SWITCHED BEAM ANTENNA ARCHITECTURE

Abstract: A multiple beam array antenna system comprises a plurality of radiating elements (82) provided from stripline-fed open-ended waveguide coupled to a Butler matrix beam forming network (86). The Butler matrix beam forming network is coupled to a switched beam combining circuit (90). The antenna can be fabricated as a single Low Temperature Co-fired Ceramic (LTCC) circuit.

Description: SWITCHED BEAM ANTENNA ARCHITECTURE FIELD OF THE INVENTION This invention relates to antenna elements and more particularly to an antenna element for use in an array antenna. BACKGROUND OF THE INVENTION As is known in the art, there is an increasing trend to include radar systems in commercially available products. For example, it is desirable to include radar systems in automobiles, tracks boats, airplanes and other vehicle. Such radar systems must be compact and relatively low cost. Furthermore, some applications have relatively difficult design parameters including restrictions on the physical size of the structure in addition to minimum operational performance requirements. Such competing design requirements (e.g. low cost, small size, high performance parameters) make the design of such radar systems relatively challenging. Among, the design challenges is a the challenge to provide an antenna system which meets the design goals of being low cost, compact and high performance. In automotive radar systems, for example, cost and size considerations are of considerable importance. Furthermore, in order to meet the performance requirements of automotive radar applications, (e.g. coverage area) an array antenna is required. Some antenna elements which have been proposed for use in antenna arrays manufactured for automotive radar applications include patch antenna elements, printed dipole antenna elements and cavity backed patch antenna elements. Each of these antenna elements have one or more drawbacks when used in an automotive radar application. For example, patch antenna elements and cavity backed patch antenna elements each require a relatively large amount of substrate area and thickness. Array antennas for automotive applications, however, are have only a limited amount of area for reasons of compactness and cost. Thus, antenna elements which can operate in a high density circuit are required. Printed dipole antennas can operate in a high density circuit environment, however, array antennas provided from printed dipole antenna elements give rise to . "blind spots" in the antenna radiation pattern. It would, therefore, be desirable to provide an antenna element which is compact, which can operate in a high density circuit environment, which is relatively low cost and which can be used to provide an array antenna having relatively high performance characteristics. SUMMARY OF THE INVENTION In accordance with the present invention, an antenna element includes a cover layer disposed over a radiator layer having a first ground plane disposed thereon with the ground plane having an aperture therein. The radiator layer is disposed over a feed circuit layer which has a second ground plane disposed thereon. A cavity is provided in the radiator and feed circuit layers by disposing a plurality of via holes between the first and second ground plane layers. An antenna element feed couples energy between the feed circuit and the antenna element. A feed circuit couples energy between the antenna element feed and a butler matrix and is provided as an elevation feed which is interlaced between each of the antenna elements. With this particular arrangement, a compact slotted antenna element which utilizes a stripline-fed open ended dielectric filled cavity is provided. In one embodiment, the antenna element is provided from LTTC substrates on which the . Multiple antenna elements can be disposed to provide a compact array antenna capable of switching between multiple antenna beams. The antenna element of the present invention requires only five layers and thus can be provided as a relatively low cost antenna. The radiator layers can be provided having capacitive windows formed therein for tuning the antenna element. By providing the feed circuit as an elevation feed which is interlaced between each of the antenna elements, a compact antenna which can operate in a densely packed environment is provided. A multiple beam array antenna was designed to radiate at 24 GHz. The entire antenna was fabricated in a single Low Temperature Co-fired Ceramic (LTCC) circuit. The design of the antenna included the radiating element (stripline-fed open-ended waveguide), the beam forming network (Butler Matrix), radiator feed circuit, quadrature hybrid, power dividers, and interlayer transitions. In accordance with a further aspect of the present invention, an array antenna comprises a plurality of slotted antenna elements, each of which utilizes a stripline-fed open ended dielectric filled cavity. With this particular arrangement, a compact array antenna which can provide multiple beams is provided. The antenna can be used in a sensor utilized in an automotive radar application. In a prefeπed embodiment, the sensor includes a transmit and a receive antenna. In a preferred embodiment, the transmit and receive antennas are provided as a bi-static antenna pair disposed on a single substrate. In other embodiments, however, a monostatic arrangement can be used. In accordance with a still further aspect of the present invention, a switched beam antenna system includes a plurality of antenna elements, a butler matrix having a plurality of antenna ports and a plurality of switch ports with each of the antenna ports coupled to a respective one of the plurality of antenna elements and a switch circuit having an input port and a plurality of output ports each of the switch output ports coupled to a respective one of the plurality of switch ports of the butler matrix. With this particular arrangement, a multiple beam switched beam antenna system is provided. By providing the antenna elements from a single Low Temperature Co-fired Ceramic (LTCC) substrate, the antenna system can be provided as a compact antenna system. In a preferred embodiment the radiating element are provided from stripline-fed open-ended waveguide fabricated in the LTTC substrate and the Butler matrix, radiator feed circuit, quadrature hybrid, power dividers, and interlayer transitions are also provided in the LTTC substrate. BRIEF DESCRIPTION OF THE DRAWINGS The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which: Fig. 1 is a block diagram of a radar system; Fig. 2 is a block diagram of an automotive near object detection (NOD) system including a plurality of radar systems; Fig. 3 is a perspective view of a side object detection (SOD) system; Fig. 4 is block diagram of a switched beam antenna system; Fig. 5 is a block diagram of a switched beam forming circuit; Fig. 6 is a block diagram of a Butler matrix beam forming circuit coupled to a plurality of antenna elements; Fig. 7 is a plot of a Butler matrix beams; Fig. 7 A is a plot of antenna system beams provided by combining predetermined ones of the Butler matrix beams; Fig. 7B is a plot of antenna system beams provided by combining predetermined ones of the Butler matrix beams; Fig. 8 is a diagrammatic view of a detection zone which can be provided by the SOD system of Figs. 2A and 11 ; Fig. 8 A is a diagrammatic view of a second detection zone which can be provided by the SOD system of Figs. 3 and 11; Fig. 9 is a top view of an array aperture formed by a plurality of antenna elements; Fig. 10 is an exploded perspective view of an antenna element; Fig. 11 is a cross-sectional view of taken across lines 11-11 of the antenna element of Fig. 10; and Fig. 12 is a detailed block diagram of a SOD system. DETAILED DESCRIPTION OF THE INVENTION Refeπing to Fig. 1, a radar system 10 includes an antenna portion 14, a microwave portion 20 having both a transmitter 22 and a receiver 24, and an electronics portion 28 containing a digital signal processor (DSP) 30, a power supply 32, control circuits 34 and a digital interface unit (DIU) 36. The transmitter 22 includes a digital ramp signal generator for generating a control signal for a voltage controlled oscillator (VCO), as will be described. The radar system 10 utilizes radar technology to detect one or more objects, or targets in the field of view of the system 10 and may be used in various applications. In the illustrative embodiment, the radar system 10 is a module of an automotive radar system (Figure 2) and, in particular, is a side object detection (SOD) module or system adapted for mounting on an automobile or other vehicle 40 for the purpose of detecting objects, including but not limited to other vehicles, trees, signs, pedestrians, and other objects which can be located proximate a path on which the vehicle is located. As will be apparent to those of ordinary skill in the art, the radar system 10 is also suitable for use in many different types of applications including but not limited to marine applications in which radar system 10 can be disposed on a boat, ship or other sea vessel. The transmitter 22 operates as a Frequency Modulated Continuous Wave (FMCW) radar, in which the frequency of the transmitted signal linearly increases from a first predetermined frequency to a second predetermined frequency. FMCW radar has the advantages of high sensitivity, relatively low transmitter power and good range resolution. However, it will be appreciated that other types of transmitters may be used. Control signals are provided by the vehicle 40 to the radar system 10 via a control signal bus 42 and may include a yaw rate signal corresponding to a yaw rate associated with the vehicle 40 and a velocity signal corresponding to the velocity of the vehicle. The DSP 30 processes these control signals and radar return signals received by the radar system 10, in order to detect objects vvithin the field of view of the radar system, as will be described in conjunction with Figures 10-16. The radar system 10 provides to the vehicle one or more output signals characterizing an object within its field of view via an output signal bus 46 to the vehicle. These output signals may include a range signal indicative of a range associated with the target, a range rate signal indicative of a range rate associated with the target and an azimuth signal indicative of the azimuth associated with the target relative to the vehicle 40. The output signals may be coupled to a control unit of the vehicle 40 for various uses such as in an intelligent cruise control system or a collision avoidance system. The antenna assembly 14 includes a receive antenna 16 for receiving RF signals and a transmit antenna 18 for transmitting RF signals. The radar system 10 may be characterized as a bistatic radar system since it includes separate transmit and receive antennas positioned proximate one another. The antennas 16, 18 provide multiple beams at steering angles that are controlled in parallel as to point a transmit and a receive beam in the same direction. Various circuitry for selecting the angle of the respective antennas 16, 18 is suitable, including a multi-position switch. Referring now to Fig. 2, an illustrative application for the radar system 10 of Figure 1 is shown in the form of an automotive near object detection (NOD) system 50. The NOD system 50 is disposed on a vehicle 52 which may be provided for example, as an automotive vehicle such as car, motorcycle, or truck, or a marine vehicle such as a boat or an underwater vehicle or as an agricultural vehicle such as a harvester. In this particular embodiment, the NOD system 50 includes a forward-looking sensor (FLS) system 54 which may be of the type described in U.S. Patent , an electro-optic sensor (EOS ) system 56, a plurality of side-looking sensor (SLS) systems 58 or equivalently side object detection (SOD) systems 58 and a plurality of rear-looking sensor (RLS) systems 60. In the illustrative embodiment, the radar system 10 of Fig. 1 is a SOD system 58. Each of the FLS, EOS, SLS, and RLS systems is coupled to a sensor processor 62. In this particular embodiment, the sensor processor 62 is shown as a central processor to which each of the FLS, EOS, SLS, and RLS systems is coupled via a bus or other means. It should be appreciated that in an alternate embodiment, one or more of the FLS, EOS, SLS, and RLS systems may include its own processors, such as the DSP 30 of Fig. 1, to perform the processing described below. In this case, the NOD system 100 would be provided as a distributed processor system. Regardless of whether the NOD system 50 includes a single or multiple processors, the information collected by each of the sensor systems 54, 56, 58 60 is shared and the processor 62 (or processors in the case of a distributed system) implements a decision or rule tree. The NOD system 50 may be used for a number of functions including but not limited to blind spot detection, lane change detection, pre-arming of vehicle air bags and to perform a lane stay function. For example, the sensor processor 62 may be coupled to the airbag system 64 of the vehicle 52. In response to signals from one or more of the FLS, EOS, SLS, and RLS systems, the sensor processor 62 determines whether it is appropriate to "pre-arm" the airbag of the vehicle. Other examples are also possible. The EOS system 56 includes an optical or IR sensor or any other sensor which provides relatively high resolution in the azimuth plane of the sensor. The pair of RLS systems 60 can utilize a triangulation scheme to detect objects in the rear portion of the vehicle. The FLS system 54 may be of the type described in U.S. Patent No. 5,929,802 entitled Automotive Forward Looking Sensor Architecture, issued July 27, 1999, assigned to the assignee of the present invention, and incorporated herein by reference. It should be appreciated that each of the SLS and RLS sensors 58, 60 may be provided having the same antenna system. Each of the sensor systems is disposed on the vehicle 52 such that a plurality of coverage zones exist around the vehicle. Thus, the vehicle is enclosed in a cocoon-like web or wrap of sensor zones. With the particular configuration shown in Fig. 2, four coverage zones 66a-66d are used. Each of the coverage zones 66a-66d utilizes one or more RF detection systems. The RF detection system utilizes an antenna system which provides multiple beams in each of the coverage zones 66a-66d. In this manner, the particular direction from which another object approaches the vehicle or vice- versa can be found. It should be appreciated that the SLS, RLS, and the FLS systems may be removably deployed on the vehicle. That is, in some embodiments the SLS, RLS, and FLS sensors may be disposed external to the body of the vehicle (i.e. on an exposed surface of the vehicle body), while in other systems the SLS, RLS, and FLS systems may be embedded into bumpers or other portions of vehicle (e.g. doors, panels, quarter panels, vehicle front ends, and vehicle rear ends). It is also possible to provide a system which is both mounted inside the vehicle (e.g., in the bumper or other location) and which is also removable. The system for mounting can be of a type described in U.S. patent application 09/ , entitled Portable Object Detection System and System filed August 16, 2001 or in U.S. patent application 09/ , entitled Technique for Mounting a Radar System on a Vehicle filed August 16, 2001. each of the above-identified patent applications assigned to the assignee of the present invention, and each incorporated herein by reference. Referring now to Fig. 3, a side object detection (SOD) system 70 includes a housing 72 in which the SOD electronics are disposed. A portion of the housing has here been removed to reveal a single substrate 76 on which a plurality of antenna elements 77 are disposed. A preferred antenna array and antenna element will be described in conjunction with Figs. 9-11 below. In this particular embodiment, the substrate 76 is provided as Low Temperature Co-fired Ceramic (LTCC) substrate 76. As will be described in detail below in conjunction with Figs. 10 and 11 , the single substrate 76 can be provided from a plurality of LTCC layers. Also provided in the LTTC substrate 76 is a Butler matrix beam forming circuit, a radiator feed circuit coupled to the antenna elements 75, a plurality of quadrature hybrid and power divider circuits as well as interlayer transition circuits. In one embodiment, the housing 72 for the antenna is provided from an injection molded plastic PBT (polybutylene terephthalate) having a relative dielectric constant (ε r ) of about 3.7. The housing 72 includes a cover 73. One characteristic of cover 73 that affects antenna performance is the cover thickness. In a preferred embodiment, the cover thickness is set to one-half wavelength (.5 λ) as measured in the dielectric. At an operating frequency of about 24 GHz, this corresponds to a cover thickness of about .125 inch. The antenna aperture is preferably spaced from the cover 73 by a distance D (measure from the antenna aperture to an inner surface of the cover 73) which coπesponds to a distance slightly less than about .5 λ as measured in air. At an operating frequency of about 24 GHz, this corresponds to about .2 inch. Other dimensions of the cover 73 are selected based on structure and manufacturing. The housing 72 is provided having a recess region in which the substrate 76 is disposed. The housing may be of the type described in co-pending U.S. Patent Application No. 09/ , entitled Highly Integrated Single Substrate MMW Multi-Beam Sensor, filed August 16, 2001, assigned to the assignee of the present invention and incorporated herein by reference in its entirety. In one embodiment, the antenna is built using Ferro's A6-M LTCC tape. The tape is provided havibg a thickness of about 0.010 inch pre-fired and 0.0074 inch post-fired and a dielectric constant of about 5.9. The LTTC tape has a loss characteristic at 24 GHz of 1.0 dB(more like l.ldB) per inch for a 0.0148 inch ground plane spacing. LTCC was chosen for this antenna for a variety of reasons including but not limited to its potential for low cost in high volume production. Furthermore, LTCC allows compact circuit design and is compatible technology (at this frequency) for multi-layer circuits having relativley large quantities of reliable, embedded vias (approximately 1200 vias in oneparticualr embodiment of this antenna). Surface-mount devices can also be integrated with LTCC. Referring now to Fig. 4, a switched beam antenna system includes an antenna 80 having a plurality of antenna elements 82. Each of the antenna elements 82 are coupled through a respective one of a plurality of elevation distribution networks 84a-84h to a respective one of a plurality of output ports 86a-86h of a Butler matrix beam foiming network 88. As will be described in conjunction with Figs. 5-7 below, a signal fed to predetennined ones of the input or beam port 88a-88h results in the antenna forming a beam which appears in a different beam location in an azimuth plane. Each of the elevation distribution networks 84 is comprised of a first two-to-one (2:1) power divider 90, which splits the power equally to two radiator feed circuits 92a, 92b. In one embodiment, the radiator feed circuit is provided from a corporate feed that provides half of the amplitude distribution in elevation. Each feed circuit 92a, 92b is coupled, respectively, to three "radiating elements" or more simply "radiators" through signal paths having differential line lengths. These differential line lengths provide the appropriate phase shift for the elevation beam steer. The Butler matrix input ports are coupled to output ports of a switched beam combining circuit 90 having a first plurality of switch ports 92a - 92h coupled to corresponding ones of the Butler matrix ports 88a - 88h and a common port 94 at which combined beams are provided as will be described below. The ports 88a - 88h of the Butler matrix 88 represent different antenna beam locations. These beams are independent and are simultaneously available. The location of the beams with respect to Butler port location is given in Fig. 7 where reference numbers 120a-120h represent the relative locations of the beams in space, and the beam numbers 1-8 (designated by reference numerals 121a- 121h in Fig. 7) refer to the Butler input beam port numbers as given in Fig. 6 and similarly in Fig. 5, designated with reference numbers 102a- 102h. It should be noted that adjacent beams (in space) are always located on opposite halves of the Butler input beam ports (left half of beams ports, 1,2,3 and 4 and right half of beam ports 5,6,7 and 8). The combination of adjacent orthogonal beams will give a resulting beam having a cosine aperture distribution, and therefore lower sidelobe levels. By placing a 4-way switch (such as switches 105, 106 in Fig. 5) each half of the Butler input beam ports access to any adjacent pair of beams is realized. The inputs of the two 4-way switches 105a and 106a (Fig. 5) are then combined through an equal power divider 108. In this way, the single input to the power divider 108a is connected to one of seven combined beams (designated as 124a-124g in Figure 7A). The beam combinations in Figure 7 A, are represented by the numbered pairs (2,6), (6,4), (4,8), (8,1), (1,5), (5,3), (3,7). That is, beam 124a (Fig. 7A) is provided from the combination of beams 2 and 6 represented as (2, 6). Similarly, beam 124b (Fig. 7 A) is provided from the combination of beams 6 and 4 represented as (6, 4); beam 124c (Fig. 7A) is provided from the combination of beams 4 and 8 represented as (4, 8); beam 124d (Fig. 7A) is provided from the combination of beams 8 and 1 represented as (8, 1); beam 12e (Fig. 7 A) is provided from the combination of beams 1 and 5 represented as (1, 5); beam 124f (Fig. 7 A) is provided from the combination of beams 5 and 3 represented as (5, 3); and beam 124g (Fig. 7A) is provided from the combination of beams 3 and 7 represented as (3, 7). These numbered pairs also represent the switch location for left and right switches 105, 106 (Fig. 5), respectively. For example, if switch 105 is set to select port 1 designated as reference number 102a in Fig. 5) (i.e. provide a low impedance signal path between port 1 and switch port 105a) and switch 106 is set to select port 8 designated by reference number 102h in Fig. 5. The resulting combined beam at power divider port 108a is beam 8,1 which is the center beam. Refeπing now to Fig. 5, a Butler matrix beam forming network 98 is shown having a plurality of antenna element ports 100a- 1 OOh (which may coπespond, for example, to ports 86a-86h of Fig. 4) and a plurality of switch ports 102a-102h (which may coπespond, for example, to ports 88a-88h of Fig. 4). The switch ports 102a-102h are coupled through transmission lines 103a-103h to a switched beam combining circuit 104. As is known, the port phasing for Butlers have 180° phase difference and the curved signal paths 103a, 103c represent 180° differential line lengths required to bring all of the ports in phase with each other. The switched beam combining circuit 104 is here provided from a pair of single pole four throw switches 105, 106, each of the switches 105, 106 having a common port 105a, 106a coupled to the output port of a power divider circuit 108. The power divider circuit 108 is provided such that a signal fed to an input port 108a has an equal phase equal power level at the output ports 108b, 108c. Refemng now to Fig. 6, a plurality of antenna elements 110a— 11 Oh are coupled to ports 100a - 1 OOh of a Butler matrix beam forming network 112. The Butler matrix beam forming network 112is here shown provided from a plurality of power divider circuits 114. The circuits 114 are provided as quadrature, or 90°, stripline ring hybrids which are coupled as shown to provide the Butler Matrix Circuit. These hybrids have a 3 dB power split with a 90° phase difference in the outputs of the through and coupled arms. The Butler matrix is a lossless beam forming network that forms orthogonal beams at fixed locations. The beam locations are a function of the spacing of the antenna elements in an antenna aπay (refeπed to as an aπay lattice spacing). In this particular embodiment, the Butler matrix 112 uses quadrature hybrid circuits 114 and fixed phase shifts to form the multiple beams. As will be explained in further detail below, the fixed phase shifts are provided from differential line lengths 115 where the path lengths are indicated by the 2n's, 3n's and ln's in Fig. 6, where n=D/8 radians. Other techniques for providing the fixed phase shifts can also be used. Typically, for narrow bandwidths, the fixed phase shifts are simply differential line lengths. The unit of phase shift is π/8 radians or λ/16. There are 2 N beams. Butler gives the following equation to calculate the beam locations: beamloc(M) := asin ■ M - I 180 N-d π where: λ is the wavelength of the center frequency; N is the number of elements; d is the element spacing; and M is the beam number. Referring now to Fig. 7, in this particular embodiment, the Butler matrix forms eight beams 120a - 120h. That is, by providing an input signal to one of the Butler matrix input ports 112a - 112h, the antenna 110 produces a coπesponding one of the beams 120a - 120h. The calculations for determining the beam locations can be found using the equations below: Wavelength (inches): λ = 11.81 24 Number of Elements : N : = 8 Element Spacing (Azimuth): d - .223 Beam Location (Degrees): beamloc(M) := asin 180 - - N-d \ 2 π N Beam Number: M : - 1.. — If the array is provided having an array lattice spacing of 0.223" in azimuth, the beam locations shown in Fig. 7 are provided. In one embodiment, the differential line length value, n is selected to be 1/16 λ which conesponds to 0.0127 inch at a frequency of 24 GHz. Fig. 7 also illustrates which beam-ports in Fig. 6 produce which beams. Refening now to Fig. 7A, a calculated antenna radiation pattern 122 includes seven beams 124a — 124g which can be used in a radar system. The seven beams are provided by combimng predetermined ones of the eight beams fonned by the Butler Matrix as discussed above. Adjacent beams (e.g. beams 120a, 120b from Fig. 7) can be combined to produce beam 124a as illustrated in Figure 7 A. Since beams out of a Butler Matrix by definition are orthogonal, combimng beams in azimuth produces a cos(θ) taper with a peak sidelobe level of 23 dB (with respect to the beam maximum). The locations of the combined beams are listed in the Table below. TABLE Combined Beam Beam Location _ _ 4,8 & 1,5 +/- 16 6,4 & 5,3 +/- 34 2,6 & 3,7 +/- 57 In elevation, there is also a 25 dB Chebyshev taper and a 15° beam steer. Referring now to Fig. 7B, the resultant combined beam aπay factor is shown The combined beams are eπorless contours in U-V space with a 20 dB floor. They assume a cos(θ) element factor. The plots are a representation of the seven combined beams in azimuth with the 15° beam steer in elevation. It should be appreciated that producing the cos(θ) taper in azimuth with the adjacent beam combining was a cost driven design choice. Instead, the taper could have been produced using attenuators. However, these would have required the use of embedded resistors in the LTCC circuit. Using embedded resistors on an LTCC tape layer would add another processing step in the manufacture of the LTCC circuit . Therefore, using attenuators to produce the azimuth distribution would have increased the cost of the antenna. Moreover, the technique of the present invention simplifies the switch network by eliminating a 2-way switch. Refeπing now to Figs. 8 and 8 A, two different examples of side detection zones are shown. In Fig. 8, a vehicle 129 has a maximum detection zone 130 disposed thereabout. The maximum detection zone 130 is defined by a detection zone boundary 131 In this example, the maximum detection zone boundary 130 is provided having a trapezoidal shape. An exemplary SOD system provides seven azimuthal beams 132a -132g each with a different maximum detection range, as indicated by the shaded region, and as determined by a detection algorithm that operates upon the beam echoes. The algorithmic control of the maximum detection range of each of the eight beams defines the shape of an actual maximum detection zone boundary 134 versus a specified nominal detection zone boundary 136. The manner in which an object is detected is described in co-pending U.S. Patent Application No. 09/ , entitled Radar Detection Method and Apparatus, filed August 16, 2001 assigned to the assignee of the present invention and incorporated herein by reference in its entirety. The exemplary SOD system of Figs. 8 and 8 A, has seven beams, each with a beam width of approximately fifteen degrees and with a total azimuth scan of about one hundred fifty degrees. It will be recognized by one of ordinary skill in the art that other numbers of beams (e.g. fewer or more than seven) and scan angles are possible without departing from the present invention. The particular number of antenna beams to use in a particular application is selected in accordance with a variety of factors including but not limited to shape of coverage zone, size of coverage zone, required Azimuth resolution, complexity and cost. Refeπing now to Fig. 8 A, a boundary 140 having a substantially rectangular shape defines a detection zone 142 about a vehicle 144. Again, an exemplary system provides seven azimuthal antenna beams 146a - 146g each of the antenna beams 146a - 146g having a different maximum detection range 148 as indicated by shading. The maximum detection ranges 148 being different from beams 132a - 132g (Fig. 8) so as to form a different actual maximum detection zone. Refeπing now to Fig. 9, an aπay antenna 150 having a length L and width W includes a transmit array 152 and a receive array 154. Each of the arrays 152, 154 includes eight rows 156a - 156g and six columns 158a- 158f. Thus each ofthe fransmit and receive aπays 152, 154 have forty-eight radiating elements (or more simply "radiators" or "elements"), generally denoted 160, with eight elements in azimuth and six elements in elevation. As will be described in detail in conjunction with Figs. 10 and 11, each radiating element 160 is a stripline-fed open-ended cavity in LTCC. The cavity is formed in the LTCC using embedded vias, generally denoted 162, that create the "cavity walls." Each of the arrays 152, 154 have a rectangular lattice spacing: 0.223" (azimuth) x 0.295" (elevation). The azimuth spacing is driven by the Butler matrix to yield desired beam locations which provided desired detection zones. The elevation spacing is driven by an elevation beamwidth requirement and the maximum spacing needed to avoid a cover induced scan blindness. In an automotive radar application, the antenna is enclosed in a plastic housing and will radiate through the housing cover (e.g. as shown in Fig. 3). In one embodiment, the housing cover can be incorporated into the radiator design. In a prefeπed embodiment, however, the antenna radiates through a housing cover spaced about one-half wavelength from the antenna aperture as shown in Fig. 3. In this approach, the antenna, can have an antenna cover disposed thereover, but the cover can be made of additional layer of LTCC (e.g. as represented by layer 176 in Fig. 10). Refeπing now to Fig. 10, in which like elements of Fig. 9 are provided having like reference designations, a radiating element 170 which may be used for example in the antennas described in conjunction with Figs. 2A, 3, 5, 8, 9 and 11 includes a ground plane layer 172 having first and second opposing surfaces with a ground plane 173 disposed over the second surface thereof. A plurality of radiating layers 174 are disposed over the second surface of the ground plane layer 172. Each of the radiating layers 174 has an antenna structure included thereon as will be described below in detail below. Suffice it here to say that structures on each of the radiating layers 174 are appropriately aligned relative to the ground plane 173. An antenna element cover layer 176 is disposed over the radiating layers 174. In one embodiment, the element cover layer 176 for the antenna 170 is incorporated into the radiator 170. In one particular embodiment in which the element operates at a frequency of about 24 GHz, the element cover layer 176 is provided having a thickness of about 0.038 inches and a dielectric constant of about 3.5 and is used to "tune" the radiator 170 (i.e. the cover 176 is utilized to help provide the antenna element 170 having an appropriate response to signals in a desired frequency range. In another embodiment, to be described below in conjunction with Fig. 10, the cover layer 176 is provided from LTCC having a thickness of about 22.2 mils. In this embodiment, the cover layer can be provided from three 10 mil (prefired) tape layers (e.g. layers 218 - 222 in Fig. 11). In the embodiment of Fig. 9, the radiating layers are provided from four layers 178 — 184. Each of the radiating layers 178 - 184 are provided as LTCC tape layers. The layers are provided having a thickness of about 10 mil (prefired) and about 7.4 mil(post fired) with a nominal ε r of about 5.9. and a loss tangent typically of about .002. Layer 178 is provided having a conductive strip 188 disposed thereon which coπesponds to an antenna element feed circuit 188. The layer 180 is provided having a conductive material 190 disposed thereon which coπesponds to a ground plane 190. Thus, feed circuit 188 is disposed between upper and lower ground planes and thus layer 178 coπesponds to a stripline feed layer 178. The ground plane 190 is provided having an aperture 192 therein and thus layer 180 coπesponds to both a ground plane layer and a capacitive layer 180. Layer 182 is provided having a conductive trace 191 disposed thereon through which vias 160 are disposed. The conductive trace 191 forms an aperture 193. Disposed over the layer 182 is the layer 184 which is provided having a ground plane 194 disposed on a top or second surface thereof . The ground plane has a portion thereof removed to form an aperture 196. Conductive vias 160 pass through each of the layers 172 and 178- 184 to form a cavity. Thus, the ground plane layer 172, radiating layers 174 (and associated structures disposed on the radiating layers 174) and cover layer 176 form the radiating element 170 as a stripline-fed open-ended cavity formed in LTCC. The cavity is formed in the LTCC by the embedded vias 160 which provide a continuous conductive path from a first or top surface of aperture layer 184 to a second or top surface of the ground plane layer 172 and is fed by the stripline probe 188. In one particular embodiment the cavity is provided having a length of 0.295 inches, a width of 0.050 inches and a height of 0.0296 inches (0.295" x 0.050" x 0.0296"). The capacitive windows 192 were used on the aperture and as internal circuit layers for tuning the radiator. In this particular embodiment, the design of the radiator 170 was driven by the desire to reduce the number of LTCC layers, and here the cost. Due to the low cost requirement of the antenna, the antenna itself was specified to have a maximum number of eight LTCC tape layers. As described above in conjunction with Fig. 5, the Butler Matrix circuit is comprised of four LTCC tape layers or two stripline circuit layers. Due to the size and circuit layout of the Butler Matrix, it was necessary to split the beam forming network between two stripline circuit layers or four LTCC tape layers. Therefore, there were four LTCC tape layers available for the radiator. In addition, these remaining four LTCC circuit layers also needed to include the elevation distribution network. This resulted in an RF circuit having a relatively high density of circuits on those layers. In this particular embodiment, the antenna element 170 includes the reactive apertures or windows 192, 193 and 196 which are used to provide the radiating element 170 having a desired impedance match to free space impedance. The reactive apertures 192, 193, 196 as well as the element cover 176 are used to match the impedance of the feed line 188 to free space impedance. Thus, the radiating element 170 includes tuning structures on a plurality of different layers, here three different layers of the radiator, which can be used to provide the antenna element 170 having a desired impedance. The cover 176 was provided from LTTC and was utilized as a tuning structure as well. However, dielectric covers are often associated with scan blindness phenomena in arrays. An analysis of the scan reflection coefficient for different cover thicknesses was performed to ensure that scan blindness effects would not hinder the performance of the antenna. The results of the scan blindness analysis for a variety of cover thicknesses for scan reflection coefficient (due to the cover 176) vs. the scan angle in degrees revealed that a prefeπed range of cover thicknesses is 0.0.020 inch to 0.030 inch. At these cover thicknesses, the scan reflection coefficient is relatively small in the azimuth scan at the farthest scan angle of 75°. Therefore, this range of covers should not produce any scan blindnesses in the array in the azimuth scan to 75°. In one embodiment, a 0..022 inch thick cover, the closest multiple of 7.4 mils to the optimum thickness using analytic techniques was used in the design of the radiator. An elevation scan analysis indicated that there would be no scan blindness effects with any of the covers at an elevation beam steer of 15 ° . Refeπing now to Fig. 11 in which like elements of Fig. 10 are provided having like reference designations, a radiating element 200 and associated feed circuits are provided from eleven LTCC tape layers 202 - 222, each of the layers having a post-fired thickness typically of about 0.0074 inch and having a (stripline) ground plane spacing of 0.0148 inch. The element 200 has an element signal port 200a provided in layer 202. The radiating element 200 is provided from the structures to be described below provided in the layers 210 - 222 as shown. It should be noted that cover layers 218-222 (which may coπespond to cover layer 176 in Fig. 10) are integral to the radiating element 200. Layer 216 has a ground plane 224 disposed thereon. Portions of the ground plane are removed to form an aperture 226. A power divider circuit 228 is coupled through conductive vias 230a, 230b to a conductive trace 232 and a strip line feed circuit 234, respectively. Thus, an elevation feed circuit is interlaced with the element 200. Capacitive windows 240 are formed on layers 214, 216 via by disposing ground planes material on the layers 214, 216 and providing openings in the ground planes. Layers 202, 204 and 208 are also provided having ground planes 242 disposed thereon.. Layers 202 - 208 are dedicated to a Butler Matrix circuit while layers 210 - 216 are dedicated to the radiator and feed circuit. The embedded vias 160 in the LTCC are used for forming the waveguide structure of the radiator in the LTCC while vias 230a, 230b, 230c, 230d are used for transitioning between the circuits on the different layers 202-216. As can been seen in Figs. 9 and 10, the embedded vias 160 form a waveguide structure and share the same layers as the power divider circuit 228 and the radiator feed circuit 234. The LTCC manufacturing flow comprises eight operations which are defined as: tape blanking, via formation, via filling, conductor deposition, lamination, wafer firing, continuity test, and dicing. The following is a brief description of each of the eight operations. Raw LTCC is supplied in tape form on spools having a standard width of either seven or ten inches. Typical tape area per roll ranges from 4200 to 6000 sq. in. and is also predetermined at time of order. The blanking of LTCC tape is performed manually with the use of an arbor blanking die. Tape is blanked to either a 5" or a 7" manufacturing format size. An orientation hole is also introduced during the blanking operation which references the LTCC tape's as-cast machine and transverse directions. This orientation hole will ultimately allow for layers to be identified and cross-plied in order to optimize total product shrinkage at firing. The creation of Z-axis via holes is performed through the use of a high speed rapid punch system. The system is driven by punch CAD/CAM data which is electronically down loaded via ethemet directly to the manufacturing work cell. The supplied punch files contain X- Y-coordinate locations for via formation. Individual tape layers, in either a 5" or 7" format, are mounted into single layer tape holders/frames. These framed layers are subsequently loaded into a handling cassette which can house a maximum of 25 LTCC tape layers. The cassette is loaded and is handled automatically at the work center when respective punch programs are activated. The high speed punch processes via holes in tape layers individually and ultimately indexes through the entire cassette. Via holes are formed at typical rates of 8 to 10 holes per second. At the completion of via formation for a particular tape layer the cassette is unloaded from the work center, processed tape layers removed, and the cassette is reloaded for continued processing. LTCC tape layers which have completed respective via formation operations require the insertion of Z-axis conductors in order to ultimately establish electrical interface with upper and lower product layers. The via filling operation requires the use of positive pressure displacement techniques to force conductive pastes into via formed holes in the dielectric tape. Mirror image stencils are manufactured for respective tape layers which feature all punched via hole locations; these stencils are fixtured on a screen printing work cell. LTCC tape layers are soft fixtured onto a porous vacuum stone. The stone is indexed under the stencil where a preset pressure head travels over the stencil forcing deposited conductor paste through the stencil and into the dielectric tape. Each tape layer is processed in a similar fashion; all layers are dried, driving off solvents, prior to follow on operations. Via filled dielectric tape layers require further processing to establish X- and Y-axis conductor paths. The deposition of these conductor mediums provides "from-to" paths on any one LTCC layer surface and originate from and terminate at filled via locations. The conductor deposition operation employs the same work center as described in the via filling operation with the exception that wire mesh, emulsion patterned screens are substituted for through hole stencils. The technique for fixturing both the screen and the tape product is also the same. All product layers are serially processed in this fashion until deposition is complete; again, all layers are dried prior to follow on operations. Prior to lamination all previous tape processing operations occur in parallel with yield fallout limited to respective layer types. The lamination operation requires the collation and maπiage of parallel processed layers into series of independent wafers. Individual layers, (layers 1,2,3, ...n), are sequentially placed upon a lamination caul plate; registration is maintained through common tooling which resides in all product layers. The collated wafer stack is vacuum packaged and placed in an isostatic work cell which provides time, temperature, and pressure to yield a leathery wafer structure. Laminated wafers are placed on firing setters and are loaded onto a belt furnace for product densification. Firing is performed in a single work cell which performs two independent tasks. The primary operation calls for the burning off of solvents and binders which had allowed the tape to remain pliable during the via formation, filling, conductor deposition, and lamination operations. This binder burnout occurs in the 350 - 450C range. The wafer continues to travel down the belt furnace and enters the peak firing zone where crystallization, and product densification occurs; temperatures ranging to 850-860C are typical. Upon cool down the wafers exit the furnace as a homogenous structure exhibiting as-fired conditions. All product firing occurs in an air environment. Post firing operations would not require wafers to be processed through an additional binder burnout steps but would only require exposure to the 850C densification temperatures. Continuity net list testing is performed on individual circuits in wafer foπn. Net list data files are ethemet down loaded to the net probe work center and are exercised against respective wafer designs. Opens and shorts testing of embedded nets, and capacitance and resistive load material measurements defines the bulk work center output. Failures are root caused to specific net paths. Net list tested wafers typically exhibit individual circuit step/repeat patterns which can range from one to fifty or more on any one particular wafer. Conventional diamond saw dicing techniques are employed to singulate and dice circuits out of the net list tested wafers. Common fixturing is in place to handle both 5" and 7" fired wafer formats. Refeπing now to Fig. 12, a radar system which may be similar to the radar systems described above in conjunction with Figs. 1 and 2 respectively for use as a SOD system is shown in greater detail. In general overview of the operation of the transmitter 22 (Fig. 1), the FMCW radar transmits a signal 250 having a frequency which changes in a predetermined manner over time. The transmit signal 250 is generally provided by feeding a VCO control or ramp signal 252 to a voltage controlled oscillator (VCO) 254. In response to the ramp signal 252, the VCO 254 generates a chirp signal 256. A measure of transmit time of the RF signal can be determined by comparing the frequency of a received or return signal 258 with the frequency of a sample 260 of the transmit signal. The range determination is thus provided by measuring the beat frequency between the frequencies of the sample 260 of the transmit signal and the return signal 258, with the beat frequency being equal to the slope of the ramp signal 252 multiplied by the time delay of the return signal 258. The measured frequency further contains the Doppler frequency due to the relative velocity between the target and the radar system. In order to permit the two contributions to the measured frequency shift to be separated and identified, the time- varying frequency of the transmit signal 250 is achieved by providing the control signal 252 to the VCO 254 in the form of a linear ramp signal. In one embodiment, the VCO control signal 252 is generated with digital circuitry and techniques. In a prefeπed embodiment, the ramp signal 252 is generated by a DSP 262 and a digital-to-analog converter (DAC) 264. Use of the DSP 262 and DAC 264 to generate the ramp signal 252 is possible in the SOD system of Fig. 12 since, it has been determined that by proper selection of the detection zone characteristics including but not limited to detection zone size, shape and resolution, precise linearity of the chirp signal 256 is not necessary. With this aπangement, the frequency of the transmit signal 250 is accurately and easily controllable which facilitates implementation of several advantageous and further inventive features. As one example, one or more characteristics of successive ramps in the ramp signal 252 are randomly varied (via random number generator 253, for example) in order to reduce interference between similar, proximate radar systems. As another example, temperature compensation is implemented by appropriately adjusting the ramp signal 252. Yet another example is compensation for non-linearity in the VCO operation. Further, changes to the SOD system which would otherwise require hardware changes or adjustments can be made easily, simply by downloading software to the DSP. For example, the frequency band of operation of the SOD system can be readily varied, as may be desirable when the SOD is used in different countries with different operating frequency requirements. An electronics portion 270 of the SOD system includes the DSP 262, a power supply 272 and a connector 274 through which signal buses 42, 46 (Fig. 1) are coupled between the SOD system and the vehicle 40 (Fig. 1). The digital interface unit 36 (Fig. 1) is provided in the form of a controller area network (CAN) transceiver (XCVR) 276 which is coupled to the DSP via a CAN microcontroller 278. The CAN controller 278 has a system clock 279 coupled thereto to provide frequency stability. In one embodiment, the system clock is provided as a crystal controlled oscillator. An analog-to-digital (A/D) converter 280 receives the output of a video amplifier 282 and converts the signal to digital form for coupling to the DSP 30 for detection processing. In one embodiment, the A/D converter is provided as a twelve bit A/D converter. Those of ordinary skill in the art will appreciate, however, that any A/D converter having sufficient resolution for the particular application may be used. A signal bus 284 is coupled to antenna switch circuits 286, 288 in order to provide control signals to drive the switches which comprise the switch circuits. Circuits 286, 288 can include switches (e.g. switches 105, 106 of Fig. 5), phase lines (e.g. lines 103a - 103h of Fig. 5) and a beamforming circuit (e.g. Butler matrix beamforming circuit 98 in Fig. 5). Also provided in the electronics portion 270 of the SOD system is a memory 190 in which software instructions, or code and data are stored. In the illustrative embodiment of Fig. 12, the memory 190 is provided as a flash memory 190. The DSP provides output signals, or words to the DAC which converts the DSP output words into respective analog signals. An analog smoothing circuit 292 is coupled to the output of the DAC in order to smooth the stepped DAC output to provide the ramp control signal to the VCO. The DSP includes a memory device 294 in which is stored a look-up table containing a set of DSP output signals, or words in association with the frequency of the transmit signal generated by the respective DSP output signal. The VCO 254 receives ramp signal 252 from the analog smoothing circuit. The VCO operates in the transmit frequency range of between 24.01 to 24.24 GHz and provides an output signal to bandpass filter 296, as shown. The output of the VCO 254 is filtered by the bandpass filter 296 and amplified by an amplifier 298. A portion of the output signal from amplifier 298, is coupled via coupler 300 to provide the transmit signal 250 to the transmit antenna 18. Another portion of the output signal from the amplifier 298 coπesponds to a local oscillator (LO) signal fed to an LO input port of a mixer 304 in the receive signal path. The switch circuits 286, 288 are coupled to the receive and transmit antennas 16, 18 through a Butler matrix. The antennas 16, 18 and switch circuits 286, 288, and Butler matrix can be of the type described above in conjunction with Figs. 1-11. Suffice it here to say that the switch circuits and Butler matrix operate to provide the antenna having a switched antenna beam with antemia beam characteristics which enhance the ability of the SOD system to detect targets. The received signal 258 is processed by an RF low noise amplifier (LNA) 306, a bandpass filter 308, and another LNA 310, as shown. The output signal of the RF amplifier 310 is down-converted by mixer 304 which receives the local oscillator signal coupled from the transmitter, as shown. Illustrative frequencies for the RF signals from the amplifier 310 and the local oscillator signal are on the order of 24 GHz. Although the illustrated receiver is a direct conversion, homodyne receiver, other receiver topologies may be used in the SOD radar system. A video amplifier 282 amplifies and filters the down-converted signals which, in the illustrative embodiment have a frequency between 1 KHz and 40 KHz. The video amplifier 64 may incorporate features, including temperature compensation, filtering of leakage signals, and sensitivity control based on frequency, as described in a co-pending U.S. Patent Application entitled Video Amplifier for a Radar Receiver, Application No. 09/ , filed on August 16, 2001, and incorporated herein by reference in its entirety. The A/D converter 280 converts the analog output of the video amplifier 320 into digital signal samples for further processing. In particular, the digital signal samples are processed by a fast Fourier transform (FFT) within the DSP in order to determine the content of the return signal within various frequency ranges (i.e., frequency bins). The FFT outputs serve as data for the rest of the signal processor 262 in which one or more algorithms are implemented to detect objects within the field of view, as described in co- pending U.S. Patent Application entitled Radar Transmitter Circuitry and Techniques, Application No. 09/ , filed on August 16, 2001, and incorporated herein by reference in its entirety. The radar system includes a temperature compensation feature with which temperature induced variations in the frequency of the transmit signal are compensated by adjusting the ramp signal accordingly. For this purpose, the transmitter 22 includes a DRO 322 coupled to a microwave signal detector 324. The output of the microwave detector is coupled to an analog-to-digital converter of the CAN controller for processing by the DSP. The details of such processing are described in the aforementioned U.S. Patent Application No. 09/ , entitled Radar Transmitter Circuitry and Techniques. It should be appreciated that in a prefeπed embodiment, the receive and transmit antenna antennas 16, 18 of the sensor system are arranged as a bi-static antenna pair. In other embodiments, however, a monostatic arrangement can be used. In one embodiment, the transmit signal path includes a signal source having an output coupled to an input port of a switch circuit having a plurality of output ports each of which is coupled through a Butler beam forming circuit to a column feed circuit of the transmit antenna. Thus, the number of output ports in the switch circuit coπesponds to the number of columns included in the transmit antenna. In one embodiment, the switch circuit is provided from a coupler circuit having an input port and a pair of output ports and a pair of switch circuits, each of the switch circuits having a common port and a plurality of output ports. Each of the coupler output ports are coupled to a one of the switch common ports. Each of the switches has a number of outputs coπesponding to a predetermined number of columns included in the transmit antenna. In a prefeπed embodiment, the number of total output ports between the two switches equals the number of columns in the antenna. Each of the switch outputs are thus coupled to inputs of the transmit antenna. With this aπangement, a combined beam transmit architecture is provided. Similarly, on a receive signal path, the receive antenna is coupled to a receiver circuit through a Butler beam forming circuit and phase lines which lead from the receive antenna output ports to the input ports of a switch circuit. The switch circuit can be provided from a coupler and a pair switches as described above in conjunction with the transmit antenna. By selectively coupling predetermined ones of the switch circuit input ports to the switch output port, combined beams are provided. With this aπangement, a combined beam receive architecture is provided. Having described the prefeπed embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may be used. It is felt therefore that these embodiments should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety. What is claimed is:

Claims: CLAIMS 1. An aπay antenna comprising: a first plurality of antenna elements disposed to provide a transmit antenna; a butler matrix beam forming circuit having a plurality of antenna ports and a plurality of switch ports with each of the antenna ports coupled to ones of said first plurality of antenna elements; and a first switched beam combining circuit having an input port and a plurality of output ports, each of the output ports coupled to a coπesponding one of the plurality of input ports of said butler matrix beamforming circuit. 2. The antenna of claim 1 further comprising: a second plurality of antenna elements disposed to provide a receive antemia; a butler matrix beam forming circuit having a plurality of antenna ports and a plurality of switch ports with each of the antenna ports coupled to ones of said second plurality of antemia elements; and a second switched beam combining circuit having a plurality of input ports and an output port, each of the plurality of input ports coupled to a coπesponding one of the plurality of the output ports of said butler matrix beamforming circuit. 3. The antenna of Claim 1 further comprising a transmit signal source having an output coupled to an input of said first switched beam combining circuit. 4. The antenna of Claim 2 further comprising a receiver coupled to an output of said second switched beam combimng circuit. 5. The antenna of Claim 1 wherein transmit antenna is provided as an aπay antenna having a first plurality of antenna columns and said first switched beam combining circuit comprises: a plurality of switch circuits, each of the switch circuits having an input port and a plurality of output ports with a total number of switch output ports provided from the combination of said plurality of switch circuits equals the number of antenna columns provided in the transmit antenna; a coupler circuit having an input port and a plurality of output ports equal to the number of input ports provided from the combination of said plurality of switch circuits with each of the output ports of said coupler circuit coupled to a coπesponding one of the input ports of said plurality of switch circuits. 6. The antenna of Claim 5 wherein the number of total output ports between said plurality of switches equals the number of columns in the antenna. 7. The antenna of Claim 6 further comprising phase lines disposed between the switch output ports and the antenna input ports. 8. The antenna of Claim 7wherein each of said antenna elements comprises: an aπay feed circuit layer having a plurality of feed circuits disposed thereon; a radiator layer having a first ground plane disposed thereon with the ground plane having an aperture therein, said radiator layer disposed over said feed circuit layer; and a cover layer disposed over said radiator layer; a plurality of via holes between the first and second ground plane layers to provide a cavity in said radiator and feed circuit layers;

Inventors: BREGLIA CAROLINE, FRENCH THOMAS W, PLEVA JOSEPH S, BREGLIA, CAROLINE, FRENCH, THOMAS, W., PLEVA, JOSEPH, S.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 2001
Main classification: H01Q21
IPC codes: B60K31, B60Q1, B60R21, G01S7, G01S13, H01Q1, H01Q3, H01Q13, H01Q21, H01Q25, H03F1, H03H7, H03H21