Abstract:
A planar polarizer feed network comprising a six port branch coupler having two input ports and four output ports. The output ports are designed to have the same amplitude while their phases are sequentially offset by 90 degrees when fed from a first input port or by minus 90 degrees when fed from a second input port. In one embodiment, each output port is coupled to an aperture coupled antenna array comprising four slots and four patch antenna elements. In this arrangement, an RF signal may be coupled to each of the two input ports to couple properly phased signals to each of the antenna elements to simultaneously form both right-hand and left-hand circularly polarized signal emitted from the planar array of antenna elements.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U. S. provisional patent application serial No. 60/200,069, filed Apr. 27, 2000, which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to circularly polarized antenna arrays and, more particularly, to feed networks for circularly polarized antenna arrays. 
     2. Description of the Related Art 
     Circularly polarized planar antennas have been widely used for various applications such as a phased array antennas, mobile antennas, and for satellite antennas. In many cases, the antennas are required to support simultaneous dual polarization, where a sequential signal rotation and phase shift technique has proven to provide wide band circular polarization and low VSWR characteristics. Such dual polarization is used in direct broadcast satellite television systems to enable a single antenna to be used to simultaneously receive multiple channels. 
     More particularly, circular polarization in planar antenna arrays is accomplished by the system having a plurality of “patch” antennas where a linearly polarized signal is coupled to each of the antenna elements. The signal is applied to the elements in a sequentially switched pattern to achieve circular polarization in either right-hand or left-hand form. However, such switched systems require sophisticated electronics and a substantial amount of microstrip or stripline circuitry to couple the RF signals to the antenna elements. Such circuit complexity results in substantial crosstalk between antenna elements and distortion of the antenna pattern. 
     Therefore, there is a need in the art for a simple feed network for a dual circular polarized antenna array. 
     SUMMARY OF THE INVENTION 
     The present invention is a planar polarizer feed network comprising a six port network having two input ports and four output ports. The output ports are designed to have the same amplitude while their phases are sequentially offset by 90 degrees when fed from a first input port or by minus 90 degrees when fed from a second input port. In one embodiment of the invention, each output port is coupled to an aperture coupled antenna element comprising a slot and a patch antenna element. In this arrangement, an RF signal may be coupled to each of the two input ports to couple properly phased signals to each of the four antenna elements to simultaneously form both right-hand and left-hand polarized signal emitted from a planar array of antenna elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     FIG. 1 depicts a top plan view of a six port planar feed network of the present invention; 
     FIG. 2 depicts a top plan view of a crossed aperture array for an antenna array incorporating the feed network of FIG. 1; 
     FIG. 3 depicts top plan view of a four antenna element array for an antenna array incorporating the feed network of FIG.  1  and the aperture array of FIG. 2; and 
     FIG. 4 depicts a cross sectional view taken along lines  4 — 4  of the antenna system depicted in FIGS. 1,  2  and  3 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention is a planar polarizer feed network for a dual circular polarized antenna array system. The planar polarized feed network distributes an RF signal to an array of four antenna elements such that both a right-hand and a left-hand polarized signal can be transmitted from the antenna system or received by the antenna system. 
     FIG. 1 depicts the top plan view of a six port, planar polarizer feed network  100  of the present invention. The feed network  100  is comprised of six ports: two input ports  104  and  106  and four output ports  108 ,  110 ,  112  and  114 . The feed network  100  is formed as a microstrip circuit (stripline may also be used). When driving the feed network  100  from input port  104  with an RF signal, output port  110  will generate a signal that is in-phase with the input signal the output port  108  will generate a signal that is 90 degrees out of phase with the input signal, output port  114  will generate a signal that is 180 degrees out of phase with the input signal, and output port  112  will generate a signal that is 270 degrees out of phase with the input signal. Similarly, when driving the network  100  through input port  106 , the feed network  100  produces a signal at port  108  that is in-phase with the input signal, output port  110  generates a signal that is 90 degrees out of phase with the input signal, output port  112  generates a signal that is 180 degrees out of phase with the input signal and output port  114  generates a signal that is 270 degrees out of phase with the input signal. As discussed below, both input ports may be driven simultaneously. 
     The feed network  100  comprises a pair of branch line couplers  102 A and  102 B that are connected together. The first branch line coupler  102 A is formed of a trunk line  116  that is connected to a distribution line  118  by a pair of branch lines  112 A and  112 B. Similarly, the second branch line coupler  102 B is formed of a trunk line  120  coupled to a second distribution line  122  by a pair of branch lines  114 A and  114 B. The ends of each trunk line are connected to one another by cross lines  124  and  126 . The input port  106  is connected to cross line  124  and input port  104  is connected to cross line  126 . The positioning of the branch lines  114  and  112  off of the trunk lines  116  and  120  are defined by the frequency and bandwidth necessary for the particular network being designed. The design of branch line couplers having phase shifted output signals is well known in the art. 
     The output ports  108 ,  110 ,  112  and  114  of network  100  may be coupled to antenna elements in one of many different ways that are well known in the art. In one specific embodiment of the invention, the output ports are coupled through apertures to square planar antenna elements. FIGS. 2,  3  and  4  depict a specific embodiment of a planar antenna array system using the feed network of FIG.  1 . 
     Specifically, FIG. 2 depicts a top plan field of a cross aperture array layer of the antenna array system, FIG. 3 depicts the top plan view of an antenna element array for the antenna array system, and FIG. 4 depicts a cross sectional view of the antenna array system. To best understand the invention the reader should simultaneously refer to FIGS. 1,  2 ,  3  and  4  while reading the following description of the antenna array system. 
     The antenna array system  400  is comprised of three dielectric layers  410 ,  402  and  304  (respectively, first, second and third dielectric layers) and three metallization layers that form the feed network  100 , the array of apertures  200  and the array of patch antenna elements  300 . The feed network  100 , including output port  112 , is formed on one surface  404  of a dielectric layer  410 . The feed network  100  is formed using conventional microstrip techniques on surface  404  of dielectric layer  410 . For example, the dielectric may be fabricated of RT-Duroid having a dielectric constant of approximately 2.2 or higher. 
     An array of cross apertures (e.g., four apertures  202 A,  202 B,  202 C and  202 D) are formed in a metal layer on surface  406  of dielectric layer  410 . Each output port of the feed network  100  is coupled to a different arm of the cross apertures. The coupling is accomplished by having the output port microstrip  112  underlie the aperture arm  204 B such that energy at the output port  112  is coupled through the aperture  202 A. 
     A dielectric  402  is formed atop the aperture layer  212 . This dielectric layer  402  may be a volume that is filled with air. Other materials having a dielectric constant of approximately 1, such as foam, can be used. Antenna elements  302 A,  302 B,  302 C and  302 D are square patches of metallization that are formed on surface  408  of dielectric layer  304 . These antenna elements  302  are formed above each of the cross coupled apertures  202 A,  202 B,  202 C and  202 D. Energy from the output ports  108 ,  110 ,  112  and  114  of the feed network  100  is coupled through the apertures  202 A,  202 B,  202 C and  202 D to each of the antenna elements  302 A,  302 B,  302 C and  302 D. The dielectric layer  304  and the antenna elements  302  are either supported above dielectric layer  410  to form an air gap  402  or formed atop of a dielectric layer  402 . The dielectric layer  304  forms an optional radome for the antenna system  400  protecting the underlying antenna components from the environmental elements. In one embodiment of the invention, the dielectric layer  304  has a dielectric constant of approximately 2.2 or higher and is fabricated of a material such a DT-Duroid or fiberglass (such as FR-4). 
     The six port planar feed network  100  is fabricated and independently tested to ensure that the output ports  108 ,  110 ,  112 ,  114  have equal amplitude output signals, and the required sequential phase distribution occurs. Phase errors can significantly degrade the axial ratio performance of the network  100 , for example, a 10-degree error can cause an axial ratio of greater than 1.5 dB. The axial ratio provided by the following formula: 
     
       
           AR ( dB )={square root over ( A   e   2 +0.02250+L φ e   2 +L ,)} 
       
     
     where A e  is the amplitude error in dB and φ e  is the phase error in degrees. 
     In one specific embodiment of the invention, the spacing of the square antenna elements is generally 0.55 λ 0  where λ 0  is the drive or received frequency for the antenna system. One particular array comprises a first dielectric layer  410  having a dielectric constant of 2.22 and thickness of 20 mils, having air as the second dielectric  402  having a thickness of 60 mils and a third dielectric  304  having a dielectric constant of 2.22 and a 20 mil thickness. The invention provides more than 18 dB return loss over a 500 MHz bandwidth and better than 20 dB isolation. The measure of radiation pattern provides less than 1.5 dB axial ratio over a 500 MHz bandwidth centered at 12.5 GHz. The measured gain of the 2×2-patch antenna system was 10.5 to 11 dB over a 500 MHz bandwidth. By driving both input ports of the feed network simultaneously forming both right-hand and left-hand circularly polarized signals. 
     Although the depicted embodiment of the invention shows the patch antenna element being at the interface of the dielectric layer  304  and the dielectric layer  402 , an alternative embodiment could have the patch antenna element positioned atop the dielectric layer  304 , or above the dielectric layer  402  and not use the radome (i.e., dielectric layer  304 ). 
     Also, in another embodiment, additional patch antenna elements can be stacked atop the patch antenna elements  302 . As such, at each location for a patch antenna element, one element is located on one side of dielectric layer  304  and another element is located on the other side of the dielectric layer  304 . Such an element  450  is shown in phantom in FIG.  4 . The dielectric layer  304  maintains the elements  302  and  450  in a parallel, spaced apart relationship. To adjust bandwidth and beam width parameters, the size of the upper patch element  450  may be different from the lower patch element  302 , and the spacing between the elements can be adjusted. Such sizing and spacing parameters vary from application to application for the antenna. Furthermore, to adjust the coupling parameters between the stacked elements  302  and  450 , the lower patch element  302  may contain a slot or other form of aperture (not shown). 
     The foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.