Patent Publication Number: US-7724200-B2

Title: Antenna device, array antenna, multi-sector antenna, high-frequency wave transceiver

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation of International Application No. PCT/JP2007/052958, filed Feb. 19, 2007, which claims priority to Japanese Patent Application No. JP2006-046749, filed Feb. 23, 2006, the entire contents of each of these applications being incorporated herein by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to antenna devices based on dipole antennas and, in particular, to a planar antenna device having dipole electrodes formed on a dielectric substrate. Furthermore, the present invention relates to an array antenna in which a plurality of these antenna devices are arranged, a multi-sector antenna having a plurality of array antennas, and a high-frequency wave transceiver. 
   BACKGROUND OF THE INVENTION 
   In the related art, Yagi-Uda antennas are one of antenna devices well known to the public. Such Yagi-Uda antennas include a planar type that employs a dielectric substrate in order to be included in a vehicle-mounted radar apparatus or the like to save space. Non-Patent Document 1 discloses an antenna device including an array of such planar Yagi-Uda antennas. 
     FIGS. 12(A)  and (B) are configuration diagrams of an antenna disclosed in Non-Patent Document 1, whereas (C) is a configuration diagram of an array antenna in which a plurality of antenna devices of (A) and (B) are arranged. Meanwhile, illustration of a ground electrode provided on a back surface is omitted in (C). 
   As shown in  FIG. 12 , in an antenna device 100 of Non-Patent Document 1, a feeder portion electrode  20 , an unbalanced-balanced transformer electrode (hereinafter, referred to as a balun electrode)  30 , a radiation portion electrode  40 , and a waveguide portion electrode  50  are formed on a top surface  111  of a dielectric substrate  101 , whereas a ground electrode  60  is formed on a back surface  112  thereof. 
   The feeder portion electrode  20  is formed like a line extending in a predetermined direction. One end thereof is connected to the balun electrode  30 . The balun electrode  30  has two U-shaped electrodes arranged so that openings thereof face each other and is formed in a shape spreading in a direction vertical to the extending direction of the feeder portion electrode  20 . One of the two U-shaped electrodes (the U-shaped electrode on the right when  FIG. 12  is viewed from the front) is formed in a shape of which the electrical length thereof is longer than that of the other one by a half wavelength (λ/2) of a transmission/reception signal. With this shape, a current path from the feeder portion electrode  20 , which is an unbalanced line, to the radiation portion electrode  40 , which is a balanced line, is maintained and transmission and reception signals are transferred. The radiation portion electrode  40  has two linear electrodes, having a predetermined length, extending in a direction vertical to the extending direction of the feeder portion electrode  20 . The electrodes thereof are connected to the two electrodes of the balun electrode  30 , respectively. This structure allows the radiation portion electrode  40  to function as a radiation portion of a dipole antenna. The waveguide portion electrode  50  is separated from the radiation portion electrode  40  by a predetermined interval and in parallel to the radiation portion electrode  40 . The ground electrode  60  is formed on the back surface  112  corresponding to an area including the feeder portion electrode  20  and the balun electrode  30 . 
   In addition, an array antenna of Non-Patent Document 1 includes antenna devices  100 A- 100 D, each having the feeder portion electrode  20 , the balun electrode  30 , the radiation portion electrode  40 , the waveguide portion electrode  50 , and the ground electrode  60 , arranged on the dielectric substrate  101  at a predetermined interval. The feeder portion electrodes of the antenna devices  100 A and  100 B are connected to a branch circuit  71 , whereas the feeder portion electrodes of the antenna devices  100 C and  100 D are connected to a branch circuit  72 . The branch circuits  71  and  72  are connected to a branch circuit  73 . This structure allows a transmission wave signal fed to the branch circuit  73  to be diverged by the branch circuit  73  into the branch circuits  71  and  72 , to be diverged by the branch circuit  71  into the antenna devices  100 A and  100 B, and to be diverged by the branch circuit  72  into the antenna devices  100 C and  100 D. On the other hand, a reflected wave signal received by the antenna devices  100 A and  100 B is transferred to a processing unit at a subsequent stage through the branch circuits  71  and  73 . A reflected wave signal received by the antenna devices  100 C and  100 D is transferred to the processing unit at the subsequent stage through the branch circuits  72  and  73 .
     Non-Patent Document 1: William R. Deal, Noritake Kaneda, James Sor, Yongxi Qian, and Tatsuo Itoh, “A New Quasi-Yagi Antenna for Planar Active Antenna Arrays”, JUNE 2000, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 48, NO. 6.   

   Nevertheless, since a feeder portion and a balun portion are separately formed in an antenna device shown in FIGS.  12 (A) and (B) and the balun portion includes two U-shaped electrodes spreading in a direction vertical to an extending direction of the feeder portion, the antenna device requires a certain size of space although the antenna device has already been miniaturized. In addition, when an array antenna is formed using these antenna devices as shown in  FIG. 12(C) , a relatively large space is needed for each antenna device. Accordingly, when the number of antennas to be arranged is increased to sharpen the directivity of a reception beam for the purpose of an improvement in the detection accuracy, the space for the feeding portion and the balun portion relative to the entire space of the array antenna increases. Thus, decreasing the space is problematic when an array antenna using a plurality of these antenna devices, a multi-sector antenna having this array antenna, and a high-frequency wave transceiver are miniaturized. In addition, since the length of a transmission line connecting each unit becomes long, a transmission loss increases and an antenna gain decreases. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a planar antenna device having a desired antenna gain and a shape smaller than conventional ones. 
   An antenna device of this invention includes a feeder electrode that is formed in a shape extending linearly on one surface of a dielectric substrate; a balanced electrode including one pair of electrodes that are connected to the feeder electrode, separated by an interval of an odd multiple of ½ of a wavelength of a transmission/reception signal, and formed in a shape extending in a direction crossing the extending direction of the feeder electrode at a predetermined angle; a radiation electrode of a predetermined length that is connected to each of the two electrodes of the balanced electrode and is formed in a shape extending in opposite directions along the extending direction of the feeder electrode; a waveguide electrode of a predetermined length that is located at a position separated from the radiation electrode by a predetermined length on a side of the radiation electrode opposite to the balanced electrode and is formed in a shape extending in parallel to the radiation electrode; and a ground electrode that is formed at an area of another surface facing an area of the one surface including at least a portion where the feeder electrode is formed but not including a portion where the radiation electrode and the waveguide electrode are formed. 
   In this configuration, upon being supplied through the feeder electrode, a transmission signal is diverged into two transmission path electrodes constituting the balanced electrode. Here, an interval between two junction points (branch points) of the feeder electrode and the balanced electrode is set to a length that is an odd multiple of ½ of a wavelength of a transmission/reception signal. More specifically, when “λ” represents the wavelength of the transmission/reception signal and N represents a natural number including “0”, the interval is ((2N+1)λ/2). By means of this, phases of transmission signals transferred to the two transmission paths of the balanced electrode are shifted from one another by λ/2 and unbalanced-balanced transform is executed. If this balanced transmission signal is supplied to the radiation electrode, the radiation electrode functions as a dipole antenna and radiates a radio wave. Here, formation of the waveguide electrode allows the radio wave to be radiated from the radiation electrode while setting the side of the waveguide electrode as the center of the directivity according to the position and shape of this waveguide electrode. On the other hand, in the case of reception of a reflected wave, the reflected wave (reception signal) received by the radiation electrode is transferred to the two transmission paths of the balanced electrode. Since the interval between the junction points of the balanced electrode and the feeder electrode is set to a length of odd multiple of ½ of a wavelength of a transmission/reception signal, the reception signal is balanced-unbalanced transformed and is transferred to the feeder electrode. 
   In addition, the antenna device of this invention is characterized in that an interval with which the two electrodes of the balanced electrode are connected to the feeder electrode is a length of ½ of a wavelength of a transmission/reception signal. 
   In this configuration, by setting the interval between the junction portions (branch portions) of the two electrodes (transmission path electrodes) of the balanced electrode and the feeder electrode to the length that is ½ of a wavelength of the transmission/reception signal (λ/2), the unbalanced-balanced transform is performed with the shortest interval. By means of this, since the unbalanced-balanced transform is performed with the shortest interval, the transmission loss is suppressed to the minimum and the antenna device is miniaturized. 
   Additionally, the antenna device of this invention is characterized by further including: a reflecting member having a reflecting surface that is separated from the other surface at an area of the other surface corresponding to a position where the radiation electrode is formed and forms a predetermined angle with the other surface. 
   In this configuration, since part of transmission waves radiated from the radiation portion electrode is reflected by a reflecting surface that is separated from the dielectric substrate by a predetermined angle, the directivity corresponding to the shape of the reflecting surface is provided. Accordingly, by appropriately setting the reflecting surface, antenna devices each having the different center direction of the directivity can be realized. For example, if the tilt angle is changed, the center direction of the directivity can be changed along the direction vertical to the two surfaces of the dielectric substrate. 
   In addition, an array antenna of this invention is characterized in that a plurality of the above-described antenna devices are formed in the extending direction of the feeder electrode at a predetermined arrangement interval. 
   In this configuration, since the above-described antenna devices are connected to the feeder electrode in series and the branch portion has functions of a branch circuit and an unbalanced-balanced transformer unit in each antenna device as described above, the array antenna is formed with a structure in which an integrated unit of the branch circuit to the radiation antenna of each antenna device and the unbalanced-balanced transformer circuit is simply arranged along the feeder electrode. 
   Additionally, a multi-sector antenna of this invention is characterized in that the plurality of array antennas are formed using a single dielectric substrate so that transmission and reception directions differ. 
   In this configuration, since the plurality of array antennas having the above-described structure and a different transmission/reception direction are included, a multi-sector antenna capable of performing detection in a plurality of directions is formed. 
   In addition, a high-frequency wave transceiver of this invention is characterized by including: at least one of the above-described antenna devices, the array antenna, and the multi-sector antenna. 
   In this configuration, by including the above-described antenna devices, the array antenna, and the multi-sector antenna, a high-frequency wave transceiver according to a desired characteristic is formed. 
   According to this invention, since a branch from a feeder electrode and unbalanced-balanced transform can be realized with two electrode branches provided at an interval of an odd multiple of ½ of a wavelength of a transmission/reception signal, an antenna device smaller than a conventional antenna can be formed. In particular, by setting the electrode branch position to ½ of the wavelength, a further smaller antenna device can be formed. In addition, since the antenna device is in such a shape, the transmission loss is reduced and an antenna device having a superior antenna gain can be formed. 
   In addition, according to this invention, by including a reflecting surface that forms a predetermined angle with a dielectric substrate on a side of the dielectric substrate different from the radiation electrode side, the transmission/reception directivity can be appropriately set and an antenna device having a desired characteristic can be formed in a small size. 
   Additionally, according to this invention, by connecting the antenna devices in series with a feeder electrode, an array antenna can be formed with a structure in which an integrated unit of a branch circuit to a radiation electrode of each antenna device and an unbalanced-balanced transform circuit is simply arranged along the feeder electrode. This allows the array antenna to be formed in a small size. 
   In addition, according to this invention, by using a plurality of array antennas, a multi-sector antenna can be formed in a small size. Furthermore, using these antenna devices, array antenna, and multi-sector antenna, a high-frequency wave transceiver can be formed in a small size. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  are a plan view and a side view showing a structure of an antenna device  1  of a first embodiment. 
       FIG. 2  is a plan view showing a structure of an antenna device including a matching circuit at a junction point of a feeder electrode and a balanced electrode. 
       FIG. 3  is a plan view showing a structure of an antenna device having balanced transmission electrodes  3 A and  3 B of a balanced electrode  3  that are not parallel. 
       FIG. 4  is a plan view showing a structure of an antenna device including a reflector electrode  9 . 
       FIG. 5  is a plan view showing a structure of an antenna device including a plurality of waveguide electrodes. 
       FIG. 6  is a plan view showing a structure of an antenna device in which lengths of a first electrode  4 A and a second electrode  4 B of a radiation electrode  4  differ. 
       FIG. 7  are an external perspective view and a side view of an antenna device of a second embodiment, and a side view showing an antenna device of a different structure. 
       FIG. 8  are results of a simulation using a conductor plate  61  having a slope portion  63 A. 
       FIG. 9  is a plan view showing a structure of an array antenna of a third embodiment. 
       FIG. 10  is an elevational view showing a structure of a multi-sector antenna of a fourth embodiment. 
       FIG. 11  is a block diagram showing a configuration of major units of a radar apparatus of a fifth embodiment. 
       FIG. 12  are configuration diagrams of an antenna disclosed in Non-Patent Document 1 and a configuration diagram of an array antenna having a plurality of these antenna devices arranged therein. 
   

   REFERENCE NUMERALS 
     1 ,  1 ′,  1 A- 1 H: antenna device,  2 ,  2 A,  2 B,  211 ,  212 : feeder electrode,  3 : balanced electrode,  3 A,  3 B: balanced transmission electrode,  23 A,  23 B: junction point,  4 : radiation electrode,  4 A: first electrode of radiation electrode  4 ,  4 B: second electrode of radiation electrode  4 ,  5 : waveguide electrode,  6 : ground electrode,  7 ,  7 A- 7 H: matching circuit,  8 : corner cut portion,  9 : reflector electrode,  10 : dielectric substrate,  11 : top surface of dielectric substrate  10 ,  12 : back surface of dielectric substrate  10 ,  61 : conductor plate,  62 : planer portion,  63 : curved portion,  63 A: slope portion,  100 ,  100 A- 100 D: antenna device,  101 : dielectric substrate,  111 : top surface,  112 : back surface,  20 : feeder electrode,  30 : balun,  40 : radiation electrode,  50 : waveguide electrode,  60 : ground electrode,  71 - 73 : branch circuit,  200 ,  201 ,  202 ,  203 : array antenna,  301 : antenna unit,  302 : signal processing unit,  303 : VCO,  304 : coupler,  305 : circulator,  306 : mixer,  307 : LNA,  308 : A/D converter 
   DETAILED DESCRIPTION OF THE INVENTION 
   An antenna device according to a first embodiment of the present invention will be described with reference to the drawings. 
     FIG. 1(A)  is a plan view showing a structure of an antenna device  1  of this embodiment, whereas (B) is a side view thereof. In  FIG. 1(A) , the horizontal axis when viewed from the front is set as an x axis, whereas a direction toward the right and a direction toward the left are set as a +x direction and a −x direction, respectively. In addition, the vertical axis is set as a y axis, whereas an upward direction and a downward direction are set as a +y direction and a −y direction, respectively. In  FIG. 1(B) , the horizontal direction when viewed from the front is set as a z axis, whereas a direction toward the left and a direction toward the right are set as a +z direction and a −z direction, respectively. In addition, the vertical axis is set as a y axis, whereas an upward direction and a downward direction are set as a +y direction and a −y direction, respectively. Hereinafter, the description of a structure is given supplementary using these x axis, y axis, and z axis. 
   The antenna device  1  of this embodiment includes a dielectric substrate  10  having a predetermined expanse in directions of two axes (the x axis and the y axis) and a predetermined thickness in a direction of an axis (the z axis) vertical to these axes. A feeder electrode  2 , a balanced electrode  3 , a radiation electrode  4 , and a waveguide electrode  5  are formed a top surface  11  (corresponding to “one surface” of the present invention), which is a surface of the dielectric substrate  10  in the +z direction. A ground electrode  6  is formed on a back surface  12  (corresponding to “another surface” of the present invention), which is a surface in the −z direction. 
   The feeder electrode  2  is a linear electrode that extends in the x-axis direction. Along the extending direction, the feeder electrode is connected to balanced transmission electrodes  3 A and  3 B of the balanced electrode  3  at an interval of ½ of a wavelength λ of a transmission/reception signal. In the description given below, a junction point of the feeder electrode  2  and the balanced transmission electrode  3 A and a junction point of the feeder electrode  2  and the balanced transmission electrode  3 B are referred to as a junction point  23 A and a junction point  23 B, respectively. 
   The balanced transmission electrodes  3 A and  3 B are connected to the feeder electrode at the junction points  23 A and  23 B vertically to the extending direction (the x axis) of the feeder electrode  2 , respectively. The balanced transmission electrodes are formed in a shape extending in parallel to each other along this vertical direction (+y direction). 
   The radiation electrode  4  includes a first electrode  4 A and a second electrode  4 B to be connected to ends of the balanced transmission electrodes  3 A and  3 B opposite to the junction points  23 A and  23 B, respectively. These first electrode  4 A and second electrode  4 B are formed in a shape extending in parallel to the extending direction (the x axis) of the feeder electrode  2 , namely, in a shape extending vertically to the extending direction (the y axis) of the balanced transmission electrodes  3 A and  3 B. At this time, the first electrode  4 A extends in the −x direction from the junction point to the balanced transmission electrode  3 A. The second electrode  4 B is formed in a shape extending in the +x direction from the junction point to the balanced transmission electrode  3 B. The length of the radiation electrode  4 , which is constituted by the first electrode  4 A, the second electrode  4 B, and a gap between the first electrode  4 A and the second electrode  4 B, is set to a length that offers predetermined directivity as a dipole antenna. 
   The waveguide electrode  5  is formed in a shape extending in parallel to the extending direction (the x axis) of the radiation electrode  4 . The waveguide electrode  5  is formed to be shorter than the length of the radiation electrode  4  at a position separated from the radiation electrode  4  by a predetermined distance on the side (+y direction) opposite to the balanced electrode  3  with respect to the radiation electrode  4 . In addition, the center of the extending direction (the x axis) of the waveguide electrode  5  is arranged to substantially match the center of the extending direction (the x axis) of the radiation electrode  4  in the x-axis direction. 
   The ground electrode  6  is formed at an area of the back surface  12  corresponding to an area including a portion of the top surface  11  where the feeder electrode  2  is formed and a part of a portion where the balanced electrode  3  is formed but excluding portions where the radiation electrode  4  and the waveguide electrode  5  are formed. More specifically, the ground electrode  6  is formed at an area facing the feeder electrode  2  when the feeder-electrode- 2 -formed portion and the position of the balanced electrode  3  separated from the feeder electrode  2  by a predetermined distance but not reaching the radiation electrode  4  are employed as a boundary. 
   In such a configuration, the dielectric substrate  10 , the feeder electrode  2 , and the ground electrode  6  constitute a microstrip line. In addition, the dielectric substrate  10 , a portion of the balanced electrode  3  near the feeder electrode  2 , and the ground electrode  6  constitute a microstrip line. The dielectric substrate  10  and a portion of the balanced electrode  3  near the radiation electrode  4  constitute a coplanar guide. 
   By means of this, a transmission signal supplied from a transmission signal generating circuit (not shown) through the microstrip line including the feeder electrode  2  is diverged into the balanced transmission electrodes  3 A and  3 B of the balanced electrode  3  at the junction points  23 A and  23 B separated from one another by λ/2, respectively. Here, since the interval between the junction points  23 A and  23 B, namely, the interval of the transmission signal branch points, is λ/2, the transmission signal diverged into the balanced transmission electrode  3 A and the transmission signal diverged into the balanced transmission electrode  3 B have opposite phases. The balanced transmission signals are then transmitted by the microstrip lines having these balanced transmission electrodes  3 A and  3 B (the balanced electrode  3 ). That is, the unbalanced-balanced transform is performed. 
   The transmission line including the balanced transmission electrodes  3 A and  3 B is transformed from the microstrip line into the coplanar type and the balanced transmission signal is transmitted. The balanced transmission signal transferred through the transmission line having the balanced transmission electrodes  3 A and  3 B in this manner is supplied to the radiation electrode  4  and is radiated to a space from the radiation electrode  4  that functions as a dipole antenna. At this time, since the waveguide electrode  5  and the ground electrode  6  are arranged to face each other while sandwiching the radiation electrode  4  at the center along the direction (the y axis) vertical to the radiation electrode  4  and the waveguide electrode  5 , this ground electrode  6  functions as a reflector, and a planar Yagi-Uda antenna including the radiation electrode  4 , waveguide electrode  5 , and ground electrode  6  is formed. With this, a transmission signal is radiated while the direction toward the waveguide electrode  5  from the radiation electrode  4  is set as the center of the directivity. Meanwhile, a reception signal having propagated through the space, received and following the path opposite to that of the transmission signal, is coupled at the two junction points of the balanced electrode  3  and the feeder electrode  2 , is transferred to the microstrip line having the feeder electrode  2 , and is output to a reception signal processing circuit (not shown) from this microstrip line. 
   As described above, the use of the structure of this embodiment allows a branch circuit (a coupled circuit) and an unbalanced-balanced transform circuit to be constituted only by the feeder electrode  2  and a transmission line having the balanced electrode  3  connected to the feeder electrode  2  at an interval of λ/2. This can simplify and miniaturize a structure of feeding a transmission signal from a feeder line, which is an unbalanced line, to a dipole antenna (planar Yagi-Uda antenna), which is a balanced antenna, and transferring a reception signal of the dipole antenna (planar Yagi-Uda antenna) to the feeder line. Furthermore, since the transmission line becomes shorter, a transmission loss is suppressed and an antenna gain is improved. 
   Meanwhile, although the interval between the junction points is set to λ/2 in the description given before, the interval between the junction points may be set to (2N+1)λ/2, where N is a natural number (including 0), which can provide similar effects and advantages. 
   In addition, the shape of each electrode constituting the above-described antenna device is one example and may be appropriately set according to a specification as shown next. 
     FIG. 2  is a plan view showing a structure of an antenna device including a matching circuit at a junction point of a feeder electrode and a balanced electrode. 
   An antenna device  1  shown in  FIG. 2  has a shape of which the width of the feeder electrode  2  is broadened by a predetermined length at a position of a junction point  23 A of a feeder electrode  2  and a balanced transmission electrode  3 A of a balanced electrode  3 . In this case, the feeder electrode  2  is formed in a shape of which the width thereof spreads to the side (−y direction) opposite to the side of the balanced transmission electrode  3 A. With this, a characteristic impedance of the line is adjusted and a matching circuit  7  of the side of the feeder electrode  2  and the side of the balanced transmission electrode  3 A can be formed. 
   In addition, the antenna device  1  shown in  FIG. 2  has a corner cut portion  8 , whose corner is cut in a shape forming a predetermined angle with the extending direction of the feeder electrode  2  at a position of a junction point  23 B of the feeder electrode  2  and a balanced transmission electrode  3 B of the balanced electrode  3 . By forming such a corner cut portion  8 , the characteristic impedance of the lines on the side of the feeder electrode  2  and the side of the balanced transmission electrode  3 B is adjusted. 
   Meanwhile, since other structures are the same as those of the antenna device  1  shown in  FIG. 1 , the description is omitted. 
   By appropriately setting the shapes of the matching circuit  7  and the corner cut portion  8  in this structure, the transmission loss of transmission/reception signals between the feeder electrode  2  and the balanced electrode  3  can be reduced. In addition, by appropriately setting the shapes of these electrodes, a signal branching ratio to the balanced transmission electrodes  3 A and  3 B can be set to a predetermined ratio. In this manner, an antenna device having desired directivity and a low loss can be formed. 
   Next,  FIG. 3  is a plan view showing a structure of an antenna device whose balanced transmission electrode  3 A and  3 B of a balanced electrode  3  are not in parallel. 
   In an antenna device  1  shown in  FIG. 3 , the balanced transmission electrodes  3 A and  3 B are formed so that an interval between the two balanced transmission electrodes  3 A and  3 B of the balanced electrode  3  gradually gets narrow toward the radiation electrode  4  from the feeder electrode  2 . Other structures are the same as those of the antenna device shown in  FIG. 2 . 
   In such a configuration, since the interval between a first electrode  4 A and a second electrode  4 B of the radiation electrode  4  becomes shorter, the directivity different from that of the above-described antenna device having the shape that the balanced transmission electrodes  3 A and  3 B extend in parallel can be obtained. In addition, by appropriately setting this approaching ratio and a gap of the radiation electrode  4 , a plurality of kinds of directivity can be obtained. 
   Next,  FIG. 4  is a plan view showing a structure of an antenna device including a reflector electrode  9 . 
   In an antenna device  1  shown in  FIG. 4 , a reflector electrode  9  is formed on a back surface facing an area where a balanced electrode  3  is formed, in parallel to a radiation electrode  4  at a position separated from the ground electrode  6  by a predetermined distance in a direction (+y direction) toward the radiation electrode  4 . This reflector electrode  9  is formed so that the center of the extending direction (the x direction) thereof substantially matches the center of the extending direction (the x axis) of the radiation electrode  4 . In addition, the length along the extending direction (the x axis) of the reflector electrode  9  is set longer than that of the radiation electrode  4  by a predetermined amount. Meanwhile, other structures are the same as those of the antenna device shown in  FIG. 1 . 
   In such a configuration, since both the reflector electrode  9  and the ground electrode  6  function as a reflector of a Yagi-Uda antenna, a component of a transmission signal radiated from the radiation electrode  4  to the side of the feeder electrode  2  is suppressed and the transmission signal is more likely to be radiated in the direction of the waveguide electrode  4 . With this, desired directivity is obtained, a reflection loss is reduced, and an effective antenna gain can be improved. 
   Meanwhile, although one reflector electrode  9  is provided in  FIG. 4 , a plurality of reflector electrodes may be provided in parallel. 
   Next,  FIG. 5  is a plan view showing a structure of an antenna device having a plurality of waveguide electrodes. 
   In an antenna device  1  shown in  FIG. 5 , two waveguide electrodes  5 A and  5 B are formed at difference distances from a radiation electrode  4  on the side (the +y direction) of the radiation electrode  4  opposite to a feeder electrode  2 . Each of the waveguide electrodes  5 A and  5 B is formed like a line extending in the same direction (the x-axis direction) as the radiation electrode  4 . The radiation electrode  4  and the waveguide electrodes  5 A and  5 B are arranged in parallel. In addition, the waveguide electrodes  5 A and  5 B are formed in the same length and to be shorter than the radiation electrode  4  by a predetermined amount as in the case of the waveguide electrode  5  of  FIG. 1 . In addition, the center of the extending direction of the waveguide electrodes  5 A and  5 B is arranged to match the center of the extending direction of the radiation electrode  4 . Meanwhile, other structures are the same as those of the antenna device shown in  FIG. 2 . 
   In such a configuration, since the directivity of a radiated transmission signal is narrowed by the two waveguide electrodes  5 A and  5 B, a narrower beam transmission signal can be radiated and, furthermore, an antenna gain can be improved. 
   Meanwhile, although two waveguide electrodes are provided in  FIG. 5 , three or more electrodes may be provided. 
   Next,  FIG. 6  is a plan view showing a structure of an antenna device having a first electrode  4 A and a second electrode  4 B of a radiation electrode  4  of different lengths. 
   In an antenna device  1  shown in  FIG. 6 , the length of the first electrode  4 A of the radiation electrode  4  is longer than the length of the second electrode  4 B. In addition, a waveguide electrode  5  is provided so that the center of the extending direction thereof matches the center of the extending direction of the radiation electrode  4 . The centers of the extending directions of these waveguide electrode  5  and radiation electrode  4  are arranged at a position shifted from a position of a line symmetric axis of balanced transmission electrodes  3 A and  3 B of a balanced electrode  3 . Here, although the length of the first electrode  4 A and the length of the second electrode  4 B are set differently, the length of the radiation electrode  4  is set to a length described above. Other structures are the same as those of the antenna device shown in  FIG. 3 . 
   In such a configuration, since the center direction of the directivity can be shifted, for example, along the x axis by the shape of the radiation electrode  4  and the position of the waveguide electrode  5 , the directivity can be changed. This can realize various kinds of directivity, such as, for example, changing the beam direction and the beam width. 
   In addition, a plurality of the above-described structures of  FIG. 2  to  FIG. 6  may be combined instead of using these individually. For example, a structure including a matching circuit and a corner cut portion, including a reflector electrode different from a ground electrode, and further including a plurality of waveguide electrodes or the like may be used. By using such a combination, the antenna device of this embodiment can realize various kinds of directivity with a simple and small structure. 
   Next, an antenna device according to a second embodiment will be described with reference to the drawings. 
     FIG. 7(A)  is an exterior perspective view of an antenna device  1 ′ of this embodiment, whereas (B) is a side view thereof. In addition,  FIG. 7(C)  is a side view showing a different structure of an antenna device of this embodiment. 
   In contrast to the antenna device  1  shown in  FIG. 1 , a conductor plate  61  is provided on a back surface  12  of a dielectric substrate  10  instead of the ground electrode  6  in the antenna device  1 ′ shown in  FIG. 7 . The structures on a top surface  11  of the dielectric substrate  10  are the same and the description regarding the top surface  11  is omitted. 
   The conductor plate  61  is formed in a shape substantially the size of the dielectric substrate  10  in a plan view of an x-y plane. A surface from one lateral face (a lateral face in the −y direction of  FIG. 7 ) to a predetermined distance is formed like a plane (a planar portion  62 ). A surface from an end of this planar portion  62  to the other lateral face (a lateral face in the +y direction of  FIG. 7 ) is formed like a curved surface (a curved portion  63 ). The curved portion  63  is a surface formed in a shape of which the thickness gradually decreases from the boundary with the planar portion  62  toward the other lateral face. The sectional shape along the thinning direction (the y-axis direction) is parabolic. In addition, the curved portion  63  makes contact with the back surface  12  of the dielectric substrate  10  at an angle θ at the boundary point with the planar portion  62  when viewed from the x-axis direction. 
   The planar portion  62  of the conductor plate  61  abuts against the back surface  12  of the dielectric substrate  10 . The size of the abutted area is substantially equal to that of the ground electrode  6  shown in  FIG. 1 . This allows the conductor plate  61  to function as a reflector for the y-axis direction as in the case of the ground electrode  6  shown in  FIG. 1 . In addition, since the curved portion  63  is not parallel to the electrode surfaces of the radiation electrode  4  and the waveguide electrode  5 , transmission signals are reflected at different angles at respective positions. Accordingly, the radiation direction of the transmission signal can be set to a direction (the +y and +z directions of the y-z plane) forming a predetermined angle with the lateral face direction of the top surface  11  according to an angle between the curved surface  63  and the radiation electrode  4  or the waveguide electrode  5 . By means of this, transmission/reception can be performed in a direction forming a predetermined angle with the top surface of the antenna device  1 ′. 
   Results of a simulation using a slope portion  63 A that is not curved but planar and forms a predetermined angle θ with the planar portion  61  as shown in  FIG. 7(C)  as the antenna device  1 ′ having such a structure are shown in  FIGS. 8(A)  and (B). 
     FIGS. 8(A)  and (B) show results of a simulation using the conductor plate  61  including the slope portion  63 A.  FIG. 8(A)  shows antenna directivity, whereas  FIG. 8(B)  shows a change in a center direction angle φ of a transmission/reception signal with respect to a tilt angle θ. In this drawing, the center direction angle of the transmission/reception signal indicates an angle φ of the center direction of the directivity of the transmission/reception signal with respect to the top surface  11  and the angle φ decreases (−value increases) as the conductor plate approaches the top surface  11  in the +z direction. 
   As shown in  FIGS. 8(A) and 8(B) , the angle φ between the center direction of the directivity of the transmission/reception signal and the top surface  11  increases as the tilt angle θ decreases. By appropriately setting the tilt angle θ using this, the center direction of the transmission/reception signal can be variably set along the z-axis. 
   In addition, by combining the structures of the antenna devices shown in  FIG. 2  to  FIG. 6  and the structure of the antenna shown in  FIG. 7 , the center direction of the directivity can be set along each of two planes, which are the x-y plane and the z-y plane, for example, in  FIG. 7 . Accordingly, an antenna device that three-dimensionally sets the center direction of the directivity of a transmission/reception signal can be formed with a simple and small structure. 
   Next, an array antenna according to a third embodiment will be described with reference to the drawing. 
     FIG. 9  is a plan view showing a structure of an array antenna  200  of this embodiment. 
   As shown in  FIG. 9 , the array antenna  200  has a feeder electrode  2  extending linearly on the top surface of a dielectric substrate  10  in the x-axis direction. In addition, the array antenna  200  includes a balanced electrode, a radiation electrode, and a waveguide electrode for each of antenna devices  1 A to  1 C on the top surface of the dielectric substrate  10 . Each of the antenna devices  1 A to  1 C is formed in the same shape as the above-described antenna device  1  shown in  FIG. 3  except for the corner cut portion. In addition, in the array antenna  200 , a junction position of the feeder electrode  2  and the balanced electrode of each of the antenna devices  1 A to  1 C is in a structure similar to the matching circuit  7  and the corner cut portion  8  shown in  FIG. 3 . Matching circuits  7 A to  7 C and a corner cut portion  8 , each set with a predetermined matching condition, are formed. 
   Intervals between respective antenna devices  1 A to  1 C are set to a length of one wavelength of a transmission/reception signal. Meanwhile, it is desirable to set the interval between the antenna devices to 0.8λ to 0.9λ, where λ represents the wavelength, in consideration of a side lobe generated by each antenna device. However, the interval is not limited particularly to this range and may be set to be substantially equal to (n+½)λ, where n is a natural number. 
   In addition, in each of the antenna devices  1 A to  1 C, the respective balanced electrode, radiation electrode, and waveguide electrode are provided in the same direction (the +y direction) with respect to the feeder electrode  2 . Such a configuration allows a transmission/reception beam of a transmission/reception signal whose center direction points the +y direction to be realized with the antenna devices  1 A to  1 C. 
   In the configuration of this embodiment, a balun for each antenna device and branch circuits that connect each antenna device in a tree structure do not have to be formed through a respective transmission line as in the case of a conventional example shown in Non-Patent Document 1. Thus, a planar array antenna can be formed with a simple and small structure. Furthermore, since the transmission distance to the radiation electrode becomes shorter, a planar array antenna having a low loss can be formed. 
   In addition, by using the structures shown in  FIG. 2  to  FIG. 7  as the shape of each antenna device and appropriately setting the interval between the antenna devices in such a configuration, a small array antenna capable of realizing desired directivity can be formed. 
   Next, a multi-sector antenna according to a fourth embodiment will be described with reference to the drawing. 
     FIG. 10  is an elevational view showing a structure of a multi-sector antenna of this embodiment. 
   As shown in  FIG. 10 , four feeder electrodes  2 A,  2 B,  211 , and  212  are formed on a top surface of a dielectric substrate  10  in a shape extending along the x-axis direction. Array antennas  201  and  202  have a structure similar to that of the array antenna  200  shown in  FIG. 9  and each of them are constituted by four antenna devices. The array antenna  201  has a structure that connects the antenna devices  1 A to  1 D to a microstrip line including the feeder electrode  2 A while performing the matching with matching circuits  7 A to  7 D and has the center direction of the directivity in the +y direction. The array antenna  202  has a structure that connects antenna devices  1 E to  1 H to a microstrip line including the feeder electrode  2 B while performing the matching with matching circuits  7 E to  7 H and has the center direction of the directivity in the −y direction. 
   The array antenna  203  is constituted by eight patch electrodes  222  formed at a predetermined interval along the feeder electrodes  211  and  212 . With this structure, the array antenna  203  has the center direction of the directivity in the +z direction substantially vertical to a top surface of the dielectric substrate  10 . 
   Here, the array antennas  201  and  202  are formed in a shape that is parallel to the feeder electrodes  2 A and  2 B and line symmetric with respect to an axis (a symmetry axis) located at the middle of the feeder electrodes  2 A and  2 B. In addition, the array antenna  203  is arranged at a position where the patch electrode  222  provided at the feeder electrode  211  and the patch electrode  222  provided at the feeder electrode  212  become symmetrical with respect to the symmetry axis. Meanwhile, such symmetry is not absolute and may be appropriately set according to the required antenna characteristic. 
   With such a configuration, a multi-sector antenna having directivity of the front direction with the array antenna  203  and directivity in lateral directions with the array antennas  201  and  202  can be formed. In this multi-sector antenna, a simple and small structure can be realized using the structures of the above-described antenna device and array antenna. In addition, since the transmission distance to each radiation electrode becomes shorter in the array antenna for the lateral direction detection, a multi-sector antenna having a low loss can be formed. Furthermore, by employing structures of the antenna devices shown in FIG.  2  to  FIG. 6  and  FIG. 7  in the multi-sector antenna, various kinds of antenna directivity can be realized in a small size. 
   Next, a radar apparatus according to a fifth embodiment will be described with reference to the drawing. 
     FIG. 11  is a block diagram showing major configurations of a radar apparatus of this embodiment. 
   A signal processing unit  302  generates a control voltage for forming a transmission beam on the basis of FMCW detection processing and supplies the voltage to a VCO  303 . The VCO  303  generates a transmission signal whose frequency is continuously modulated in a triangular shape in a time series according to the supplied control voltage. A coupler  304  outputs the input transmission signal to a circulator  305  and also supplies part thereof to a mixer  306  as a local signal. The circulator  305  outputs the transmission signal fed from the coupler  304  to an antenna unit  301 . 
   The antenna unit  301  includes the array antenna shown in  FIG. 9  or the multi-sector antenna shown in  FIG. 10 . Each antenna of the array antenna and the multi-sector antenna are constituted by the antennas shown in  FIG. 1  to  FIG. 7 . 
   The circulator outputs a reception signal fed from the antenna unit  301  to the mixer  306 . The mixer  306  mixes the local signal fed from the coupler  304  and the reception signal fed from the circulator  305 , thereby generating a beat signal. The mixer then outputs the beat signal to an LNA  307 . The LNA  307  amplifies the beat signal and supplies the beat signal to an A/D converter  308 . The A/D converter  308  performs A/D conversion on the amplified beat signal and supplies the signal to the signal processing unit  302 . The signal processing unit  302  calculates a relative speed and a relative distance of a target using a known FMCW data processing method on the basis of the digitalized beat signal. 
   With such a configuration, since the antenna unit  301  is miniaturized, the radar apparatus can be miniaturized. In addition, since the loss of the antenna unit  301  decreases, a radar apparatus having a low antenna loss can be formed and a detection ability can be improved. 
   Meanwhile, although an FMCW radar apparatus is described in this embodiment, radar apparatuses according to other methods may employ the planar antenna, the array antenna using these planar antennas, or the multi-sector antenna.