Abstract:
A system and method for a center fed reflector feed for a parabolic antenna where the feed is configured to include an output portion that is curved in two directions to thereby enhance the E and H plane patterns of the signal directed rearwardly towards a parabolic reflector.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a system and method for a microwave antenna feed for a parabolic reflector. More specifically, the present invention relates to a system and method for a center fed, parabolic reflector feed with enhanced electric (“E”) and magnetic (“H”) plane patterns. 
     In general, a waveguide is utilized to direct a high-frequency electromagnetic signal rearwardly toward a parabolic reflector for forward reflection. Common reflectors include the two-reflector Cassagrain system in which a horn shaped waveguide directs the signal away forwardly to a sub-reflector from which the signal is directed rearwardly towards the main parabolic reflector for forward reflection. Back-feed, center waveguide systems are also common in which the horn directs the signal rearwardly and directly onto the parabolic reflector for forward reflection. 
     The signal transmission path from the center of the main reflector is the most important region of the path for obtaining a desired radiation pattern. If there are physical obstructions in this transmission path, an undesirable radiation pattern with side lobes may result. Inherent in the design of the Cassagrain antenna is the problem that the energy transmitted from the main parabolic reflector is blocked by the sub-reflector, and in turn substantial side lobes are generally created. Side lobes also may be formed when there is an uncontrolled destructive combination of two waves or improper control of the E and H plane patterns. 
     One known back-feed antenna feed system includes a waveguide which has a cap at the distal end for directing the signal rearwardly towards the parabolic reflector from which the waveguide extends. There is generally less obstruction of the transmission path by the cap as compared with a sub-reflector, but waveguide caps generally do not allow for good control and balancing of the E and H plane patterns. Proper control of the E and H plane patterns to avoid large side lobes requires careful placement of the cap with respect to the end of the waveguide and performance problems generally arise because the structurally required proximity of the cap to the waveguide removes the option of strategically locating the cap to control the E and H plane patterns. 
     Another known back-feed antenna feed system uses a center waveguide bent so as to point toward the parabolic reflector from which it extends. This configuration generally allows for the balancing of E and H plane patterns; however, the obstruction caused by the waveguide geometry generally creates substantial side lobes. 
     It is also known to split a single waveguide of an antenna feed system into two waveguides that collectively direct an electromagnetic signal rearwardly towards the main reflector. A conventional apparatus with such dual output waveguides is disclosed in U.S. Pat. No. 2,824,305 and includes an input waveguide of rectangular shape that connects to a rectangular head which defines two output waveguides that are substantially parallel to the input waveguide and are of substantially rectangular shape. In such systems, the waveguide is located in the center of the parabolic reflector and the head obstructs the transmission path resulting in a radiation pattern with significant side lobes. In addition, there are two distinctly separate beams of electromagnetic energy directed towards the parabolic reflector; the effects of a point source illuminating the parabolic reflector with a single beam of electromagnetic energy cannot be replicated by such a waveguide. 
     Accordingly, it is an object of the present invention to obviate many of the above problems in the known systems and to provide a novel system and method for a center fed reflector feed with enhanced E and H plane patterns. 
     It is another object of the present invention to provide a novel center fed reflector feed apparatus and method with reduced obstruction of the transmitted signal by the waveguide. 
     It is yet another object of the present invention to provide a novel center fed reflector feed apparatus and method that matches the impedance between a single input waveguide and dual output waveguides. 
     It is still another object of the present invention to provide a novel center fed reflector feed apparatus and method that matches the impedance between the waveguide and free space. 
     It is a further object of the present invention to provide a novel dual waveguide center fed reflector feed apparatus and method that generates an improved radiation pattern. 
    
    
     These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiment. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of one embodiment of the center fed reflector feed in longitudinal cross-section showing the tapering of the input waveguide and the choke and tongue components of the output waveguides. 
     FIG. 2 is an enlarged pictorial view of the head of the reflector feed of FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to the drawings, where like numerals represent like components, the proximate end  16  of the input portion  12  of the waveguide  10  receives the electromagnetic signal from a source located behind the parabolic reflector. The distal end of the input portion  12  extends into the head  18  where it is divided into two output portions  14 ,  15 . 
     As shown in FIG. 1, the input portion  12  is tapered over the length thereof so as to reduce the obstruction to the transmission path, the narrowest portion  20  of the waveguide  10  having a cross-sectional area approximately equal to one half of the cross-sectional area of the widest portion of the input portion  12  at the proximate end  16  of the waveguide  10 . Furthermore, by tapering the input portion  12  the distance between the output portions  14 ,  15  is reduced and the dual electromagnetic beams emitted from the output portions  14 ,  15  are sufficiently close together to approximately reproduce the effects of a point source illuminating the parabolic reflector with a single beam of electromagnetic energy. 
     Immediately after the narrowest portion  20 , the input portion  12  of the waveguide is gradually expanded to achieve a cross-sectional area  30  equivalent to the cross-sectional area of the proximate end  16  of the input waveguide  12 . The input portion  12  is divided into two generally U-shaped output portions  14 ,  15 . Expanding the input portion  12  before splitting into two output portions  14 ,  15  effectively matches the impedance between the single input portion  12  and the dual output portions  14 ,  15  reducing the loss of electromagnetic energy and the size of the side lobes within the radiation pattern. 
     FIG. 2 provides an enlarged view of the head  18  of the waveguide  10  where the tapered and expanded portions of the input waveguide  12  are more clearly illustrated. In addition, the U-shape of the output portions  14 ,  15  may be more readily seen. 
     As shown in FIG. 2, suitable conventional chokes  24 ,  25  located intermediate the length of the output portions  14 ,  15  on the outside of the head  18  are provided to improve the E and H plane pattern for the electromagnetic energy directed towards the parabolic reflector. There is often a small amount of energy that is emitted from the output waveguides  14 ,  15  into free space that does not radiate rearwardly towards the parabolic reflector, but radiates forwardly away from the reflector. The chokes  24 ,  25  couple the forwardly radiating energy and re-direct such energy rearwardly towards to the parabolic reflector. When the re-directed signal unites with the original signal radiating towards the parabolic reflector the phases in the E-field of each signal are such that the amplitude of the combined E-field signal is tapered from the center of the parabolic reflector to the edge of the parabolic reflector. In turn, the side lobes of the resultant radiation pattern are improved. 
     With continued reference to FIG. 2, the termination of the output portion includes tongues  22 ,  23  which are smoothly curved in both the E plane and H plane, the E plane curve being approximately one third of the free space wavelength of the transmitted signal. The smooth curves of the tongues  22 ,  23  effectively match the impedance between the output portions  14 ,  15 , respectively, and free space over a very broad band, for example, over a bandwidth that is approximately thirty-five percent of the center frequency of the transmitted signal. For example, if the center frequency of the transmitted signal is 29 GHz, the bandwidth would be approximately 10 GHz (i.e., 29 GHz * 0.35=10.15). Therefore, the bandwidth would include frequencies up to approximately 5 GHz lower than the center frequency and frequencies up to approximately 5 GHz greater than the center frequency. Specifically, the bandwidth would be approximately 24 GHz to 34 GHz for a transmitted signal with 29 GHz center frequency. By effectively matching the impedance of free space, a negligible amount of electromagnetic energy is reflected and the resultant radiation pattern has reduced side lobes. 
     While preferred embodiments of the present invention have been described, it is to be understood that the embodiments described are illustrative only and that the scope of the invention is to be defined solely by the appended claims when accorded a full range of equivalence, many variations and modifications naturally occurring to those of skill in the art from a perusal hereof.