Abstract:
An antenna system and method for generating a desired contiguous spot beam pattern, and a signal is disclosed. The contiguous spot beam generating antenna system comprises a reflector system configured in a side-fed Cassegrain (SFOC) configuration, an array of feed horns comprising at least a subset of feed horns for illuminating the reflector system, and a beamforming network, communicatively coupled to the array of feed horns, for controlling an excitation of the subset of the feed horns in the array of feed horns. The method comprises illuminating a side fed offset Cassegrain reflector system with an RF signal emanating from an array of feed horns, and controlling an excitation of the subset of the feed horns. The present invention provides a beamformer network that can produce uniform performance over wide scan angles, is easier to integrate and test, that can change the beam pattern on orbit, and that provides a more complete utilization of space assets without dramatically increasing the cost of manufacturing and operating a satellite. The use of a SFOC configuration or other wide scanning antenna permits the use of a simple beamforming network while maintaining excellent beam beam scanning characteristics.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to antenna systems, and in particular to a multi-beam reflector antenna system with a simple beamforming network. 
     2. Description of Related Art 
     Communications satellites have become commonplace for use in many types of communications services, e.g., data transfer, voice communications, television spot beam coverage, and other data transfer applications. As such, satellites must provide signals to various geographic locations on the Earth&#39;s surface. As such, typical satellites use customized antenna designs to provide signal coverage for a particular country or geographic area. 
     The primary design constraints for communications satellites are antenna beam coverage and radiated Radio Frequency (RF) power. These two design constraints are typically thought of to be paramount in the satellite design because they determine which customers on the earth will be able to receive satellite communications service. Further, the satellite weight becomes a factor, because launch vehicles are limited as to how much weight can be placed into orbit. 
     Many satellites operate over fixed coverage regions and employ polarization techniques, e.g., horizontal and vertical polarized signals, to increase the number of signals that the satellite can transmit and receive. These polarization techniques use overlapping reflectors where the reflector surfaces are independently shaped to produce substantially congruent coverage regions for the polarized signals. This approach is limited because the coverage regions are fixed and cannot be changed on-orbit, and the cross-polarization isolation for wider coverage regions is limited to the point that many satellite signal transmission requirements cannot increase their coverage regions. 
     Many satellite systems would be more efficient if they contained antennas with high directivity of the antenna beam and had the ability to have the coverage region be electronically configured on-orbit to different desired beam patterns. These objectives are typically met using a phased array antenna system. However, phased array antennas carry with them the problems of large signal losses between the power amplifiers and the beam ports, because of the beamforming network interconnections and long transmission lines. Further, the beamforming network is heavy, difficult to integrate and test, and is difficult to repair or replace without large time and labor costs. 
     The need to change the beam pattern provided by the satellite has become more desirable with the advent of direct broadcast satellites that provide communications services to specific areas. As areas increase in population, or additional subscribers in a given area subscribe to the satellite communications services, e.g., DirecTV, satellite television stations, local channel programming, etc., the satellite must divert resources to deliver the services to the new subscribers. Without the ability to change beam patterns and coverage areas, additional satellites must be launched to provide the services to possible future subscribers, which increases the cost of delivering the services to existing customers. Further, such systems typically have beamforming networks that are heavy, complex, and difficult to design, test, and integrate onto a spacecraft, and can be difficult to design to produce a uniform performance over a wide scan angle for the antenna. 
     There is therefore a need in the art for a beamformer that can produce uniform performance over wide scan angles. There is also a need in the art for a beamformer that is easier to integrate and test. There is also a need in the art for a beamforming network that can change the beam pattern on orbit. There is also a need in the art for a beamformer that to provide more complete utilization of space assets without dramatically increasing the cost of manufacturing and operating a satellite. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses an antenna system and method for generating a desired contiguous spot beam pattern, and a signal. The contiguous spot beam generating antenna system comprises a reflector system configured in a side-fed Cassegrain (SFOC) configuration, an array of feed horns comprising at least a subset of feed horns for illuminating the reflector system, and a beamforming network, communicatively coupled to the array of feed horns, for controlling an excitation of the subset of the feed horns in the array of feed horns. 
     The method comprises illuminating a side fed offset Cassegrain reflector system with an RF signal emanating from an array of feed horns, and controlling an excitation of the subset of the feed horns. The present invention provides a beam former that can produce uniform performance over wide scan angles. The present invention also provides a beamformer that is easier to integrate and test. The present invention also provides a beamforming network that can change the beam pattern on orbit. The present invention also provides a beamformer that to provide more complete utilization of space assets without dramatically increasing the cost of manufacturing and operating a satellite. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIGS. 1A and 1B illustrate a typical satellite environment for the present invention; 
     FIG. 2 illustrates a typical spot beam coverage of the Continental United States; 
     FIGS. 3-5 illustrate antenna systems that generate spot beam coverage pattern as shown in FIG. 2; 
     FIG. 6 illustrates an isometric view of the SFOC geometry of the present invention; 
     FIG. 7 illustrates a side view of a conventional single offset antenna geometry; 
     FIG. 8 illustrates the scan performance for the system illustrated in FIG.  7 . 
     FIG. 9 illustrates the scan performance for the system illustrated in FIG. 6; 
     FIG. 10 illustrates the feed excitations required for the SFOC and single offset beamforming networks to generate the focal and scanned beams; and 
     FIG. 11 is a flow chart illustrating the steps used to practice the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following description of the preferred embodiment, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration a specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     Satellite Environment 
     FIGS. 1A and 1B illustrate a typical satellite environment for the present invention. 
     Spacecraft  100  is illustrated with four antennas  102 - 108 . Although shown as dual reflector antennas  102 - 108 , antennas  102 - 108  can be direct fed single reflector antennas  102 - 108  without departing from the scope of the present invention. Antenna  102  is located on the east face of the spacecraft bus  110 , antenna  104  is located on the west face of spacecraft bus  110 , antenna  106  is located on the north part of the nadir face of the spacecraft bus  110 , and antenna  108  is located on the south part of the nadir face of the spacecraft bus  110 . Solar panels  112  are also shown for clarity. 
     Feed horns  114 - 120  are also shown. Feed horn  114  illuminates antenna  102 , feed horn  116  illuminates antenna  104 , feed horn  118  illuminates antenna  108 , and feed horn  120  illuminates antenna  106 . Feed horn  114  is directed towards subreflector  122 , which is aligned with antenna  102 . Feed horn  116  is directed towards subreflector  124 , which is aligned with antenna  104 . Feed horns  114 - 120  can be single or multiple sets of feed horns as desired by the spacecraft designer or as needed to produce the beams desired for geographic coverage. For example, feed horns  114  and  116  are shown as two banks of feed horns, but could be a single bank of feed horns, or multiple banks of feed horns, as desired. Antennae  102  and  104  are shown in a side-fed offset Cassegrain (SFOC) configuration, which are packaged on the East and West sides of the spacecraft bus  110 . Antennas  106  and  108  are shown as offset Gregorian geometry antennas, but can be of other geometric design if desired. Further, antennas  102 - 108  can be of direct fed design, where the subreflectors are eliminated and the feed horns  114 - 120  directly illuminate reflectors  102 - 108  if desired. Further, any combination of Cassegrainian, Gregorian, SFOC, or direct illumination designs can be incorporated on spacecraft  100  without departing from the scope of the present invention. 
     Feed horn  118  illuminates subreflector  130  with RF energy, which is aligned with antenna  108  to produce output beam  132 . Feed horn  120  illuminates subreflector  134  with RF energy, which is aligned with antenna  106  to produce beam  136 . Beams  132  and  136  are used to produce coverage patterns on the Earth&#39;s surface. Beams  132  and  136  can cover the same geographic location, or different geographic locations, as desired. Further, feed horns  118  and  120  can illuminate the antennae  102 - 108  with more than one polarization of RF energy, i.e., left and right hand circular polarization, or horizontal and vertical polarization, simultaneously. 
     Although described with respect to satellite installations, the antennas described herein can be used in alternative embodiments, e.g., ground-based systems, mobile-based systems, etc., without departing from the scope of the present invention. Further, although the spacecraft  100  is described such that the feed horns  114 - 120  provide a transmitted signal from spacecraft  100  via the reflectors  102 - 108 , the feed horns  114 - 120  can be diplexed such that signals can be received on the spacecraft  100  via reflectors  102 - 108 . 
     Overview of the Related Art 
     Current day satellites are required to generate contiguous spot beam coverages in order to provide continuous geographical coverage of a large geographic area. A typical Continental United States (CONUS) spot beam coverage pattern generated by a satellite is shown in FIG.  2 . Outline  200  of the CONUS geographical area is shown as being overlapped by spot beams  202 , which have intersection areas  204  to provide contiguous signal coverage over the CONUS outline  200 . 
     There are several antenna configurations that can be used to provide spot beam coverage as shown in FIG.  2 . FIG. 3 illustrates one approach, which uses a single aperture antenna without a beamforming network. 
     System  300  comprises a single aperture antenna  302  and a bank of feed horns  304 . The bank of feed horns  304  comprises signal  1  feed horns  306 , signal  2  feed horns  308 , signal  3  feedhorns  310 , and signal  4  feed horns  312 . These feed horns  306 - 312  each generate a separate spot beam  314 - 320 , i.e., signal  1  feed horns  306  generate spot beam  1   314 , signal  2  feed horns  308  generate spot beam  2   316 , signal  3  feed horns  310  generate spot beam  3   318 , and signal  4  feed horns  312  generate spot beam  4   320 . 
     Due to the high overlap requirement, e.g., 3 to 6 dB, of the beams  314 - 320 , the feed horn  306 - 312  size for system  300  is relatively small, which results in a very high spillover and associated degradation in performance for the system  300 , e.g., 2 to 3 dB. 
     Further, there is no control of the beam pattern  322  shape, and, therefore, the scan performance of system  300  is limited by the reflector  302  geometry. 
     FIG. 4 illustrates a multiple aperture antenna system that generates a spot beam coverage pattern as shown in FIG.  2 . System  400  comprises reflectors  402 - 408  and feed horn banks  410 - 416 . Each reflector  402 - 408  has a dedicated feed horn bank  410 - 416 ; i.e., reflector  402  is only illuminated by feed horn bank  410 , reflector  404  is only illuminated by feed horn bank  412 , reflector  406  is only illuminated by feed horn bank  414 , and reflector  408  is only illuminated by feed horn bank  416 . 
     Each reflector  402 - 408  and the associated feed horn bank  410 - 416  generates a spot beam  418 - 424  for the beam pattern  426 . For example, reflector  402  and associated feed bank  410  generate spot beam  418 , reflector  404  and associated feed bank  412  generate spot beam  420 , reflector  406  and associated feed bank  414  generate spot beam  422 , and reflector  408  and associated feed bank  416  generate spot beam  424 . As such, adjacent beams in beam pattern  426  are generated from alternate reflectors (apertures)  402 - 408 . Hence, a larger feed horn can be used within feed horn banks  410 - 416  to generate each of the beams  418 - 424 , which allows for better spillover and gain performance of system  400 . Further, since system  400  uses a single feed horn bank  410 - 416  per beam  418 - 424 , no Beam Forming Network (BFN) is required for system  400 . However, system  400  requires multiple apertures (reflectors)  402 - 408  to generate beam pattern  426 . 
     FIG. 5 illustrates a single aperture system  500  that uses a BFN to generate the beam pattern shown in FIG.  2 . System  500  comprises reflector  502 , feed horn bank  504 , and BFN  506 . The feed horn bank comprises feed horns  508 - 520 . For each beam input  522 - 528  to the BFN  506 , a subset of the feed horns  508 - 520  in feed horn bank  504  is used to generate each of the beams  530 - 536  in beam pattern  538 . For example, feed horns  508 - 520  are used when beam  1  input  522  is activated to generate beam  1   530 . A different set of feed horns in feed horn bank  504  can be used to generate the other spot beams  532 - 536  in beam pattern  538 . 
     The advantage of this approach is that all of the spot beams  530 - 536  in beam pattern  538  are generated from a single aperture. The antenna geometry of system  500  is typically a single offset reflector as shown in FIG. 5, and each spot beam  530 - 536  is generated by a cluster of feed horns  508 - 520 , typically a combination of 7 feed horns  508 - 520  or a combination of 19 feed horns  508 - 520 , for each spot beam  530 - 536 . 
     Each of the feed horns  508  in the cluster of feed horns  508 - 520  is excited according to an optimum excitation amplitude value and an optimum excitation phase value to obtain the best gain and scan performance for system  500 . The cluster of feed horns  508 - 520  simulates a larger single feed horn, resulting in better spillover and gain performance. The subset of excited feed horns  508 - 520  includes a primary feed horn (e.g. horn  520 ) and a plurality of secondary feed horns (e.g. horns  508 - 518 ). In system  500 , adjacent beams, e.g., beam  530  and beam  532 , need to share feed horns  508 - 520 . In a design using a 7 feed horn cluster, e.g., feed horns  508 - 520 , each feed horn  508 - 520  can be shared by up to 7 beams  530 - 536 . The disadvantage of this approach is the complexity associated with the BFN  506 , which is necessary to produce the amplitude and phase excitations for all the beams  530 - 536 . In a typical reflector antenna system  500 , the optimum excitations for the different spot beams  530 - 536  are typically different, since the component beams generated from each feed horn  508 - 520  in the feed horn bank  504  vary as a function of the scan angle. This is shown in columns  1002 - 1008  of FIG. 10 discussed later in this disclosure. This leads to additional complexity in the BFN  506  since many different designs of the components, e.g., couplers, phase shifters, etc. within the BFN  506  have to be created. 
     Overview of the Present Invention 
     The present invention describes an antenna system using a simplified BFN, which will give uniform performance over a wide scan angle with identical cluster excitations for all the spot beams, independent of the scan angle, thus simplifying the BFN design. The antenna system comprises a dual reflector system such as a Side-Fed Offset Cassegrain (SFOC) system, which is illuminated by a feed horn array, controlled by a relatively simple BFN. In many applications, the present invention reduces the complexity of the BFN to providing only amplitude variations to the feed horn array without using phase variations, which further simplifies the BFN of the present invention. 
     Conventional multi-beam antennas that use BFNs require a unique design for the BFN, and, within the unique BFN, unique component designs to account for scan angle differences and other geometry and spot beam size differences for a given satellite. These unique component and BFN designs require not only additional complex circuitry during the fabrication process, they require additional testing to determine if the design is properly functioning. The present invention standardizes the BFN design, and reduces the number of unique circuit and component designs over the related art. Further, the present invention, in many applications, provides optimal spot beam performance without any phase-variation circuitry, relying solely on amplitude variation to generate the spot beams. Conventional antenna systems such as a single offset reflector or a Gregorian antenna, without the present invention, would typically require a more complex BFN to generate the required varying amplitude and phase excitations of the feed horn cluster as a function of the scan angle. 
     FIG. 6 illustrates an isometric view of the SFOC geometry of the present invention. 
     System  600  illustrates subreflector  602  and main reflector  604  being illuminated by feed horn array  606 , mounted on the East face of spacecraft  608 . The diameters of the main reflector  604  and the subreflector  602  are approximately 61 inches, but can be larger or smaller without departing from the scope of the present invention. At an operating frequency of 30 GHz, these diameters correspond to an antenna half-power beamwidth of 0.5 deg. 
     FIG. 7 illustrates a side view of a conventional single offset antenna geometry. System  700  comprises a single reflector  702 , illuminated by feed horn array  704  to generate beam  706 . 
     FIG. 8 illustrates the scan performance of the system illustrated in FIG.  7 . 
     The scan performance  800 , with peak performance at point  802  of 50.07 dB and point  804  of 49.64 dB, was obtained using a 7 feed horn approach to illuminate the reflector  702 . The performance  800  used optimum excitations in both amplitude and phase for each beam position. 
     FIG. 9 illustrates the scan performance for the system illustrated in FIG.  6 . 
     Scan performance  900 , with peak performance at point  902  of 50.14 dB and point  804  of 50.01 dB, was obtained using a 7 feed horn approach to illuminate the reflectors  602  and  604 . The scan performances  800  and  900  for the beams are comparable in terms of coverage gain. However, the scan loss is less in scan performance  900 , which demonstrates the superior performance of the present invention. Further, the simplified BFN used to generate scan performance  900  makes the BFN and associated system of the present invention even more attractive, since it is easier to build and test than a fully optimized single offset reflector system with a complex BFN as described with respect to FIGS. 5 and 7. 
     Another advantage of using the geometry of FIG. 6, e.g., a SFOC geometry with a simplified BFN, is that the geometry of the present invention allows the same set of feed excitations to generate all the beams with smaller performance degradation characteristics. The single offset configuration of FIG. 7 requires a different set of feed horn excitations, e.g., different outputs from the BFN, to generate each of the different beams. 
     A direct comparison between the SFOC and a single offset configuration, for both focal beams and scanned beams, shows that the SFOC configuration meets or exceeds the performance of the single offset configuration. For a focal beam, the SFOC configuration has an edge of beam at 46.1 dBi, which is the same as the single offset configuration. The sidelobe levels to the nearest neighbor beam is −28 dBr for the SFOC, and −31 dBr for the single offset configuration. For scanned beams, the SFOC configuration has an edge of beam at 46.0 dBi, whereas the single offset configuration has an edge of beam at 45.6 dBi. The sidelobe levels to the nearest neighbor beam is −26 dBr for the SFOC, which is the same for the single offset configuration. 
     FIG. 10 illustrates the feed excitations required for the SFOC and single offset beamforming networks to generate the focal and scanned beams. 
     Chart  1000  illustrates the feed excitations required to generate any single beam for the beam pattern shown in FIG.  2 . In the single offset configuration of FIG. 7, in order to form any one beam, the beamforming network needs a total of fifteen unique coupler designs to give the appropriate excitations to the seven feeds to form that beam. For example, to generate a focal beam in a single offset configuration, column  1002  and column  1006  illustrate that there are fourteen unique feed excitations (seven in amplitude and seven in phase) for a seven feed horn configuration, plus an additional coupler to couple all of the feed horns together. Thus, a total of fifteen unique coupler designs are required to generate the necessary feed excitations. The same situation also applies to the scanned beams. A different set of fifteen coupler designs are required to form the scanned beams, as the required feed excitations change, as shown in columns  1004  and  1006 . 
     In the single offset reflector design, since each beam would need a different set of feed excitations, the BFN would require 15 couplers times the number of beams to generate the beams. As an example, if 100 beams need to be formed, single offset geometry would require approximately 1500 different unique coupler designs. 
     For the SFOC geometry, these 15 couplers take on only four different coupler values, as shown in columns  1010  and  1012 . This occurs because the outer six feed elements use the same feed horn excitations, e.g., 0.03 watts. In other words, it would require only four unique coupler designs to build up the entire BFN for any number of beams since the same set of feed excitations can be applied to all the beams. Further, since there are no phase shifters required, as shown in columns  1014  and  1016 , the BFN of the present invention is dramatically simplified over previous BFN designs. 
     To compare the BFN of the present invention with the BFN of the related art, if 100 beams were required, the BFN of the related art would require approximately 1500 unique coupler designs, whereas the BFN of the present invention would only require four unique coupler designs. Such a generic approach using the present invention results in cost reductions and faster construction times without sacrificing quality of the spacecraft. 
     Process Chart 
     FIG. 11 is a flow chart illustrating the steps used to practice the present invention. 
     Block  1100  illustrates performing the step of illuminating a side fed offset Cassegrain reflector system with an RF signal emanating from at least a subset of an array of feed horns. Block  1102  illustrates performing the step of controlling an excitation of the subset of feed horns. 
     Conclusion 
     The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.