Patent Publication Number: US-6342865-B1

Title: Side-fed offset cassegrain antenna with main reflector gimbal

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to antennas for satellites and more particularly, to a side-fed reflector antenna for a satellite which provides a steerable antenna beam for full Earth field-of-view coverage with little degradation in the beam quality over the scan range. 
     In satellite communications systems, the antenna architecture has been to attach the entire antenna, comprising a parabolically curved main reflector, a feed horn, and a subreflector, to a positioning mechanism, such as a gimbal which moves the entire antenna to position or scan the antenna beam over the earth. Two factors contribute to the heavy weight of such a system. First, to maneuver a large mass and therefore the momentum, a heavy duty gimbal system is necessary. Second, to secure the entire antenna assembly in place during the launching vibration requires the use of a heavy latching structure during launch. 
     One antenna that addresses the above concerns is described in U.S. Pat. No. 5,870,060 and is depicted in FIG.  1 . The antenna has a fixed non-moving feed  3  and associated electronics  5  and, a gimbaled  7 , 9  main reflector  10 . Only the reflector  10  is moved to scan the beam, depicted by the dotted lines and arrows marked  11 . The shortfall of this antenna is that it incurs high scan losses which is compensates for by special design of the reflector  10  and feed  3 , which is expensive. This antenna additionally utilizes a long focal length to minimize the scan loss which results in the antenna requiring a substantial amount of real estate on a spacecraft which is typically at a premium. The antenna also uses an oversized reflector  10  to compensate for the gain loss. These compensations however do not solve the high cross-polarization level, high sidelobe level, and beam distortion problems which occurs when the reflector  10  is scanned off axis, particularly when the antenna is scanned to high scan angles such as the +/−11 degrees required for earth coverage from a geosynchronous satellite. The long focal length additionally results in the antenna requiring a substantial amount of real estate on a spacecraft which is typically at a premium. 
     What is needed therefore is a light weight antenna which has a low cross-polarization level and low beam distortion when scanned over a field of view, particularly when scanned over the Earth from a geosynchronous orbiting satellite. 
     SUMMARY OF THE INVENTION 
     The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides a steerable antenna. In a first aspect, the steerable antenna assembly comprises a main reflector, a feed and a subreflector which together are oriented to define a side-fed dual reflector geometry where the feed is to a side of both the subreflector and the main reflector. The feed, subreflector and main reflector together producing an antenna beam which is directed in a preselected direction by the main reflector. A gimbal is coupled to the main reflector for positioning the main reflector and scanning the antenna beam over a preselected coverage area. The feed and subreflector remain substantially fixed in position when the main reflector is positioned and the antenna beam is scanned. 
     In a second aspect, the steerable antenna is coupled to a satellite in a geosynchronous orbit about the earth where the earth subtends approximately a twenty two degree cone of coverage from the satellite. The main reflector and gimbal are configured to scan the antenna beam over the earth field of view. 
    
    
     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 steerable reflector antenna; 
     FIGS. 2 &amp; 3 are isometric drawings, each of which shows a portion of a satellite having a steerable side-fed dual reflector antenna assembly coupled thereto in accordance with the present invention; and 
     FIG. 4 is a side plane view of a side-fed dual reflector antenna system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 2, a portion  20  of a spacecraft having a reduced weight antenna system  22  for scanning an antenna beam is illustrated. The antenna system  22  of the present invention is preferably used for communications between the spacecraft and the Earth where the spacecraft is preferably located in a geosynchronous or near geosynchronous orbit and the antenna beam is scanned over an earth field of view. 
     Referring to FIGS. 2-4, an embodiment of a scanning antenna assembly configured according to the invention is illustrated. FIGS. 2 &amp; 3 depict the antenna assembly  22  in an isometric view fashion whereas FIG. 4 depicts the antenna assembly  22  in a side plane view fashion. The antenna assembly  22  includes a feed assembly  24 , a subreflector  26  and a main reflector  28 . The feed assembly  24  preferably contains a single feed horn and associated electronics but can also contain a feed array. The feed assembly  22 , subreflector  26  and main reflector  28  are configured in a side-fed dual reflector antenna configuration. The location of the feed assembly  24  to the side of both the subreflector  26  and main reflector  28  define the antenna assembly  22  as being “side-fed.” 
     The side-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  26  with the main reflector  28  in a side-fed dual reflector configuration enables an optical system to be packaged into an extremely small envelope while providing an antenna  22  free of blockage. A more detailed discussion of side-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. Note that the above description of the antenna pertains to the antenna being configured in a transmit mode. As is well known to one skilled in the art, the antenna can also be configured to operate in a receive mode. 
     Table 1 below gives an example of the parameters of the antenna  22  in accordance with a first embodiment of the invention. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Main Reflector 
                 Subreflector 
               
               
                   
               
             
            
               
                 Vertex: x = 0, y = 0, z = 0 
                 Focus: x = 0, y = 0, z = 120″ 
               
               
                 Focal Length: 120″ 
                 Focus Distance: 70.9355″ 
               
               
                 RIM: 
                 Rotation: 128.7101° 
               
               
                 Center: x = 90.2374″, y = 0, z = 0 
                 RIM: 
               
               
                 Diameter: 24″ 
                 Center: x = 18.31052″, y = 0, z = 0 
               
               
                   
                 Diameter: 20″ 
               
               
                   
               
            
           
         
       
     
     The geometry and configuration of feed assembly  24 , the subreflector  26  and the main reflector  28  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, ψ is the angle from the subreflector axis to the focal axis, and M is the magnification factor. 
     In the side-fed configuration, the illumination beam, depicted by the lines marked  30 , are provided by the feed assembly  24  and are reflected by the subreflector  26  which directs the illumination beam  30  towards the main reflector  28 . The illumination beam  30  is reflected from the main reflector  28  which produces an antenna beam. As indicated by the arrows marked  32 , the antenna beam is directed in a preselected direction which is substantially or totally free of blockage by the subreflector  26  and feed assembly  24 . 
     A gimbal  34  is coupled to the main reflector  28  and angularly moves the main reflector  28 . The gimbal  34  is a conventional electrical positioning and sensor device which steers the main reflector  28  over a preselected scan area; that is, positions the main reflector&#39;s attitude and elevation. Since the electronic controls and electrical leads and accompany electrical circuits for supplying driving current to the gimbal and sending position information therefrom are known and not necessary to an understanding of the invention, they are not illustrated or further described. As those skilled in the art recognize, many gimbal arrangements may be used to steer the reflector, such as a bi-axial gimbal attached to the back side of the main reflector  28 . 
     Only the main reflector  28  is gimbaled while the feed assembly  24  and subreflector  26  remain stationary in position. Through the gimbal controls, the direction of the antenna beam  32  is changed in attitude and elevation just like a mirror would deflect an incident light beam. For example, FIG. 2 depicts a boresight scan of the antenna  22 , denoted as z=0° whereas FIG. 3 depicts a 10° scan of the antenna  22  denoted as z=10°. Since the main reflector  28  weighs only a fraction of the total assembly weight, a small size gimbal  34  and light weight holding device is sufficient to steer the antenna beam  32  and survive the vibration during satellite launch. That alone results in considerable weight savings. 
     The feed assembly  24  and subreflector  26  are each positioned in preselected, fixed locations and do not move with the main reflector  28 . The feed assembly  24  and subreflector  26  are preferably mounted to separate brackets  36 ,  38 , respectively, which are each mounted to the bulkhead  40  of a spacecraft  20 . The brackets  36 ,  38  serve to fix the location of the feed assembly  24  and subreflector  26  thereby maintaining substantially fixed the relative distance between the feed assembly  24  and subreflector  26 . 
     The main reflector  28  may be formed from a solid piece of metal that is concavely shaped into one of the conventional curves used for reflector type microwave antennas, such as parabolic or a section of a parabolic, or may be so formed of wire mesh or of composite graphite material, all of which are known structures. 
     The subreflector  26  may also comprise a solid piece of metal or be formed of wire mesh or a composite material. The subreflector  26  preferably has the shape of a portion of a hyperbola having a concave side  42  with an associated focal point  44  and a convex side  46  with an associated focal point  48 . 
     The main reflector  28  has a main reflector focal point  50  and the subreflector  26  provides a secondary focus  52  for the main reflector  28 . The position of the feed assembly  24  is preferably selected so that the feed assembly  24  is approximately co-located with the secondary focus  52  when the antenna beam  32  is directed to the center of the area to be scanned. This is known to one skilled in the art as a boresight scan and is indicated in FIG. 2 as z=0°. This positioning of the feed assembly  24  minimizes the displacement of the secondary focus  52  from the feed assembly  24  during scanning which minimizes the loss in gain of the antenna  22  over the area to be scanned. For example, if the scan area is a twenty two degree cone, the antenna must scan +/−11 degrees from the center of the scan area. Placing the feed assembly  24  at the secondary focus  52  when the antenna  22  is at a zero degree scan angle will result in the secondary focus  52  being displaced from the feed assembly  24  by only a small amount over the entire scan area. 
     As depicted in FIGS. 2 &amp; 3, the feed assembly  24  becomes displaced from the secondary focus  52  of the main reflector  28  as the main reflector  28  is moved since the feed assembly  24  and subreflector  26  are held stationary during positioning of the main reflector  28 . Displacing the feed assembly  24  from the secondary focus  52  of the main reflector  28  is normally associated with a large loss in gain, a high cross polarization level, a high sidelobe level and distortion in the beam shape. It was found that by using a side-fed antenna configuration, superior scanning performance can be realized even though the feed assembly  24  is displaced from the secondary focus  52  during scanning. For example, it was found that the scan loss was only 0.6 dB, the cross-polarization level increased by only 2.5 dB and the sidelobe level increase only about 3 dB when the main reflector  28  was scanned +/−11 degrees for a total scan of twenty two degrees. Good performance over an approximate twenty two degree scan angle is particularly desirable for an antenna used on a gyosynchronous satellite since the earth subtends approximately a twenty two degree cone angle from a geosynchronous orbit. 
     In addition to the superior scanning performance, the side-fed configuration has the additional advantage that the subreflector  26  does not block the main reflector  28 . As such, the subreflector  26  can be made to be oversized without incurring gain loss and distortion associated with subreflector blockage of the main reflector  28 . Typical subreflectors  26  are sized to be approximately ten to twenty wavelengths in diameter at a frequency of operation. The feed assembly  24  is typically designed to illuminate the edge of the subreflector  26  at a −8 to −14 dB level. Energy which does not illuminate the subreflector  26  is lost. This lost energy is known in the art as “spillover loss”. It has been determined that an oversized subreflector, preferably between 50 and 100 wavelengths in diameter at a frequency of operation, will significantly reduce spillover loss and thereby increase overall antenna gain. 
     An additional benefit of the present invention is an improved long-term reliability of the antenna assembly  22 . The gimbaled main reflector  28  eliminates any RF moving parts, such as RF rotary joint or flexible waveguide and cables, which are needed in some of the prior art gimbaled antenna approaches. The life, and consequently the performance degradation over life, of high frequency RF parts constantly flexing over a long period of time is always a design concern for a space-based system. 
     The antenna assembly described above offers significant improvements over those antenna system 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 satellite earth coverage from a geosynchronous satellite. 
     It is believed that the foregoing description of the preferred embodiments of the invention is sufficient in detail to enable one skilled in the art to make and use the invention. However, it is expressly understood that the detail of the elements presented for the foregoing purposes is not intended to limit the scope of the invention, in as much as equivalents to those elements and other modifications thereof, all of which come within the scope of the invention, will become apparent to those skilled in the art upon reading this specification. Thus the invention is to be broadly construed within the full scope of the appended claims. 
     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.