Abstract:
A reflect array antenna comprises a non-electrically conductive substrate with an array of antenna elements supported on the substrate. Each antenna element comprises a plurality of patch radiating elements arranged in rows and columns. Each patch radiating element comprises a plurality of notches formed in the element, the notches being angularly displaced around the circumference of the element. A plurality of stub short transmission lines are individually positioned in each of the plurality of notches and a plurality of switches individually couple one end of a notch to one of the plurality of stub short transmission lines.

Description:
RELATED PATENTS 
     This application is related to U.S. application Ser. No. 09/181,591, entitled Microstrip Phase Shifting Reflect Array Antenna, filed on Oct. 28, 1988, now U.S. Pat. No. 6,020,853. This application is also related to U.S. application Ser. No. 09/181,457, entitled Integrated Microelectromechanical Phase Shifting Reflect Array Antenna, filed on Oct. 28, 1988, now U.S. Pat. No, 6,195,047. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates to reflect array antennas, and more particularly to a microstrip asymmetric-element phase shifting reflect array antenna. 
     BACKGROUND OF THE INVENTION 
     Many radar, electronic warfare and communication systems require a circularly polarized antenna with high gain and low axial ratio. Conventional mechanically scanned reflector antennas are available to meet these specifications. However, such antennas are bulky, difficult to install, and subject to performance degradation in winds. Planar phased arrays may also be employed in these applications. However, these antennas are costly because of the large number of expensive GaAs Monolithic microwave integrated circuit components, including an amplifier and phase shifter at each array element as well as a feed manifold and complex packaging. Furthermore, attempts to feed each microstrip element from a common input/output port becomes impractical due to the high losses incurred in the long microstrip transmission lines, especially in large arrays. 
     Conventional microstrip reflect array antennas use an array of microstrip antennas as collecting and radiating elements. Conventional reflect array antennas use either delay lines of fixed lengths connected to each microstrip element to produce a fixed beam or use an electronic phase shifter connected to each microstrip element to produce an electronically scanning beam. These conventional reflect array antennas are not desirable because the fixed beam reflect arrays suffer from gain ripple over the reflect array operating bandwidth, and the electronically scanned reflect array suffer from high cost and high phase shifter losses. 
     It is also known that a desired phase variation across a circularly polarized array is achievable by mechanically rotating the individual circularly polarized array elements. Miniature mechanical motors or rotators have been used to rotate each array element to the appropriate angular orientation. However, the use of such mechanical rotation devices and the controllers introduce mechanical reliability problems. Further, the manufacturing process of such antennas are labor intensive and costly. 
     In U.S. Pat. No. 4,053,895 entitled “Electronically Scanned Microstrip Antenna Array” issued to Malagisi on Oct. 11, 1977, antennas having at least two pairs of diametrically opposed short circuit shunt switches placed at different angles around the periphery of a microstrip disk is described. The shunt switches connect the periphery of the microstrip disk to a ground reference plane. Phase shifting of the circularly polarized reflect array elements is achieved by varying the angular position of the short-circuit plane created by diametrically opposed pairs of diode shunt switches. This antenna is of limited utility because of the complicated labor intensive manufacturing process required to connect the shunt switches and associated bias network between the microstrip disk and ground, as well as the cost of the circuitry required to control the diodes. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a reflect array antenna providing electronic beam scanning at low cost. The reflect array antenna of the present invention enables an increase in the number of phase states for the reflect array elements, while reducing the number of switches required to provide electronic beam scanning. The reflect array antenna of the present invention provides increased performance for a given frequency, that is, a greater number of discreet phase states for a given number of switches. Alternatively, the described reflect array antenna provides improved performance (number of phase states) at a higher frequency due to the ability to utilize fewer switches and therefore provide phase shift integration. This enables the claimed reflect array antenna to be used as an electronically steered array (ESA) at millimeter wave frequencies for applications requiring low cost, for example, millimeter wave communication apertures, and millimeter wave missile seekers. 
     In accordance with the present invention, there is provided a reflect array antenna comprising a non-electrically conductive substrate with the antenna array supported on the substrate. Each array of the antenna comprises patch antenna elements having a plurality of notches formed in the antenna element, the notches are angularly displaced around the circumference of the element. A plurality of stub short transmission lines are individually positioned in each of the plurality of notches. A plurality of switches are individually coupled to an end of one notch and to one of the plurality of stub short transmission lines. 
     Further in accordance with the present invention, there is provided an antenna element for a reflect array antenna comprising as an element thereof a non-electrically conductive substrate. Supported on the substrate is a patch antenna element having a plurality of notches formed in the element, the notches are angularly displaced around a circumference of the element. A plurality of stub short transmission lines are individually positioned in each of the plurality of notches and a plurality of switches individually couple an end of one notch to one of the plurality of stub short transmission lines. 
     Further in accordance with the present invention, there is provided a circularly polarized reflect array antenna comprising a support base and plurality of antenna. subarrays mounted to the support base. Each antenna subarray comprises a non-electrically conductive substrate with a patch antenna supported on the substrate. Each patch antenna of the array comprises a patch antenna element having a plurality of notches formed in the antenna element, the notches are angularly displaced around the circumference of the element. A plurality of stub short transmission lines are individually positioned in each of the plurality of notches, and a plurality of switches are individually coupled to an end of one notch and to one of the plurality of stub short transmission lines. In addition, the circular polarized reflect array antenna comprises a feed horn coupled to the support base for transmitting or receiving radio frequency energy to a subreflector, the subreflector focusing the radio frequency energy received by the plurality of antenna subarrays to the feed horn. 
     A technical advantage of the present invention is a simplified method for building an electronic scanning reflect array antenna. The advantages of the present invention are achieved by an antenna containing a lattice of circular patch antennas with perimeter stubs connected to the patches by switches. A further advantage of the present invention is a reduction of the number of stub short transmission lines and switches required to control beam steering. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a Cassegrain configured reflect array antenna utilizing monolithic fabrication; 
     FIG. 2 is a perspective view of one of the subarrays of the reflect array antenna of FIG. 1; 
     FIG. 3 is a plan view of a reflect array antenna element with asymmetric inset stubs in accordance with one embodiment of the invention; 
     FIG. 4 is a cross-sectional view of an embodiment of an array element constructed according to the teachings of the present invention; 
     FIG. 5 is a schematic representation of an array element as part of the subarray of FIG. 2 for the reflect array antenna of FIG. 1; 
     FIG. 6 is a schematic representation of the use of  16  segment decoder/driver integrated circuits for control of the diode switches for the patch antenna elements as illustrated in FIG. 5; and 
     FIG. 7 is an alternate embodiment of an asymmetric antenna element for a reflect array antenna as illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A preferred embodiment of the present invention is illustrated in FIGS. 1 through 6 where like reference numerals are used to refer to like and corresponding parts of the various drawings. 
     Referring to FIG. 1, there is illustrated a microstrip phase shifting reflect array antenna  10  in accordance with the present invention. As illustrated, the antenna  10  includes a substantially flat circular disk  12  supporting a plurality of subarrays  14  where each subarray  14  supports a plurality of array elements  16  disposed in a regular and repeating pattern as illustrated in FIG.  2 . The array elements  16  may be etched on the top side of an insulating dielectric sheet, which may be supported and strengthened by a thicker flat panel. For high frequencies, the array elements may be constructed as thin or thick film metallization on a semiconductor substrate. 
     As illustrated in FIG. 1, the subarrays  14  supporting antenna elements  16  are arranged in rows and columns on the disk  12 . A subreflector  18  is located above the disk  12 , either centered (as shown) or offset over the plurality of subarrays  14 . The subreflector  18  is supported from the disk  12  by supports  20 . Energy captured by the subreflector  18  is focused onto a feed horn  22  connected to processing circuitry for the radio frequency energy captured by the antenna elements  16  of the subarrays  14 . 
     Although the antenna  10  is shown on a substantially flat substrate  12 , it will be understood that the invention contemplates substrates that may be curved or formed to some physical contour to meet installation requirements or space limitations. The variation in the substrate plane geometry and the spherical wavefront from the feed and steering of the beam may be corrected by modifying the phase shift state of array elements  16 . Further, the subarrays  14  may be fabricated separately and then assembled on site to increase the portability of the antenna and facilitate its installation and deployment. 
     Referring to FIGS. 2 and 3, the reflect array antenna of FIG. 1 utilizes antenna elements  16  comprising switched microstrip stubs  24  arranged around the perimeter of circular microstrip patch radiating elements  26 . Incident circularly polarized energy is captured by the patch radiating elements  26  and reflected with a phase shift that depends on which stub is electrically short circuited to the patch radiating element. Each circular microstrip patch radiating element  26  has an odd number of microstrip stubs  24  arranged at uniform angular increments around the perimeter of the antenna element. Each of the microstrip stubs  24  are inset into notches  28  extending from the perimeter of the antenna element  26  for impedance matching as will be explained. Electronic switches  30  such as PIN diodes FETS or MEMS are interconnected to a respective microstrip stub  24  by means of bond wires  32 . The requirement of the electronic switches  30  is that when a switch is in the “off” or “on” state, it is a good RF open or short circuit, respectively. 
     As illustrated in FIG. 3, PIN diodes are utilized as the electronic switches  30  and function as the reflect array control elements. The chip diodes shown in FIG. 3 are mounted to the surface of the radiating element  26  typically by means of a conducting adhesive. The top surface of each diode is connected to one of the microstrip stubs  24  by means of the bond wires  32  and to a DC bias connection (not shown in FIG. 3) using bond wires  34 . When a positive voltage is applied to one of the DC bias connections, the respective electronic switch  30  is forward biased, thereby creating an RF short circuit by operation of the electronic switch thereby allowing a current to flow between one of the microstrip stubs  24  and the respective patch radiating element  26 . Thus, the electronic switches  30  control the phase of the reflective energy, for example, with five stubs as illustrated in FIG. 3, relative phase shifts of 0 degrees, 72 degrees, 144 degrees, 216 degrees, and 280 degrees, may be achieved. An alternative fabrication method uses a semiconductor substrate  14  with all of the PIN diodes constructed at once using established semiconductor manufacturing process. This method would make it possible to use the reflect array at millimeter wave frequencies, where the small dimensions of the patches and stubs would make individually-placed and wire-bonded diodes impractical. 
     A feature of the present invention is the use of asymmetric inset microstrip stubs  24 . As previously mentioned, the stubs are inset into the perimeter of the radiating element  26  for impedance matching since the stubs  24  serve as short transmission line sections. For best operation, the microstrip stubs  24  are impedance matched to the patch radiating element  26  at the connection points of the electronic switches  30 . Typically, the input impedance of a circular patch radiating element  26  is 300 to 500 Ohms at the perimeter, while the microstrip stubs typically have a 100 Ohm characteristic impedance. The insets place the attachment points inside the patch perimeter, where its input impedance is nearer to 100 Ohms. 
     Referring to FIGS. 3 and 4, an individual antenna element  16  comprises a metallic disk member  26 , a metallic ground plane member  36 , and dielectric medium  38  and  39  functioning as insulating layers (the RF substrate  38  and DC substrate  39 ). Also comprising each of the radiating elements  26  is a DC bias connection metallized conductor  40  on the bottom side of the insulating layer  39 . As illustrated in FIG. 4, the two dielectric medium substrates  38  and  39  are isolated from each other by means of the ground plane  36  which comprises metallization on either the bottom of the RF substrate  38  or the top of the DC substrate  39 . A DC bias connection from the conductor  40  to the bond wires  34  are by means of vias  42  passing through small holes in the ground plane  36 . Also metallized on the top surface of the RF substrate  38  are the microstrip stubs  24 . 
     The antenna elements  16  either singly or in an array are fabricated by etching a printed circuit board or semiconductor substrate using conventional microcircuit techniques. The center of each circular radiating element  26  is short-circuited to the ground plane  36  by an RF ground via  44 . As illustrated in FIG. 4, the electronic switch  30  is bonded to the radiating element  26  and connected to the microstrip stub  24  by means of a bond wire  32  and to the via  42  by means of the bond wire  34 . 
     Also as illustrated in FIG. 4 is a DC control circuit  46  on the DC substrate  39  and connected to the DC bias connector  40 . The function of the DC control circuit  46  is to demultiplex beam steering controls that are distributed to the reflect array antenna elements  16  by a bus (parallel conductors) as will be described. The second function of the control circuit  46  is to generate an output to drive the electronic switch  30  thereby providing the current required to turn the electronic switches “off” or “on”. Typically, the DC control circuit  46  is a conventional decoder and diode driver such as those extensively used in digital displays. 
     Referring to FIG. 5, there are five dimensions that must be considered in the fabrication of a reflect array element  16 . The five dimensions vary with four parameters of the reflect array element. The four parameters are the operating frequency (f) and associated wavelength (λ); the permittivity of the supporting dielectric substrate (ε τ ); and the thickness of the substrate (h). 
     The resonant frequency of a microstrip patch antenna element with radius “a”, is approximately given by the following equation:              f   =       ck   11       2                 π                   a   eff            ɛ   r                   (   1   )                                
     where a eff  is the effective radius, given by                a   eff     =     a          {     1   +         2      h       π                 a                   ɛ   r              [       ln        (       π                 a       2      h       )       +   1.7726     ]         }       1   /   2                 (   2   )                                
     and k  11 =1.841 (the first zero of the derivative of the Bessel function J 1 ). The constant k 11  is selected in place of the more general K mm  because the circular patch antenna element  16  is intended to function as a cavity resonator in the TM 11  mode. To ensure that other modes are not excited, a via  44  will be placed at the center of the patch, shorting it to the ground plane. 
     The stub width (W s ), FIG. 5, is selected to yield a characteristic impedance between 50 and 150 Ohms. This selection depends on the substrate material and the resulting sensitivity of impedance to the line width (some choices may result in excessively wide or narrow lines). The following approximate formula gives the characteristic impedance (Z 0 ) in terms of an effective relative permittivity ε eff :                Z   0     ≈     {               60       ɛ   eff            ln                   (         8      h     w     +     w     4      h         )                   for                   w   /   h       ≤   1                         120                 π         ɛ   eff              [       w   h     +   1.393   +     .667                   ln        (       w   h     +   1.444     )           ]         -   1                     for                   w   /   h       ≥   1                     (   3   )                                
     and the effective relative permittivity is                ɛ   eff     =           ɛ   r     +   1     2     +           ɛ   r     -   1     2            (     1   +     12        h   w         )         -   1     /   2                   (   4   )                                
     Next, the stub length (L s )is chosen to be approximately one quarter wavelength, to provide a two-way path length of λ/2. However, the length must account for the fact that an open-ended microstrip line is electrically longer than its physical length due to field fringing at the open end. An approximate formula for the length extension due to fringing is:                Δ                 L     =     .412                   h        (         ɛ   eff     +   .3         ɛ   eff     -   .258       )            (         w   /   h     +   .264         w   /   h     +   .8       )               (   5   )                                
     The stub length also includes the length of the switch itself, as indicated by the shaded areas in FIG.  5 . 
     The input impedance of a circular microstrip patch varies from zero at the center to 250 Ohms or more at the edge. The depth of the inset notch  28  (a-r s ) is chosen such that the input impedance of the radiating element  26  at the radius r s  is equal to the characteristic impedance of the microstrip stub  24 . For a characteristic impedance of 50 Ohms and 100 Ohms, r s  will be approximately a/3 and a/2, respectively. 
     Last, the gap width (w g ) of the notch  28  is chosen to be wide enough so as not significantly change the characteristic impedance of the microstrip stub  24 . For example, if the gap width (w g ) is only slightly wider than the microstrip stub, then the inset portion of the stub will essentially be a coplanar waveguide instead of a microstrip. The result would be a characteristic impedance of the inset portion that will be different from that of the portion of the microstrip stub outside the perimeter of the radiating element  26 . A rule of thumb is that w g  should be greater than or equal to the substrate thickness (h). 
     Referring to FIG. 6, reflect array antenna beam steering involves two considerations, the electronic switches  30 , and the control of the switching elements. The switches  30  are controlled by the circuit of FIG. 6 that activates the individual switches in essentially the same operation as for memory or display bits. The address and data bus provides switching commands to multiple decoder/driver circuits mounted on a control circuit layer beneath the reflect array. FIG. 6 is an example of the use of  16 -segment decoder/driver integrated circuits  50  and  52 . The decoder/driver chip  50  is interconnected to three reflect array antenna elements  16 . The decoder/driver chip  52  also interconnects to three reflect array antenna elements  16 . The control circuit of FIG. 6 is repeated with additional decoder/driver chips sequentially connected by the address lines  56  and data lines  54  until all of the reflect array antenna elements  16  of the antenna  10  are connected to a decoder/driver chip. The address and data lines originate at the parallel output port of a computer. 
     In operation, varying the phase shift at each array antenna element  16  is achieved by operating the electronic switches  30  from the control circuit of FIG.  6 . Only one of the electronic switches  30  for each antenna element  16  is “on”, that is, connecting a microstrip stub  24  to ground at any instant of time. Phase shifting of the circularly polarized reflect array antenna elements  16  is achieved by varying the angular position of the short-circuited plane created by switching between different electronic switches  30 . Operating in this manner, array antenna elements  16  collectively form a circularly polarized antenna. 
     Referring to FIG. 7, there is shown another embodiment of an array antenna element for use with a reflect array antenna as illustrated in FIG.  1 . As illustrated in FIG. 7, the bond wires  34  connected by means of the via  42  to the DC bias connector  40  are at the ends of the stubs  24 . The bond wire  34  is also attached to an electronic switch  30  fabricated to the end of the microstrip stub  24 . Each of the microstrip stubs  24  are permanently joined to the radiating element  26  at the base of the notch  28 . Again, the radiating element  26  couples to the ground plane  36  (FIG. 4) by means of the via  44 . 
     In operation of the embodiment of FIG. 7, the electronic switches  30  create either an open circuit or a short circuit boundary condition at the end of a microstrip stub  24 , depending on whether the switch is in the “off” or “on” state, respectively. In accordance with this embodiment the electronic switches  30  and the DC control vias  42  are outside the perimeter of the radiating element  26 , and therefore less likely to alter the RF performance of the antenna element. 
     Although several embodiments of the present invention and the advantages thereof have been described in detail, it should be understood that changes, substitutions, transformations, modifications, variations, and alterations may be made without departing from the teachings of the present invention, or the spirit and scope of the invention as set forth in the appended claims.