line is selected as a true edge of the target. The lateral position of the target is derived on the basis of the position of the edge that has been selected as a true edge. This permits a further improvement on the accuracy of detection of a lateral position of the target. 
     A related technique is described in the specification of U.S. Pat. No. 8,610,620, the entire disclosure of which is incorporated herein by reference. 
     In a second processing apparatus, in determining the presence of a target, the processing section alters a determination threshold to be used in checking for a target presence in radar information, on the basis of image information. Thus, if a target image that may be an obstacle to vehicle travel has been confirmed with a camera or the like, or if the presence of a target has been estimated, etc., for example, the determination threshold for the target detection by the millimeter wave radar detection section can be optimized so that more accurate target information can be obtained. In other words, if the possibility of the presence of an obstacle is high, the determination threshold is altered so that this processing apparatus will surely be activated. On the other hand, if the possibility of the presence of an obstacle is low, the determination threshold is altered so that unwanted activation of this processing apparatus is prevented. This permits appropriate activation of the system. 
     Furthermore in this case, based on radar information, the processing section may designate a region of detection for the image information, and estimate a possibility of the presence of an obstacle on the basis of image information within this region. This makes for a more efficient detection process. 
     A related technique is described in the specification of U.S. Pat. No. 7,570,198, the entire disclosure of which is incorporated herein by reference. 
     In a third processing apparatus, the processing section performs combined displaying where images obtained from a plurality of different imaging devices and a millimeter wave radar detection section and an image signal based on radar information are displayed on at least one display device. In this displaying process, horizontal and vertical synchronizing signals are synchronized between the plurality of imaging devices and the millimeter wave radar detection section, and among the image signals from these devices, selective switching to a desired image signal is possible within one horizontal scanning period or one vertical scanning period. This allows, on the basis of the horizontal and vertical synchronizing signals, images of a plurality of selected image signals to be displayed side by side; and, from the display device, a control signal for setting a control operation in the desired imaging device and the millimeter wave radar detection section is sent. 
     When a plurality of different display devices display respective images or the like, it is difficult to compare the respective images against one another. Moreover, when display devices are provided separately from the third processing apparatus itself, there is poor operability for the device. The third processing apparatus would overcome such shortcomings. 
     A related technique is described in the specification of U.S. Pat. No. 6,628,299 and the specification of U.S. Pat. No. 7,161,561, the entire disclosure of each of which is incorporated herein by reference. 
     In a fourth processing apparatus, with respect to a target which is ahead of a vehicle, the processing section instructs an image acquisition section and a millimeter wave radar detection section to acquire an image and radar information containing that target. From within such image information, the processing section determines a region in which the target is contained. Furthermore, the processing section extracts radar information within this region, and detects a distance from the vehicle to the target and a relative velocity between the vehicle and the target. Based on such information, the processing section determines a possibility that the target will collide against the vehicle. This enables an early detection of a possible collision with a target. 
     A related technique is described in the specification of U.S. Pat. No. 8,068,134, the entire disclosure of which is incorporated herein by reference. 
     In a fifth processing apparatus, based on radar information or through a fusion process which is based on radar information and image information, the processing section recognizes a target or two or more targets ahead of the vehicle. The “target” encompasses any moving entity such as other vehicles or pedestrians, traveling lanes indicated by white lines on the road, road shoulders and any still objects (including gutters, obstacles, etc.), traffic lights, pedestrian crossings, and the like that may be there. The processing section may encompass a GPS (Global Positioning System) antenna. By using a GPS antenna, the position of the driver&#39;s vehicle may be detected, and based on this position, a storage device (referred to as a map information database device) that stores road map information may be searched in order to ascertain a current position on the map. This current position on the map may be compared against a target or two or more targets that have been recognized based on radar information or the like, whereby the traveling environment may be recognized. On this basis, the processing section may extract any target that is estimated to hinder vehicle travel, find safer traveling information, and display it on a display device, as necessary, to inform the driver. 
     A related technique is described in the specification of U.S. Pat. No. 6,191,704, the entire disclosure of which is incorporated herein by reference. 
     The fifth processing apparatus may further include a data communication device (having communication circuitry) that communicates with a map information database device which is external to the vehicle. The data communication device may access the map information database device, with a period of e.g. once a week or once a month, to download the latest map information therefrom. This allows the aforementioned processing to be performed with the latest map information. 
     Furthermore, the fifth processing apparatus may compare between the latest map information that was acquired during the aforementioned vehicle travel and information that is recognized of a target or two or more targets based on radar information, etc., in order to extract target information (hereinafter referred to as “map update information”) that is not included in the map information. Then, this map update information may be transmitted to the map information database device via the data communication device. The map information database device may store this map update information in association with the map information that is within the database, and update the current map information itself, if necessary. In performing the update, respective pieces of map update information that are obtained from a plurality of vehicles may be compared against one another to check certainty of the update. 
     Note that this map update information may contain more detailed information than the map information which is carried by any currently available map information database device. For example, schematic shapes of roads may be known from commonly-available map information, but it typically does not contain information such as the width of the road shoulder, the width of the gutter that may be there, any newly occurring bumps or dents, shapes of buildings, and so on. Neither does it contain heights of the roadway and the sidewalk, how a slope may connect to the sidewalk, etc. Based on conditions which are separately set, the map information database device may store such detailed information (hereinafter referred to as “map update details information”) in association with the map information. Such map update details information provides a vehicle (including the driver&#39;s vehicle) with information which is more detailed than the original map information, thereby rending itself available for not only the purpose of ensuring safe vehicle travel but also some other purposes. As used herein, a “vehicle (including the driver&#39;s vehicle)” may be e.g. an automobile, a motorcycle, a bicycle, or any autonomous vehicle to become available in the future, e.g., an electric wheelchair. The map update details information is to be used when any such vehicle may travel. 
     (Recognition Via Neural Network) 
     Each of the first to fifth processing apparatuses may further include a sophisticated apparatus of recognition. The sophisticated apparatus of recognition may be provided external to the vehicle. In that case, the vehicle may include a high-speed data communication device that communicates with the sophisticated apparatus of recognition. The sophisticated apparatus of recognition may be constructed from a neural network, which may encompass so-called deep learning and the like. This neural network may include a convolutional neural network (hereinafter referred to as “CNN”), for example. A CNN, a neural network that has proven successful in image recognition, is characterized by possessing one or more sets of two layers, namely, a convolutional layer and a pooling layer. 
     There exists at least three kinds of information as follows, any of which may be input to a convolutional layer in the processing apparatus: 
     (1) information that is based on radar information which is acquired by the millimeter wave radar detection section; 
     (2) information that is based on specific image information which is acquired, based on radar information, by the image acquisition section; or 
     (3) fusion information that is based on radar information and image information which is acquired by the image acquisition section, or information that is obtained based on such fusion information. 
     Based on information of any of the above kinds, or information based on a combination thereof, product-sum operations corresponding to a convolutional layer are performed. The results are input to the subsequent pooling layer, where data is selected according to a predetermined rule. In the case of max pooling where a maximum value among pixel values is chosen, for example, the rule may dictate that a maximum value be chosen for each split region in the convolutional layer, this maximum value being regarded as the value of the corresponding position in the pooling layer. 
     A sophisticated apparatus of recognition that is composed of a CNN may include a single set of a convolutional layer and a pooling layer, or a plurality of such sets which are cascaded in series. This enables accurate recognition of a target, which is contained in the radar information and the image information, that may be around a vehicle. 
     Related techniques are described in the U.S. Pat. No. 8,861,842, the specification of U.S. Pat. No. 9,286,524, and the specification of US Patent Application Publication No. 2016/0140424, the entire disclosure of each of which is incorporated herein by reference. 
     In a sixth processing apparatus, the processing section performs processing that is related to headlamp control of a vehicle. When a vehicle travels in nighttime, the driver may check whether another vehicle or a pedestrian exists ahead of the driver&#39;s vehicle, and control a beam(s) from the headlamp(s) of the driver&#39;s vehicle to prevent the driver of the other vehicle or the pedestrian from being dazzled by the headlamp(s) of the driver&#39;s vehicle. This sixth processing apparatus automatically controls the headlamp(s) of the driver&#39;s vehicle by using radar information, or a combination of radar information and an image taken by a camera or the like. 
     Based on radar information, or through a fusion process based on radar information and image information, the processing section detects a target that corresponds to a vehicle or pedestrian ahead of the vehicle. In this case, a vehicle ahead of a vehicle may encompass a preceding vehicle that is ahead, a vehicle or a motorcycle in the oncoming lane, and so on. When detecting any such target, the processing section issues a command to lower the beam(s) of the headlamp(s). Upon receiving this command, the control section (control circuit) which is internal to the vehicle may control the headlamp(s) to lower the beam(s) therefrom. 
     Related techniques are described in the specification of U.S. Pat. No. 6,403,942, the specification of U.S. Pat. No. 6,611,610, the specification of U.S. Pat. No. 8,543,277, the specification of U.S. Pat. No. 8,593,521, and the specification of U.S. Pat. No. 8,636,393, the entire disclosure of each of which is incorporated herein by reference. 
     According to the above-described processing by the millimeter wave radar detection section, and the above-described fusion process by the millimeter wave radar detection section and an imaging device such as a camera, the millimeter wave radar can be constructed so as to have a small size and high performance, whereby high performance and downsizing, etc., can be achieved for the radar processing or the entire fusion process. This improves the accuracy of target recognition, and enables safer travel control for the vehicle. 
     Application Example 2: Various Monitoring Systems (Natural Elements, Buildings, Roads, Watch, Security) 
     A millimeter wave radar (radar system) incorporating an array antenna according to an embodiment of the present disclosure also has a wide range of applications in the fields of monitoring, which may encompass natural elements, weather, buildings, security, nursing care, and the like. In a monitoring system in this context, a monitoring apparatus that includes the millimeter wave radar may be installed e.g. at a fixed position, in order to perpetually monitor a subject(s) of monitoring. Regarding the given subject(s) of monitoring, the millimeter wave radar has its resolution of detection adjusted and set to an optimum value. 
     A millimeter wave radar incorporating an array antenna according to an embodiment of the present disclosure is capable of detection with a radio frequency electromagnetic wave exceeding e.g. 100 GHz. As for the modulation band in those schemes which are used in radar recognition, e.g., the FMCW method, the millimeter wave radar currently achieves a wide band exceeding 4 GHz, which supports the aforementioned Ultra Wide Band (UWB). Note that the modulation band is related to the range resolution. In a conventional patch antenna, the modulation band was up to about 600 MHz, thus resulting in a range resolution of 25 cm. On the other hand, a millimeter wave radar associated with the array antenna according to the present disclosure has a range resolution of 3.75 cm, indicative of a performance which rivals the range resolution of conventional LIDAR. Whereas an optical sensor such as LIDAR is unable to detect a target in nighttime or bad weather as mentioned above, a millimeter wave radar is always capable of detection, regardless of daytime or nighttime and irrespective of weather. As a result, a millimeter wave radar associated with the array antenna according to the present disclosure is available for a variety of applications which were not possible with a millimeter wave radar incorporating any conventional patch antenna. 
       FIG. 31  is a diagram showing an exemplary construction for a monitoring system  1500  based on millimeter wave radar. The monitoring system  1500  based on millimeter wave radar at least includes a sensor section  1010  and a main section  1100 . The sensor section  1010  at least includes an antenna  1011  which is aimed at the subject of monitoring  1015 , a millimeter wave radar detection section  1012  which detects a target based on a transmitted or received electromagnetic wave, and a communication section (communication circuit)  1013  which transmits detected radar information. The main section  1100  at least includes a communication section (communication circuit)  1103  which receives radar information, a processing section (processing circuit)  1101  which performs predetermined processing based on the received radar information, and a data storage section (storage medium)  1102  in which past radar information and other information that is needed for the predetermined processing, etc., are stored. Telecommunication lines  1300  exist between the sensor section  1010  and the main section  1100 , via which transmission and reception of information and commands occur between them. As used herein, the telecommunication lines may encompass any of a general-purpose communications network such as the Internet, a mobile communications network, dedicated telecommunication lines, and so on, for example. Note that the present monitoring system  1500  may be arranged so that the sensor section  1010  and the main section  1100  are directly connected, rather than via telecommunication lines. In addition to the millimeter wave radar, the sensor section  1010  may also include an optical sensor such as a camera. This will permit target recognition through a fusion process which is based on radar information and image information from the camera or the like, thus enabling a more sophisticated detection of the subject of monitoring  1015  or the like. 
     Hereinafter, examples of monitoring systems embodying these applications will be specifically described. 
     [Natural Element Monitoring System] 
     A first monitoring system is a system that monitors natural elements (hereinafter referred to as a “natural element monitoring system”). With reference to  FIG. 31 , this natural element monitoring system will be described. Subjects of monitoring  1015  of the natural element monitoring system  1500  may be, for example, a river, the sea surface, a mountain, a volcano, the ground surface, or the like. For example, when a river is the subject of monitoring  1015 , the sensor section  1010  being secured to a fixed position perpetually monitors the water surface of the river  1015 . This water surface information is perpetually transmitted to a processing section  1101  in the main section  1100 . Then, if the water surface reaches a certain height or above, the processing section  1101  informs a distinct system  1200  which separately exists from the monitoring system (e.g., a weather observation monitoring system), via the telecommunication lines  1300 . Alternatively, the processing section  1101  may send information to a system (not shown) which manages the water gate, whereby the system if instructed to automatically close a water gate, etc. (not shown) which is provided at the river  1015 . 
     The natural element monitoring system  1500  is able to monitor a plurality of sensor sections  1010 ,  1020 , etc., with the single main section  1100 . When the plurality of sensor sections are distributed over a certain area, the water levels of rivers in that area can be grasped simultaneously. This allows to make an assessment as to how the rainfall in this area may affect the water levels of the rivers, possibly leading to disasters such as floods. Information concerning this can be conveyed to the distinct system  1200  (e.g., a weather observation monitoring system) via the telecommunication lines  1300 . Thus, the distinct system  1200  (e.g., a weather observation monitoring system) is able to utilize the conveyed information for weather observation or disaster prediction in a wider area. 
     The natural element monitoring system  1500  is also similarly applicable to any natural element other than a river. For example, the subject of monitoring of a monitoring system that monitors tsunamis or storm surges is the sea surface level. It is also possible to automatically open or close the water gate of a seawall in response to a rise in the sea surface level. Alternatively, the subject of monitoring of a monitoring system that monitors landslides to be caused by rainfall, earthquakes, or the like may be the ground surface of a mountainous area, etc. 
     [Traffic Monitoring System] 
     A second monitoring system is a system that monitors traffic (hereinafter referred to as a “traffic monitoring system”). The subject of monitoring of this traffic monitoring system may be, for example, a railroad crossing, a specific railroad, an airport runway, a road intersection, a specific road, a parking lot, etc. 
     For example, when the subject of monitoring is a railroad crossing, the sensor section  1010  is placed at a position where the inside of the crossing can be monitored. In this case, in addition to the millimeter wave radar, the sensor section  1010  may also include an optical sensor such as a camera, which will allow a target (subject of monitoring) to be detected from more perspectives, through a fusion process based on radar information and image information. The target information which is obtained with the sensor section  1010  is sent to the main section  1100  via the telecommunication lines  1300 . The main section  1100  collects other information (e.g., train schedule information) that may be needed in a more sophisticated recognition process or control, and issues necessary control instructions or the like based thereon. As used herein, a necessary control instruction may be, for example, an instruction to stop a train when a person, a vehicle, etc. is found inside the crossing when it is closed. 
     If the subject of monitoring is a runway at an airport, for example, a plurality of sensor sections  1010 ,  1020 , etc., may be placed along the runway so as to set the runway to a predetermined resolution, e.g., a resolution that allows any foreign object that is 5 cm by 5 cm or larger to be detected. The monitoring system  1500  perpetually monitors the runway, regardless of daytime or nighttime and irrespective of weather. This function is enabled by the very ability of the millimeter wave radar according to an embodiment of the present disclosure to support UWB. Moreover, since the present millimeter wave radar device can be embodied with a small size, a high resolution, and a low cost, it provides a realistic solution for covering the entire runway surface from end to end. In this case, the main section  1100  keeps the plurality of sensor sections  1010 ,  1020 , etc., under integrated management. If a foreign object is found on the runway, the main section  1100  transmits information concerning the position and size of the foreign object to an air-traffic control system (not shown). Upon receiving this, the air-traffic control system temporarily prohibits takeoff and landing on that runway. In the meantime, the main section  1100  transmits information concerning the position and size of the foreign object to a separately-provided vehicle, which automatically cleans the runway surface, etc., for example. Upon receive this, the cleaning vehicle may autonomously move to the position where the foreign object exists, and automatically remove the foreign object. Once removal of the foreign object is completed, the cleaning vehicle transmits information of the completion to the main section  1100 . Then, the main section  1100  again confirms that the sensor section  1010  or the like which has detected the foreign object now reports that “no foreign object exists” and that it is safe now, and informs the air-traffic control system of this. Upon receiving this, the air-traffic control system may lift the prohibition of takeoff and landing from the runway. 
     Furthermore, in the case where the subject of monitoring is a parking lot, for example, it may be possible to automatically recognize which position in the parking lot is currently vacant. A related technique is described in the specification of U.S. Pat. No. 6,943,726, the entire disclosure of which is incorporated herein by reference. 
     [Security Monitoring System] 
     A third monitoring system is a system that monitors a trespasser into a piece of private land or a house (hereinafter referred to as a “security monitoring system”). The subject of monitoring of this security monitoring system may be, for example, a specific region within a piece of private land or a house, etc. 
     For example, if the subject of monitoring is a piece of private land, the sensor section(s)  1010  may be placed at one position, or two or more positions where the sensor section(s)  1010  is able to monitor it. In this case, in addition to the millimeter wave radar, the sensor section(s)  1010  may also include an optical sensor such as a camera, which will allow a target (subject of monitoring) to be detected from more perspectives, through a fusion process based on radar information and image information. The target information which was obtained by the sensor section  1010 ( s ) is sent to the main section  1100  via the telecommunication lines  1300 . The main section  1100  collects other information (e.g., reference data or the like needed to accurately recognize whether the trespasser is a person or an animal such as a dog or a bird) that may be needed in a more sophisticated recognition process or control, and issues necessary control instructions or the like based thereon. As used herein, a necessary control instruction may be, for example, an instruction to sound an alarm or activate lighting that is installed in the premises, and also an instruction to directly report to a person in charge of the premises via mobile telecommunication lines or the like, etc. The processing section  1101  in the main section  1100  may allow an internalized, sophisticated apparatus of recognition (that adopts deep learning or a like technique) to recognize the detected target. Alternatively, such a sophisticated apparatus of recognition may be provided externally, in which case the sophisticated apparatus of recognition may be connected via the telecommunication lines  1300 . 
     A related technique is described in the specification of U.S. Pat. No. 7,425,983, the entire disclosure of which is incorporated herein by reference. 
     Another embodiment of such a security monitoring system may be a human monitoring system to be installed at a boarding gate at an airport, a station wicket, an entrance of a building, or the like. The subject of monitoring of such a human monitoring system may be, for example, a boarding gate at an airport, a station wicket, an entrance of a building, or the like. 
     If the subject of monitoring is a boarding gate at an airport, the sensor section(s)  1010  may be installed in a machine for checking personal belongings at the boarding gate, for example. In this case, there may be two checking methods as follows. In a first method, the millimeter wave radar transmits an electromagnetic wave, and receives the electromagnetic wave as it reflects off a passenger (which is the subject of monitoring), thereby checking personal belongings or the like of the passenger. In a second method, a weak millimeter wave which is radiated from the passenger&#39;s own body is received by the antenna, thus checking for any foreign object that the passenger may be hiding. In the latter method, the millimeter wave radar preferably has a function of scanning the received millimeter wave. This scanning function may be implemented by using digital beam forming, or through a mechanical scanning operation. Note that the processing by the main section  1100  may utilize a communication process and a recognition process similar to those in the above-described examples. 
     [Building Inspection System (Non-Destructive Inspection)] 
     A fourth monitoring system is a system that monitors or checks the concrete material of a road, a railroad overpass, a building, etc., or the interior of a road or the ground, etc., (hereinafter referred to as a “building inspection system”). The subject of monitoring of this building inspection system may be, for example, the interior of the concrete material of an overpass or a building, etc., or the interior of a road or the ground, etc. 
     For example, if the subject of monitoring is the interior of a concrete building, the sensor section  1010  is structured so that the antenna  1011  can make scan motions along the surface of a concrete building. As used herein, “scan motions” may be implemented manually, or a stationary rail for the scan motion may be separately provided, upon which to cause the movement by using driving power from an electric motor or the like. In the case where the subject of monitoring is a road or the ground, the antenna  1011  may be installed face-down on a vehicle or the like, and the vehicle may be allowed to travel at a constant velocity, thus creating a “scan motion”. The electromagnetic wave to be used by the sensor section  1010  may be a millimeter wave in e.g. the so-called terahertz region, exceeding 100 GHz. As described earlier, even with an electromagnetic wave over e.g. 100 GHz, an array antenna according to an embodiment of the present disclosure can be adapted to have smaller losses than do conventional patch antennas or the like. An electromagnetic wave of a higher frequency is able to permeate deeper into the subject of checking, such as concrete, thereby realizing a more accurate non-destructive inspection. Note that the processing by the main section  1100  may also utilize a communication process and a recognition process similar to those in the other monitoring systems described above. 
     A related technique is described in the specification of U.S. Pat. No. 6,661,367, the entire disclosure of which is incorporated herein by reference. 
     [Human Monitoring System] 
     A fifth monitoring system is a system that watches over a person who is subject to nursing care (hereinafter referred to as a “human watch system”). The subject of monitoring of this human watch system may be, for example, a person under nursing care or a patient in a hospital, etc. 
     For example, if the subject of monitoring is a person under nursing care within a room of a nursing care facility, the sensor section(s)  1010  is placed at one position, or two or more positions inside the room where the sensor section(s)  1010  is able to monitor the entirety of the inside of the room. In this case, in addition to the millimeter wave radar, the sensor section  1010  may also include an optical sensor such as a camera. In this case, the subject of monitoring can be monitored from more perspectives, through a fusion process based on radar information and image information. On the other hand, when the subject of monitoring is a person, from the standpoint of privacy protection, monitoring with a camera or the like may not be appropriate. Therefore, sensor selections must be made while taking this aspect into consideration. Note that target detection by the millimeter wave radar will allow a person, who is the subject of monitoring, to be captured not by his or her image, but by a signal (which is, as it were, a shadow of the person). Therefore, the millimeter wave radar may be considered as a desirable sensor from the standpoint of privacy protection. 
     Information of the person under nursing care which has been obtained by the sensor section(s)  1010  is sent to the main section  1100  via the telecommunication lines  1300 . The main section  1100  collects other information (e.g., reference data or the like needed to accurately recognize target information of the person under nursing care) that may be needed in a more sophisticated recognition process or control, and issues necessary control instructions or the like based thereon. As used herein, a necessary control instruction may be, for example, an instruction to directly report a person in charge based on the result of detection, etc. The processing section  1101  in the main section  1100  may allow an internalized, sophisticated apparatus of recognition (that adopts deep learning or a like technique) to recognize the detected target. Alternatively, such a sophisticated apparatus of recognition may be provided externally, in which case the sophisticated apparatus of recognition may be connected via the telecommunication lines  1300 . 
     In the case where a person is the subject of monitoring of the millimeter wave radar, at least the two following functions may be added. 
     A first function is a function of monitoring the heart rate and/or the respiratory rate. In the case of a millimeter wave radar, an electromagnetic wave is able to see through the clothes to detect the position and motions of the skin surface of a person&#39;s body. First, the processing section  1101  detects a person who is the subject of monitoring and an outer shape thereof. Next, in the case of detecting a heart rate, for example, a position on the body surface where the heartbeat motions are easy to detect may be identified, and the motions there may be chronologically detected. This allows a heart rate per minute to be detected, for example. The same is also true when detecting a respiratory rate. By using this function, the health status of a person under nursing care can be perpetually checked, thus enabling a higher-quality watch over a person under nursing care. 
     A second function is a function of fall detection. A person under nursing care such as an elderly person may fall from time to time, due to weakened legs and feet. When a person falls, the velocity or acceleration of a specification site of the person&#39;s body, e.g., the head, will reach a certain level or greater. When the subject of monitoring of the millimeter wave radar is a person, the relative velocity or acceleration of the target of interest can be perpetually detected. Therefore, by identifying the head as the subject of monitoring, for example, and chronologically detecting its relative velocity or acceleration, a fall can be recognized when a velocity of a certain value or greater is detected. When recognizing a fall, the processing section  1101  can issue an instruction or the like corresponding to pertinent nursing care assistance, for example. 
     Note that the sensor section(s)  1010  is secured to a fixed position(s) in the above-described monitoring system or the like. However, the sensor section(s)  1010  can also be installed on a moving entity, e.g., a robot, a vehicle, a flying object such as a drone. As used herein, the vehicle or the like may encompass not only an automobile, but also a smaller sized moving entity such as an electric wheelchair, for example. In this case, this moving entity may include an internal GPS unit which allows its own current position to be always confirmed. In addition, this moving entity may also have a function of further improving the accuracy of its own current position by using map information and the map update information which has been described with respect to the aforementioned fifth processing apparatus. 
     Furthermore, in any device or system that is similar to the above-described first to third detection devices, first to sixth processing apparatuses, first to fifth monitoring systems, etc., a like construction may be adopted to utilize an array antenna or a millimeter wave radar according to an embodiment of the present disclosure. 
     Application Example 3: Communication System 
     [First Example of Communication System] 
     The waveguide device and antenna device (array antenna) according to the present disclosure can be used for the transmitter and/or receiver with which a communication system (telecommunication system) is constructed. The waveguide device and antenna device according to the present disclosure are composed of layered conductive members, and therefore are able to keep the transmitter and/or receiver size smaller than in the case of using a hollow waveguide. Moreover, there is no need for dielectric, and thus the dielectric loss of electromagnetic waves can be kept smaller than in the case of using a microstrip line. Therefore, a communication system including a small and highly efficient transmitter and/or receiver can be constructed. 
     Such a communication system may be an analog type communication system which transmits or receives an analog signal that is directly modulated. However, a digital communication system may be adopted in order to construct a more flexible and higher-performance communication system. 
     Hereinafter, with reference to  FIG. 32 , a digital communication system  800 A in which a waveguide device and an antenna device according to an embodiment of the present disclosure are used will be described. 
       FIG. 32  is a block diagram showing a construction for the digital communication system  800 A. The communication system  800 A includes a transmitter  810 A and a receiver  820 A. The transmitter  810 A includes an analog to digital (A/D) converter  812 , an encoder  813 , a modulator  814 , and a transmission antenna  815 . The receiver  820 A includes a reception antenna  825 , a demodulator  824 , a decoder  823 , and a digital to analog (D/A) converter  822 . The at least one of the transmission antenna  815  and the reception antenna  825  may be implemented by using an array antenna according to an embodiment of the present disclosure. In this exemplary application, the circuitry including the modulator  814 , the encoder  813 , the A/D converter  812 , and so on, which are connected to the transmission antenna  815 , is referred to as the transmission circuit. The circuitry including the demodulator  824 , the decoder  823 , the D/A converter  822 , and so on, which are connected to the reception antenna  825 , is referred to as the reception circuit. The transmission circuit and the reception circuit may be collectively referred to as the communication circuit. 
     With the analog to digital (A/D) converter  812 , the transmitter  810 A converts an analog signal which is received from the signal source  811  to a digital signal. Next, the digital signal is encoded by the encoder  813 . As used herein, “encoding” means altering the digital signal to be transmitted into a format which is suitable for communication. Examples of such encoding include CDM (Code-Division Multiplexing) and the like. Moreover, any conversion for effecting TDM (Time-Division Multiplexing) or FDM (Frequency Division Multiplexing), or OFDM (Orthogonal Frequency Division Multiplexing) is also an example of encoding. The encoded signal is converted by the modulator  814  into a radio frequency signal, so as to be transmitted from the transmission antenna  815 . 
     In the field of communications, a wave representing a signal to be superposed on a carrier wave may be referred to as a “signal wave”; however, the term “signal wave” as used in the present specification does not carry that definition. A “signal wave” as referred to in the present specification is broadly meant to be any electromagnetic wave to propagate in a waveguide, or any electromagnetic wave for transmission/reception via an antenna element. 
     The receiver  820 A restores the radio frequency signal that has been received by the reception antenna  825  to a low-frequency signal at the demodulator  824 , and to a digital signal at the decoder  823 . The decoded digital signal is restored to an analog signal by the digital to analog (D/A) converter  822 , and is sent to a data sink (data receiver)  821 . Through the above processes, a sequence of transmission and reception processes is completed. 
     When the communicating agent is a digital appliance such as a computer, analog to digital conversion of the transmission signal and digital to analog conversion of the reception signal are not needed in the aforementioned processes. Thus, the analog to digital converter  812  and the digital to analog converter  822  in  FIG. 32  may be omitted. A system of such construction is also encompassed within a digital communication system. 
     In a digital communication system, in order to ensure signal intensity or expand channel capacity, various methods may be adopted. Many such methods are also effective in a communication system which utilizes radio waves of the millimeter wave band or the terahertz band. 
     Radio waves in the millimeter wave band or the terahertz band have higher straightness than do radio waves of lower frequencies, and undergoes less diffraction, i.e., bending around into the shadow side of an obstacle. Therefore, it is not uncommon for a receiver to fail to directly receive a radio wave that has been transmitted from a transmitter. Even in such situations, reflected waves may often be received, but a reflected wave of a radio wave signal is often poorer in quality than is the direct wave, thus making stable reception more difficult. Furthermore, a plurality of reflected waves may arrive through different paths. In that case, the reception waves with different path lengths might differ in phase from one another, thus causing multi-path fading. 
     As a technique for improving such situations, a so-called antenna diversity technique may be used. In this technique, at least one of the transmitter and the receiver includes a plurality of antennas. If the plurality of antennas are parted by distances which differ from one another by at least about the wavelength, the resulting states of the reception waves will be different. Accordingly, the antenna that is capable of transmission/reception with the highest quality among all is selectively used, thereby enhancing the reliability of communication. Alternatively, signals which are obtained from more than one antenna may be merged for an improved signal quality. 
     In the communication system  800 A shown in  FIG. 32 , for example, the receiver  820 A may include a plurality of reception antennas  825 . In this case, a switcher exists between the plurality of reception antennas  825  and the demodulator  824 . Through the switcher, the receiver  820 A connects the antenna that provides the highest-quality signal among the plurality of reception antennas  825  to the demodulator  824 . In this case, the transmitter  810 A may also include a plurality of transmission antennas  815 . 
     [Second Example of Communication System] 
       FIG. 33  is a block diagram showing an example of a communication system  800 B including a transmitter  810 B which is capable of varying the radiation pattern of radio waves. In this exemplary application, the receiver is identical to the receiver  820 A shown in  FIG. 32 ; for this reason, the receiver is omitted from illustration in  FIG. 33 . In addition to the construction of the transmitter  810 A, the transmitter  810 B also includes an antenna array  815   b , which includes a plurality of antenna elements  8151 . The antenna array  815   b  may be an array antenna according to an embodiment of the present disclosure. The transmitter  810 B further includes a plurality of phase shifters (PS)  816  which are respectively connected between the modulator  814  and the plurality of antenna elements  8151 . In the transmitter  810 B, an output of the modulator  814  is sent to the plurality of phase shifters  816 , where phase differences are imparted and the resultant signals are led to the plurality of antenna elements  8151 . In the case where the plurality of antenna elements  8151  are disposed at equal intervals, if a radio frequency signal whose phase differs by a certain amount with respect to an adjacent antenna element is fed to each antenna element  8151 , a main lobe  817  of the antenna array  815   b  will be oriented in an azimuth which is inclined from the front, this inclination being in accordance with the phase difference. This method may be referred to as beam forming. 
     The azimuth of the main lobe  817  may be altered by allowing the respective phase shifters  816  to impart varying phase differences. This method may be referred to as beam steering. By finding phase differences that are conducive to the best transmission/reception state, the reliability of communication can be enhanced. Although the example here illustrates a case where the phase difference to be imparted by the phase shifters  816  is constant between any adjacent antenna elements  8151 , this is not limiting. Moreover, phase differences may be imparted so that the radio wave will be radiated in an azimuth which allows not only the direct wave but also reflected waves to reach the receiver. 
     A method called null steering can also be used in the transmitter  810 B. This is a method where phase differences are adjusted to create a state where the radio wave is radiated in no specific direction. By performing null steering, it becomes possible to restrain radio waves from being radiated toward any other receiver to which transmission of the radio wave is not intended. This can avoid interference. Although a very broad frequency band is available to digital communication utilizing millimeter waves or terahertz waves, it is nonetheless preferable to make as efficient a use of the bandwidth as possible. By using null steering, plural instances of transmission/reception can be performed within the same band, whereby efficiency of utility of the bandwidth can be enhanced. A method which enhances the efficiency of utility of the bandwidth by using techniques such as beam forming, beam steering, and null steering may sometimes be referred to as SDMA (Spatial Division Multiple Access). 
     [Third Example of Communication System] 
     In order to increase the channel capacity in a specific frequency band, a method called MIMO (Multiple-Input and Multiple-Output) may be adopted. Under MIMO, a plurality of transmission antennas and a plurality of reception antennas are used. A radio wave is radiated from each of the plurality of transmission antennas. In one example, respectively different signals may be superposed on the radio waves to be radiated. Each of the plurality of reception antennas receives all of the transmitted plurality of radio waves. However, since different reception antennas will receive radio waves that arrive through different paths, differences will occur among the phases of the received radio waves. By utilizing these differences, it is possible to, at the receiver side, separate the plurality of signals which were contained in the plurality of radio waves. 
     The waveguide device and antenna device according to the present disclosure can also be used in a communication system which utilizes MIMO. Hereinafter, an example such a communication system will be described. 
       FIG. 34  is a block diagram showing an example of a communication system  800 C implementing a MIMO function. In the communication system  800 C, a transmitter  830  includes an encoder  832 , a TX-MIMO processor  833 , and two transmission antennas  8351  and  8352 . A receiver  840  includes two reception antennas  8451  and  8452 , an RX-MIMO processor  843 , and a decoder  842 . Note that the number of transmission antennas and the number of reception antennas may each be greater than two. Herein, for ease of explanation, an example where there are two antennas of each kind will be illustrated. In general, the channel capacity of an MIMO communication system will increase in proportion to the number of whichever is the fewer between the transmission antennas and the reception antennas. 
     Having received a signal from the data signal source  831 , the transmitter  830  encodes the signal at the encoder  832  so that the signal is ready for transmission. The encoded signal is distributed by the TX-MIMO processor  833  between the two transmission antennas  8351  and  8352 . 
     In a processing method according to one example of the MIMO method, the TX-MIMO processor  833  splits a sequence of encoded signals into two, i.e., as many as there are transmission antennas  8352 , and sends them in parallel to the transmission antennas  8351  and  8352 . The transmission antennas  8351  and  8352  respectively radiate radio waves containing information of the split signal sequences. When there are N transmission antennas, the signal sequence is split into N. The radiated radio waves are simultaneously received by the two reception antennas  8451  and  8452 . In other words, in the radio waves which are received by each of the reception antennas  8451  and  8452 , the two signals which were split at the time of transmission are mixedly contained. Separation between these mixed signals is achieved by the RX-MIMO processor  843 . 
     The two mixed signals can be separated by paying attention to the phase differences between the radio waves, for example. A phase difference between two radio waves of the case where the radio waves which have arrived from the transmission antenna  8351  are received by the reception antennas  8451  and  8452  is different from a phase difference between two radio waves of the case where the radio waves which have arrived from the transmission antenna  8352  are received by the reception antennas  8451  and  8452 . That is, the phase difference between reception antennas differs depending on the path of transmission/reception. Moreover, unless the spatial relationship between a transmission antenna and a reception antenna is changed, the phase difference therebetween remains unchanged. Therefore, based on correlation between reception signals received by the two reception antennas, as shifted by a phase difference which is determined by the path of transmission/reception, it is possible to extract any signal that is received through that path of transmission/reception. The RX-MIMO processor  843  may separate the two signal sequences from the reception signal e.g. by this method, thus restoring the signal sequence before the split. The restored signal sequence still remains encoded, and therefore is sent to the decoder  842  so as to be restored to the original signal there. The restored signal is sent to the data sink  841 . 
     Although the MIMO communication system  800 C in this example transmits or receives a digital signal, an MIMO communication system which transmits or receives an analog signal can also be realized. In that case, in addition to the construction of  FIG. 34 , an analog to digital converter and a digital to analog converter as have been described with reference to  FIG. 32  are provided. Note that the information to be used in distinguishing between signals from different transmission antennas is not limited to phase difference information. Generally speaking, for a different combination of a transmission antenna and a reception antenna, the received radio wave may differ not only in terms of phase, but also in scatter, fading, and other conditions. These are collectively referred to as CSI (Channel State Information). CSI may be utilized in distinguishing between different paths of transmission/reception in a system utilizing MIMO. 
     Note that it is not an essential requirement that the plurality of transmission antennas radiate transmission waves containing respectively independent signals. So long as separation is possible at the reception antenna side, each transmission antenna may radiate a radio wave containing a plurality of signals. Moreover, beam forming may be performed at the transmission antenna side, while a transmission wave containing a single signal, as a synthetic wave of the radio waves from the respective transmission antennas, may be formed at the reception antenna. In this case, too, each transmission antenna is adapted so as to radiate a radio wave containing a plurality of signals. 
     In this third example, too, as in the first and second examples, various methods such as CDM, FDM, TDM, and OFDM may be used as a method of signal encoding. 
     In a communication system, a circuit board that implements an integrated circuit (referred to as a signal processing circuit or a communication circuit) for processing signals may be stacked as a layer on the waveguide device and antenna device according to an embodiment of the present disclosure. Since the waveguide device and antenna device according to an embodiment of the present disclosure is structured so that plate-like conductive members are layered therein, it is easy to further stack a circuit board thereupon. By adopting such an arrangement, a transmitter and a receiver which are smaller in volume than in the case where a hollow waveguide or the like is employed can be realized. 
     In the first to third examples of the communication system as described above, each element of a transmitter or a receiver, e.g., an analog to digital converter, a digital to analog converter, an encoder, a decoder, a modulator, a demodulator, a TX-MIMO processor, or an RX-MIMO processor, is illustrated as one independent element in  FIGS. 32, 33, and 34 ; however, these do not need to be discrete. For example, all of these elements may be realized by a single integrated circuit. Alternatively, some of these elements may be combined so as to be realized by a single integrated circuit. Either case qualifies as an embodiment of the present invention so long as the functions which have been described in the present disclosure are realized thereby. 
     A slot array antenna according to the present disclosure is applicable to any technological field where antennas are used. For example, it is available to various applications where transmission/reception of electromagnetic waves of the gigahertz band or the terahertz band is performed. In particular, it is suitably used in onboard radar systems, various types of monitoring systems, indoor positioning systems, wireless communication systems, and the like where downsizing and gain enhancement are desired. 
     While the present invention has been described with respect to exemplary embodiments thereof, it will be apparent to those skilled in the art that the disclosed invention may be modified in numerous ways and may assume many embodiments other than those specifically described above. Accordingly, it is intended by the appended claims to cover all modifications of the invention that fall within the true spirit and scope of the invention.