Planar polarizer feed network for a dual circular polarized antenna array

[0002] 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.

[0003] 1. Field of the Invention

[0004] The present invention generally relates to circularly polarized antenna arrays and, more particularly, to feed networks for circularly polarized antenna arrays.

[0005] 2. Description of the Related Art

[0006] 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.

[0007] 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.

[0008] Therefore, there is a need in the art for a simple feed network for a dual circular polarized antenna array.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] FIG. 1 depicts a top plan view of a six port planar feed network of the present invention;

[0013] FIG. 2 depicts a top plan view of a crossed aperture array for an antenna array incorporating the feed network of FIG. 1;

[0014] 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

[0015] FIG. 4 depicts a cross sectional view taken along lines 4—4 of the antenna system depicted in FIGS. 1, 2 and 3.

[0016] 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.

[0017] 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.

[0018] The feed network 100 comprises a pair of branch line couplers 102A and 102B that are connected together. The first branch line coupler 102A is formed of a trunk line 116 that is connected to a distribution line 118 by a pair of branch lines 112A and 112B. Similarly, the second branch line coupler 102B is formed of a trunk line 120 coupled to a second distribution line 122 by a pair of branch lines 114A and 114B. 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] An array of cross apertures (e.g., four apertures 202A, 202B, 202C and 202D) 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 204B such that energy at the output port 112 is coupled through the aperture 202A.

[0023] 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 302A, 302B, 302C and 302D 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 202A, 202B, 202C and 202D. Energy from the output ports 108, 110, 112 and 114 of the feed network 100 is coupled through the apertures 202A, 202B, 202C and 202D to each of the antenna elements 302A, 302B, 302C and 302D. 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).

[0024] 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:

[0025] AR(dB)={square root over (A_e²+0.02250+L φ_e²+L ,)}

[0026] where A_eis the amplitude error in dB and φ_eis the phase error in degrees.

[0027] In one specific embodiment of the invention, the spacing of the square antenna elements is generally 0.55 λ₀where λ₀is 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.

[0028] 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).

[0029] 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).

[0030] 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.