Patent Publication Number: US-6215452-B1

Title: Compact front-fed dual reflector antenna system for providing adjacent, high gain antenna beams

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to three commonly assigned applications entitled: “A Compact Folded Optics Antenna System For Providing Adjacent, High Gain Antenna Beams”, U.S. patent application Ser. No. 09/232,454, having inventors Romulo F. Jimenez Broas, David L. Brundrett, Charles W. Chandler and Te-Kao Wu; “A Compact Side-Fed Dual Reflector System For Providing Adjacent, High Gain Antenna Beams”, U.S. patent application Ser. No. 09/232,452, having inventors Ann L. Peebles, Charles W. Chandler and Louis C. Wilson; and, “A Compact Offset Gregorian Antenna System For Providing Adjacent, High Gain Antenna Beams”, U.S. patent application Ser. No. 09/232,450, having inventors Charles W. Chandler, Gregory P. Junker and Ann L. Peebles; filed on the same date as this application. These applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to antennas for satellites and more particularly, to a reflector antenna system for a satellite which provides a plurality of antenna beams for full Earth field-of-view coverage from a geosynchronous orbit with each antenna beam having approximately equivalent beam characteristics and being substantially symmetrically shaped. 
     Communications satellites in a geosynchronous orbit require high gain antennas for uplink and downlink communications with the Earth. A satellite uplink communications signal is transmitted to a satellite from one or more ground stations located on the Earth; and, a satellite downlink communications signal is transmitted from a satellite to one or more ground stations located on the Earth. The uplink and downlink signals are received and transmitted respectively at particular frequency bands which are typically in the ratio of about 3:2 (uplink frequency band: downlink frequency band) for Ka band. The signals are also typically coded. A satellite is equipped with antennas or antenna systems to receive and transmit the uplink and downlink signals respectively. To minimize the number of satellites in a constellation and maximize communications capabilities, it is desirable for each satellite to have the capability to communicate with the locations on the Earth within the satellite&#39;s field of view and to do so with high gain antenna beams. 
     FIG. 1 shows a simplified plan view of one antenna  10  used for high gain communications from satellites. This antenna  10  was detailed in the article Jorgensen, Rolf, et. al., “A Dual Offset Reflector Multibeam Antenna for International Communications Satellite Applications”, IEEE Transactions on Antennas and Propagation, Vol. AP-33, No. 12, December 1985. The antenna  10  is a front-fed dual reflector antenna having a main reflector  11 , a subreflector  12  and a feed array  13 . The feed array  13  consists of multiple feed horns with each feed horn generating an illumination beam  14  which is reflected from the subreflector  12  and main reflector  11  and directed toward a defined coverage cell on the Earth. The disadvantage with this antenna  10  is that it does not provide symmetrically shaped beams at wide scan angles. 
     The antenna  10  disclosed above has the additional disadvantage that it cannot provide high gain, adjacently located antenna beams. The above antenna  10  provides a single beam from each feed horn in the feed array  13 . To provide high gain beams, the main reflector  11  must be efficiently illuminated. To do so requires large feed horns, with the location of each feed horn determining the location of a corresponding beam on the Earth. To provide beams which are adjacently located and completely cover the Earth&#39;s field-of-view requires that all the feeds in the feed horn array  13  be physically positioned close together. If the feeds are not physically close together, the corresponding antenna beams will not be adjacently located and will be spaced too far apart on the Earth, with locations between antenna beams having no coverage. Large feed horns typically cannot be physically spaced close enough together within the antenna  10  to produce adjacent beams on the Earth. The above referenced antenna attempts to address this problem by using feed horns which are physically small so that the feed horns can be physically spaced close together. These smaller feed horns can produce adjacent beams but do not efficiently illuminate the reflectors  12 ,  11  resulting in high spillover losses and lower gain beams. 
     What is needed therefore is an efficient antenna system that provides a plurality of high gain, adjacent located antenna beams which cover the entire Earth field-of-view. 
     SUMMARY OF THE INVENTION 
     The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides an antenna system for use on a spacecraft. In a first aspect, the antenna system comprises a feed array, a subreflector and a main reflector which are oriented to define a front-fed dual reflector geometry where the feed array is in front of the subreflector. 
     The feed array is comprised of a plurality of separate feeds which are aligned along a predetermined contour. Each feed is coupled to a feed network which acts to combine the illumination beams of clusters of a preselected number of feeds to produce a plurality of composite illumination beams each of which has a central ray. The central ray of each composite illumination beam is directed to be incident upon a separate preselected location on the subreflector. The subreflector is configured to receive each composite illumination beam at the preselected location and direct the central ray of each composite illumination beam towards the main reflector. 
     The main reflector is positioned to receive each composite illumination beam from the subreflector and direct each composite illumination beam in a preselected direction so that each composite illumination beam forms an antenna beam that impinges a predetermined coverage area on the Earth. Each antenna beam defines a separate coverage cell within a preselected coverage area. 
     In a second aspect, the position and orientation of the feeds, the subreflector and the main reflector provides adjacent antenna beams over a full Earth field of view coverage area where each antenna beam is approximately symmetrically shaped. 
     In a third aspect, the antenna system comprises a plurality of subreflector and main reflector combinations and a feed array associated with each subreflector and main reflector combination. Each subreflector and main reflector combination and associated feed array is oriented to define a separate front-fed dual reflector antenna geometry with each subreflector and main reflector combination and associated feed array together comprising a single front-fed dual reflector antenna. 
     Each feed array generates a plurality of illumination beams which form a plurality of associated antenna beams therefrom. The antenna beams from all the front-fed dual reflector antennas within the antenna system are interleaved. Each antenna beam defines a separate coverage cell in a coverage area with the coverage cells being arranged so that no coverage cell defined by an antenna beam associated with one fronted dual reflector antenna is contiguous with another coverage cell defined by another antenna beam associated with the same front-fed dual reflector antenna. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Reference is now made to the detailed description of the preferred embodiments illustrated in the accompanying drawings, in which: 
     FIG. 1 is a prior art front-fed dual reflector antenna; 
     FIG. 2 is an isometric drawing showing a satellite having a front-fed dual reflector antenna system in accordance with the present invention; 
     FIG. 3 is a side plane view of a front-fed dual reflector antenna system in accordance with a first embodiment of the invention; 
     FIG. 4 is a schematic drawing of a feed network used in the front-fed dual reflector antenna system shown in FIG. 3; 
     FIG. 5 is an illustration of a feed array used in the front-fed dual reflector antenna system shown in FIG.  3  and antenna beams generated therefrom; 
     FIG. 6 is a front plane view showing a satellite having a plurality of front-fed dual reflector antennas which together form a front-fed dual reflector antenna system in accordance with a second embodiment of the invention; and, 
     FIG. 7 is a depiction of antenna beams on the Earth provided by the front-fed dual reflector antenna system shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, a spacecraft  17  having an antenna system  18  for providing adjacent high gain antenna beams  19  on Earth  20  is illustrated. The antenna system  18  of the present invention is used for communications between the spacecraft  17  and the Earth  20  where the spacecraft  17  is preferably located in a geosynchronous or near geosynchronous orbit. The antenna system  18  provides symmetrically shaped adjacent antenna beams  19  on the Earth  20  from a single spacecraft  17 . 
     Referring to FIG. 3, for a first embodiment of the invention, the antenna system  21  is comprised of a main reflector  25 , a subreflector  27  and a feed array  22  configured in a front-fed dual reflector antenna configuration so that the illumination beams, depicted by the lines marked  23 , provided by the feed array  22  are reflected towards Earth from the main reflector  25  in a compact manner which is substantially or totally free of blockage by the subreflector  27  or feed array  22 . A more detailed discussion of front-fed dual reflector antenna configurations can be found in the article Jorgenson et. al. “Development of dual reflector multibeam spacecraft antenna system,” IEEE Transactions of Antennas and Propagation, vol. AP-32, pp. 30-35, 1984. The location of the feed array  22  in front of the subreflector  27  define the antenna system  21  as being “front-fed”. For the preferred embodiment of the invention, the subreflector  27  is a portion of a hyperbola which has a concave side  24  with an associated focal point  26  and a convex side  28 . The main reflector  25  is a portion of a parabola having a main reflector focal point  30 . The subreflector  27  and main reflector  25  are positioned so that the focal point  30  of the main reflector  25  is approximately coincident with the focal point  26  associated with the concave side  24  of the subreflector  27 . The feed array  22  is placed in the proximity of the focal point  26  associated with the concave side  24  of the subreflector  27  with the exact location of each feed in the array  22  being determined as detailed below. The antenna system  21  is configured so that the illumination beams  23  are incident on the concave side  24  of the subreflector  27 , redirected towards the main reflector  25 , and, directed towards the Earth free of blockage by the subreflector  27  or feed array  22 . 
     The front-fed dual reflector configuration provides an optical system having a long effective focal length in a compact structure. A relatively long effective focal length of the optical system ensures low beam squint and virtually distortionless scanning to wide scan angles. Coupling a subreflector  27  with the main reflector  25  in a front-fed dual reflector configuration enables an optical system to be packaged into an extremely small envelope while providing an antenna  21  free of blockage. Table 1 below gives an example of the parameters of the subreflector  27  and the main reflector  25  for the front-fed dual reflector configuration in accordance with the invention. The main reflector coordinate system is defined by the axes XmZm as shown in FIG.  3 . 
     
       
         
           
               
               
               
             
               
                 TABLE I 
               
               
                   
               
               
                 Main Reflector 
                 Subreflector 
                 Minimum Spotsize 
               
               
                   
               
             
            
               
                 Eccentricity (E) 1.0 
                 Eccentricity 2.11111 
                 .0° x,z = 
               
               
                 Focal Length 137,958λ 
                 Focal Length 66.4329λ 
                 213.63, 133.95λ 
               
               
                 Rotation 0° 
                 Rotation −302.4° 
                 11.0° x,z = 
               
               
                 Xmin,Zmin = 
                 Xmin,Zmin = 
                 244.36, 134.87λ 
               
               
                 279.17, −2.29λ 
                 1.86.98 −4.00λ 
                 −11.0° x,z = 
               
               
                 Xmax,Zmax = 
                 Xmax,Zmax = 
                 188.32, 124.09λ 
               
               
                 336.67, 60.72λ 
                 269.97, 62.31λ 
                 Overall Box Size 
               
               
                 Xo,Zo = 
                 Xo,Zo = 
                 X,Z = 
               
               
                 5.56, −137.96λ 
                 157.05, 99.67λ 
                 149.69, 64.60λ 
               
               
                   
               
            
           
         
       
     
     The geometry and configuration of select feeds in the feed array  22 , the subreflector  27  and the main reflector  25  discussed above preferably satisfy the cross-polarization cancellation condition,          tan                   γ   2       =       1   M     ×   tan                   ϕ   2                       
     where y is the angle from the main reflector axis to the subreflector axis, θ is the angle from the subreflector axis to the focal axis, and M is the magnification factor. 
     Referring to FIGS. 3 and 4, the feed array  22  is comprised of a plurality of feeds. A practical feed array  22  contains many feeds, however, for illustration purposes, only five feeds of the feed array are shown in FIG.  3 . Each feed in the feed array  22  is coupled to a feed network  49  which provides a plurality of feed is signals to the feeds to produce a plurality of illumination beams  23 . The feeds can have any desirable configuration, such as circular, square, hexagonal and the like appropriate for a particular application. As will be subsequently discussed, the signal intensity and phase of each feed signal is preselected to produce illumination beams  23  having desirable beam characteristics. In addition, the feed signals are selectively provided to the feeds either separately or in combination depending on the particular application and the desired coverage area. 
     Each feed in the feed array  22  is positioned so that the central ray of each illumination beam  23  is incident on a separate preselected location on the subreflector  27 . According to the present invention shown in FIG. 3, the central ray of each illumination beam  23  is directed towards a preselected location on the subreflector  27 , thus, the subreflector  27  is oversized and approximately 50-150 wavelengths at the frequency of operation of the antenna system to accommodate the desired location of each illumination beam  23  on the subreflector  27 . The subreflector  27  is configured to direct each illumination beam  23  towards the main reflector  25 . For the preferred embodiment of the invention, the central ray of each illumination beam  23  is incident on the center  64  of the main reflector  25 . As a result, a circularly symmetrical illumination on the main reflector  25  is obtained and nearly circularly symmetric antenna beams  19  (FIG. 2) can be achieved, even when the antenna beam is scanned more than thirteen beamwidths away from the antenna  21  boresight. For example, computer simulations have shown that antenna beams can be created over the entire EFOV where each antenna beam has a level of −6 dB±0.5 dB relative to the peak signal level occurring within approximately 0.45 degrees ±0.05 degrees of the peak signal location of each antenna beam. 
     The position, orientation and pointing direction of each feed in the feed array  22  relative to the subreflector  27  and the main reflector  25  determines the location of each antenna beam  19  (FIG. 2) on the Earth  20 . For a first embodiment of the invention, the feed array  22  is comprised of a plurality of feeds each of which is placed in a predetermined location and orientation, and which are then combined in groups of seven to provide antenna beams over approximately ±8.7 degrees. As an example, the feed array  22  may include about 700 feeds that provide approximately 650 antenna beams covering an approximately 8.7 degree half-cone angle thereby covering the full extent of EFOV from a satellite in a geosynchronous orbit. 
     In prior art antenna designs, the feeds in a feed array  22  would be oriented relative to each other along a focal plane which is perpendicular to the propagation direction of the boresight or center beam to achieve the desirable pointing direction. Computer simulations have shown that locating the feeds on a common plane will not provide symmetrical antenna beams over the entire EFOV. Therefore, according to the present invention, each feed in the feed array  22  is positioned relative to each other on a predetermined contour rather than on a common plane. Computer simulations have shown that by locating the feeds at predetermined locations on a predetermined contour, adjacent, symmetrically shaped antenna beams can be created over the entire EFOV. 
     To determine the proper location for each feed in the feed array  22 , an optical ray-tracing program is used. The desired location of each antenna beam  19  (FIG. 2) on the Earth  20  is preselected and input into the ray tracing program. The desired location of the phase center and the pointing angle of each feed is then output from the program. This information is then input into an antenna analysis computer code which calculates the beam characteristics such as gain, beam shape, sidelobe level and beamwidth of each antenna beam. These antenna beams are examined to determine if the beams have the desired characteristics such as gain, beam shape, sidelobe level, and beamwidth. If so, the locations of the feeds are output from the ray tracing program. If the antenna beams do not occur at the desired locations with the desired performance characteristics, the location of the feeds require adjustment. The locations of the feeds are then incrementally adjusted and input into the antenna analysis program. The process is continued until the beams occur in the predetermined locations with the desired beam characteristics. Once the antenna beams occur in the predetermined locations with the desired beam characteristics, the locations of the phase centers of feeds and the pointing angle of the feeds which is used to manufacture the resulting antenna are output from the ray tracing program. 
     Referring to FIGS. 3 and 5 for the preferred embodiment of the invention, each illumination beam  23 , and thus, each antenna beam  69  is generated by a selected plurality of feeds within the feed array  22 . For one embodiment of the invention, each illumination beam  23  and corresponding antenna beam  69  is generated by a cluster of seven feeds, such as clusters  70 ,  72 , configured so that one of the feeds in the cluster is the central feed and the remaining six feeds in the cluster surround the central feed. For cluster  70 , the central feed is feed  74  and for cluster  72 , the central feed is feed  76 . Feeds in the feed array  22  can be shared by more than one cluster. For example, feed  76  can simultaneously be shared by three different feed clusters, the first of which comprises central feed horn  76  and surrounding feeds  74 ,  90 ,  92 ,  100 ,  102  and  104 , the second of which is comprised of central feed  74  and surrounding feeds  76 ,  100 ,  102 ,  105 ,  106  and  108 , the third of which comprises central feed  102  and surrounding feeds  74 ,  76 ,  104 ,  108 ,  110  and  112 . 
     The feeds in a cluster combine to approximate a single feed to produce a single composite illumination beam  23  which when directed towards the Earth by the front-fed dual reflector antenna system  21 , creates a single antenna beam  69  that defines a single coverage cell on the Earth. Each antenna beam  69  is associated with a different feed cluster with the location of each antenna beam  69  being determined by the location of the phase center of a cluster producing the respective antenna beam. Clusters having adjacent central feeds will produce adjacent antenna beams. For example, adjacent antenna beams  114 ,  116  are created by clusters  70 ,  72  respectively which have adjacent central feed horns  74 ,  76  respectively. In this embodiment, clusters of feeds are combined to define single antenna beams because of the size of the feeds and the size of the desired coverage area for existing satellite systems. Particularly, if single feed horns were used to provide antenna beams  69  of the same gain and beamwidth characteristics as that provided by the feed clusters, the single feed horns would not be able to be positioned close enough together in a single front-fed dual reflector antenna to provide adjacent, high gain antenna beams  69  on the Earth. For example, a single feed horn having a diameter of 5.6λ would be required to provide the same antenna beam  114  as that provided by the feed cluster  70 . In contrast each feed in a cluster  70  has a diameter of 2.8λ. By producing each composite illumination beam from a plurality of feeds, each composite illumination beam and corresponding antenna beam  69  appears to have been created by a physically large feed so that higher gain antenna beams can be created than that created by the prior art. Clusters of more than seven feed horns can also be used depending on the gain, beamwidth and number of beams desired. If antenna beams of predetermined shapes are desired, each feed cluster can be comprised of a preselected number of feeds to produce antenna beams of a desired shape. 
     Referring to FIG. 4 and 5, to provide downlink antenna beams, the feed network  49  provides the feeds in the feed array  22  with feed signals  124  to create composite illumination beams  23  (FIG. 3) having the desired beam characteristics which will produce antenna beams  69  which are symmetrically shaped. The feed network  49  divides an input signal  126  into a plurality of feed signals  124  and weights the feed signals  124  with predetermined signal intensities and phases. To provide antenna beams  69  having approximately the same gain and being approximately symmetrical in shape, each central feed in a cluster is provided with a feed signal  124  having a relatively high signal level intensity, and, the six surrounding feeds in a cluster are each provided with approximately equal strength feed signals  124  each of a lower signal strength than that provided to the central feed horn. The feed signals  124  input to the feeds are also phase delayed relative to each other by a predetermined amount so that each individual feed generates an illumination beam having the proper phase to create composite illumination beams and corresponding antenna beams  69  having good beam symmetry. The phase delaying can be accomplished with fixed or variable phase shifters. For simplicity, fixed value phase shifters in the form of different lengths of transmission lines may be used to create the desired phase delays. The weighting and phasing of the feed signals  124  can be conducted in such a manner as to provide feed signals  124  which produce antenna beams having lower sidelobes than that of a single antenna feed. Lower sidelobes provide less interference with adjacent antenna beams  114 ,  116 . If the feed signals  124  are provided to different clusters at different times, the corresponding antenna beams  69  will be created at different times such that the antenna beams  69  scan over the EFOV. Alternatively, if the feed signals  124  are provided to different clusters at the same time, continuous adjacent antenna beams  69  are created over the entire EFOV. By selectively controlling each of the feed signals  124 , the antenna feeds in a feed array  22  can be selectively activated to control the beam coverage area. The same concept works for reception purposes where the feeds receive a signal. 
     The above described embodiments provide adjacent, symmetrically shaped antenna beams which cover the EFOV from a single front-fed dual reflector antenna. Referring to FIG. 6, for another embodiment of the invention, a plurality of front-fed dual reflector antennas  130 - 136  provide antenna beams which are interleaved to provide full EFOV coverage. Each antenna  130 - 136  has a subreflector and main reflector combination  137 - 144  and a separate feed array  162 - 168  associated with each subreflector and main reflector combination. Each subreflector and main reflector combination and associated feed array together define a separate front fed dual reflector antenna configuration. Preferably each antenna  130 - 136  has approximately similarly sized and configured subreflectors  146 - 152  as well as similarly sized and configured main reflectors  154 - 160  so that the antenna beams generated by each antenna  130 - 136  are approximately equivalent. 
     A separate feed array  162 - 168  is associated with each subreflector and main reflector combination  137 - 144 . These feed arrays  162 - 168  are different than the feed array described above in that each feed within a feed array  162 - 168  is physically larger than the feeds described in the embodiments above, and, each illumination beam is generated by a smaller number of feeds than the embodiment described above. For one embodiment of the invention, each feed in a feed array  162 - 168  generates one illumination beam instead of a combination of feeds producing an illumination beam. For this embodiment, the feed network (not shown) coupled to each feed array  162 - 168  can be relatively uncomplicated in that the feed signal producing an illumination beam is not divided between multiple feeds thereby resulting in high gain antenna beams since every division of the feed signal results in signal loss. 
     The location of each feed within each feed array  162 - 166  with respect to a subreflector and main reflector combination  137 - 144  is determined in the same manner as described above. As mentioned above, to provide high gain, adjacently located antenna beams over the EFOV from single feeds would require the feeds to be positioned too close together to be implemented in a single front-fed dual reflector antenna. Therefore, feeds which produce adjacently located antenna beams are positioned within different feed arrays  162 - 168 . 
     Referring to FIGS. 6 and 7, the antenna beams  170  from each front-fed dual reflector antenna  130 - 136  are interleaved to provide adjacent antenna beams  170  over the EFOV. No two adjacent antenna beams are created from the same antenna  130 - 136 . FIG. 7 shows a portion of a coverage area on the Earth provided by the system. Only a few of the antenna beams are shown in that the entire coverage area would include many more cells. Each cell is labeled with an A, B, C or D to show which of the four antennas  130 - 136  actually provided that particular antenna beam. For example, each antenna beam labeled with an “A” is provided from the first antenna  130 . Similarly, each antenna beam labeled with a “B”, “C”, or “D” is provided from the second  132 , third  134  and fourth  136  antennas respectively. 
     The number of front-fed dual reflector antennas  130 - 136  are chosen so that a feed can physically be located at every desired position so that adjacent beams are provided over the desired coverage area where each beam exhibits the desired beam characteristics such as gain, beam width and shape. For the example shown in FIG. 6, four antennas  130 - 136  are required to position a feed at every location necessary to provide high gain, adjacently located antenna beams  170  over the full EFOV coverage. By using more front-fed dual reflector antennas, the size of the feeds in a feed array can be increased with the desired size and gain of each antenna beam determining the size of each feed and thus the number of antennas needed. 
     Like the embodiments described above, each feed in a given feed array  162 - 164  is located at a predetermined position on a contour and configured so that each illumination beam is incident on a preselected location on the subreflector  146 - 152  and directed towards the main reflector  152 - 158  within the same subreflector and main reflector combination  137 - 144  respectively. This embodiment requires additional area on a spacecraft  172  but has the advantage of a relatively uncomplicated, low loss feed network resulting in higher gain antenna beams from the same sized main reflector. 
     The antenna systems described above offer significant improvements over those antenna systems known in the art for use on satellites. The antenna systems of the invention are able to generate high gain, low scan loss, nearly undistorted, symmetrically shaped antenna beams for many uses, such as cellular satellite global coverage. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been shown and described hereinabove. The scope of the invention is limited solely by the claims which follow.