Abstract:
A technique for reducing cross-polarized signals in the axis of four-port al-polarized antennas. The resulting lower cross-polarized signals reduce a tracking antenna&#39;s susceptibility to tracking errors and to breaklocks. Some energy from the transmission line connected to the (V) port is coupled, after proper attenuation and phase adjustment, and transmitted through the (H) port, thereby nulling out the (V) cross-polarized pattern.

Description:
RIGHTS OF THE GOVERNMENT 
     The invention described herein may be manufactured, used, and licensed by or for the U.S. Government for Governmental purposes without the payment of royalty to me of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to dual-polarized tracking antennas, and more particularly to reducing cross-polarized signals in the vicinity of such antennas. 
     Dual-polarized tracking antennas have four difference ports (Δ)-one pair for each polarization. FIG. 1 illustrates in schematic form a dual-polarized monopulsetype tracking antenna in which the polarizations are horizontal (H) and vertical (V). While any pair of orthogonal polarization may be used, the six ports in this example are: 
     Σ(H)-sum (horizontally Polarized) 
     Δ a  (H)-azimuth difference (horizontally polarized) 
     Δ a  (H)-elevation difference (horizontally polarized) 
     Σ(V)-sum (vertically polarized) 
     Δ a  (V)-azimuth difference (vertically polarized) 
     Δ e  (V)-elevation difference (vertically polarized) 
      When a single polarization is used, a set of three ports is chosen, either the horizontal or vertical. 
     Radiation patterns when these ports are energized, if, for example, the polarizations are horizontal and vertical, are illustrated in FIGS. 2(a)-2(d). The solid patterns represent the desired co-polarized radiation and the dashed patterns represent the undesired cross-polarized radiation. The cross-polarized lobes usually are about 10dB to 20dB lower than the co-polarized lobes for a conventional antenna. The cross-polarization results in the antenna&#39;s susceptibility to tracking errors and to breaklocks. 
     Prior art techniques to reduce the cross-polarized radiation in the vicinity of the axis of a dual-polarized tracking antenna involved choosing antenna and feed geometries, but satisfactory performance could not always be achieved and some degradation of performance had to be accepted. Nulling techniques for communication systems use a single dual-polarized antenna beam to achieve their nulls and hence they are effective over only a narrow angular region of the beam and these techniques are not applicable to cross-polarization in the difference patterns of tracking antennas. 
     Variable-ratio microwave power dividers, utilizing two spaced-apart 90 degree hybrids directionally coupling two microwave channels and an adjustible phase shifting device disposed between the two 90 degree hybrids, have been known in the art for many years. For example, such a variable power divider is described in a paper entitled &#34;A Variable-ratio Microwave Power Divider and Multiplexer&#34;, by W. L. Teeter and K. R. Bushore, IRE Transactions on Microwave Theory and Techniques, October, 1957, pages 227-229. In this power divider, an incoming microwave signal may be directed solely to either of two outputs or may be divided in any disired ratio between the two outputs by adjustment of the phase shifting device. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to reduce cross-polarized signals in the vicinity of the axis of dual-polarized tracking antennas. 
     Another object of the present invention is to reduce tracking errors and breaklocks in tracking antennas. 
     Still another object of the present invention is to provide a technique to reduce cross-polarization in the difference patterns of tracking antennas. 
     These and other objects of the present invention are provided by applying energy from the cross-polarized orthogonal-axis difference pattern to reduce crosspolarized signals in the desired difference pattern over a relatively large angular region around the tracking axis. Directional couplers are provided between the difference patterns and the signals appropriately phase shifted. 
     Other objects and further aspects of the present invention will become apparent during the course of the following description and by reference to the accompanying drawings wherein like numerals represent like parts throughout the several views, wherein 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a dual-polarized monopulse-type tracking antenna and the six ports feeding the antenna; 
     FIGS. 2(a)-2(d) illustrate in schematic form the far-field patterns from the four difference (Δ) ports as viewed along the (Σ) sum beam axis; 
     FIG. 3 is a schematic diagram of the nulling circuit according to one embodiment of this invention; 
     FIGS. 4 and 5 are schematic diagrams of nulling circuits according to two other embodiments of this invention; and 
     FIG. 6 is a schematic diagram illustrating the nulling circuit of FIG. 3 in a dual-polarized monopulse-type tracking antenna. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     To understand how the nulling technique works, suppose that the transmitted signal is on the vertically polarized radiation through the Δ e  (V) port. It is desired to transmit only on the co-polarized pattern (solid in FIG. 2(b)), and not on the cross-polarized pattern (dashed in FIG. 2 (b)). 
     The co-polarized pattern of the Δ a  (H) port is the same as the cross-polarized pattern of the Δ e  (V) port. If some energy from the transmission line connected to the Δ e  (V) port, properly adjusted for amplitude and phase, is transmitted through the Δ a  (H) port, the cross-polarized pattern of Δ e  (V) can be nulled out. In actual practice, not all of the Δ e  (V) cross-polarized pattern can be nulled out everywhere, because the two patterns under consideration are not identical. However, the cross-polarized pattern in the region of interest, namely around the tracking axis, is thoroughly nulled out. Thus, if one desires to receive on a vertically polarized elevation difference pattern, he would use signals from the horizontally polarized azimuth difference pattern to destructively interfere with the cross-polarized signals in the desired pattern. 
     Referring now to FIG. 3, there is illustrated a schematic diagram of the nulling circuit with the Δ e  (V) and Δ a  (H) ports coupled. 
     A coupling circuit disposed in the Δ e  (V) and Δ a  (H) transmission lines includes two spaced-apart 90 degree hybrid directional couplers 10 and two phase shifters 14 respectively disposed in the two transmission lines between the two 90 degree hybrids 10. The two phase shifters 14 of the coupling circuit have settings of -α and +α, respectively. FIG. 3 also includes two additional phase shifters 14, one disposed in the Δ a  (H) transmission line ahead of the coupling circuit and having a setting of -β, and the other disposed in the Δ a  (H) transmission line behind the coupling circuit and having a setting of +β. In practice, α determines the magnitude of the coupling, and β determines the relative phase between the Δ e  (V) and Δ a  (H) ports. The circuit of FIG. 3 is designed so that the main signal phase shift is a constant value, independent of the α and β settings. 
     The embodiment of FIG. 4 can be used when a constant phase shift is not required in the main channels. FIG. 4 includes a coupling circuit disposed in the Δ e  (V) and Δ a  (H) transmission lines which consists of two spaced-apart 90 degree hybid directional couplers 10 and an α phase shifter 14 disposed in one of the transmission lines between the two 90 degree hybrids 10. FIG. 4 also includes a β phase shifter 14 disposed in the Δ a  (H) line after the coupling circuit. 
     The embodiment of FIG. 5, which can be used when insertion loss is not critical, includes four 90 degree hybrid directional couplers 10, two attenuators 12, and two phase shifters 14. FIG. 5 includes two coupling circuits for directionally coupling the two transmission lines Δ e  (V) and Δ a  (H) in respective opposite directions. Each coupling circuit includes a 90 degree hybrid 10 coupled to the Δ e  (V) line which is connected in series through an attenuator 12 and phase shifter 14 to a 90 degree hybrid coupled to the Δ a  (H) line. 
     When the nulling circuit of FIG. 3, 4, or 5 is used in a dual-polarized monopulse-type tracking antenna, the ports are coupled as pairs, Δ e  (V) with Δ a  (H), and Δ e  (H) with Δ a  (V), as shown in FIG. 6 which utilizes two of the nulling circuits of FIG. 3. 
     Typical elements for the various embodiments described herein that may be employed as follows: 
     Attentuator 12-- Hewlett Packard Model G382A 
     Phase Shifter 14-- ARRA Inc., Bayshore, N.Y. Model No. 9428A-28 
     Directional Coupler 10--ARRA Model No. 5164-90 
     There has therefore been described a technique for greatly reducing cross-polarized radiation in the vicinity of the axis of a dual-polarized tracking antenna. This greatly reduces or eliminates tracking errors and breaklocks due to cross-polarized signals. 
     It is to be understood that the above embodiments are simply illustrative of the invention. Various other modifications and changes may be made by those skilled in the art which will employ the principles of the invention and fall within the spirit and scope thereof.