Abstract:
Variable beamwidth antenna systems for use on spacecraft that is capable of changing their beamwidths while the spacecraft in on orbit. The variable beamwidth antenna systems include a main reflector, a subreflector, a feed horn, a main reflector displacement mechanism and a feed horn (or subreflector) displacement mechanism. For broaden the beamwidth, the RF feed horn and the subreflector are moved close together by proper distance. The main reflector is moved away from the subreflector along a line through centers of their respective surface by a distance given by a predetermined equation. Another embodiment of the present invention provides for a zoom contour beam antenna system that radiates a contour beam and whose beam is variable or zoomable.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part application of U.S. patent application Ser. No. 09/531,613, filed Mar. 21, 2000. 
     
    
     
       BACKGROUND  
         [0002]    The present invention relates generally to antennas for use on spacecraft, and more particularly, to variable beamwidth and zoom contour beam antenna systems designed for use on spacecraft.  
           [0003]    The present invention relates to improvements in offset Gregorian reflector antenna systems for use on communication satellites. Due to unpredictability of communication traffic, it is desirable that the beamwidth of the antenna radiation pattern be changeable when the spacecraft is on orbit.  
           [0004]    When a satellite is in an elliptical orbit, the view of the coverage area (CONUS, for example) from the satellite changes as the satellite travels. The size of the coverage area viewed from the satellite is inversely proportional to the distance between the satellite and the earth. Under such circumstances, in order that a communication system on the satellite is to be operated efficiently, it is required that (1) the antenna radiates a contour beam (the beam contour matches the boundary of the coverage) and (2) the contour beam is variable or zoomable (size of the beam changes) in space.  
           [0005]    Previously known zoom antennas are limited to radiate (1) a circular beams (not a contour beam) wherein the radiation contour of antenna matches the boundary of the coverage area, such as is disclosed in U.S. patent application Ser. No. 09/531,613, filed Mar. 21, 2000, assigned to the assignee of the present invention, and (2) a defocused elliptical beam, not a zoomed contour beams such as is disclosed in U.S. Pat. No. 5,977,923, entitled “Reconfigurable, zoomable, turnable, elliptical-beam antenna”, by Contu, et al, issued Nov. 2, 1999.  
           [0006]    Accordingly, it would be advantageous to have improved variable beamwidth and zoom contour beam antenna systems that may be used on a spacecraft that has a changeable beamwidth. It would also be advantageous to have improved zoom contour beam antenna systems that radiate a contour beam, and wherein the contour beam is variable or zoomable.  
         SUMMARY OF THE INVENTION  
         [0007]    The variable beamwidth antenna system comprises a main reflector, a reflector displacement mechanism, a subreflector, a feed horn, and a feed horn displacement mechanism. The reflector displacement mechanism can place the main reflector at any desired location, while the feed displacement mechanism can place the feed horn at any desired location. More specifically, the main reflector displacement mechanism controls the spacing between the main reflector and the subreflector; while the feed displacement mechanism controls the spacing between the feed and the subreflector.  
           [0008]    To implement the present invention, two mechanical movements are required. First, the RF feed horn and the subreflector are moved closer together. Second, the main reflector is moved away from the subreflector. Alternatively, the subreflector is moved closer to the RF feed horn, and the main reflector is moved away from the subreflector.  
           [0009]    Two mechanical movements are not independent. They are related by  
       y   =         d   2        x         c   2     -     x        (     c   +   d     )                                 
 
           [0010]    where “x” is the distance of RF feed horn displacement, and “y” is the distance of the main reflector displacement.  
           [0011]    When x=y=0, the antenna arrangement is in the focused condition. Under the focused condition, the focal point of the paraboloidal main reflector is coincident with one of the foci of the elliptical subreflector, and the feed horn is located at the other focus of the subreflector. The terms “c” and “d” in the above equation are the distance between the RF feed horn and the subreflector, and the distance between the focal point of the main reflector and subreflector, respectively, when the antenna is in focus.  
           [0012]    A reduced-to-practice embodiment of the present variable beamwidth antenna has 3 dB beamwidth that can be changed while the spacecraft is on orbit by proper displacements of any two components among the main reflector, the subreflector and the feed.  
           [0013]    Another embodiment of the present invention comprises a zoom contour beam antenna system that radiates a contour beam and whose beam is variable or zoomable. The present invention produces a zoom contour beam, wherein the radiation contour of antenna matches the boundary of the coverage area. This is possible because the present invention utilizes two mechanical displacement mechanisms respectively coupled to a main shaped reflector and a shaped subreflector that are moveable to provide zoomable contoured beams. The zoom contour beam provides higher gain in the coverage area, and thus improves the efficiency of the satellite communication system in which it is employed.  
           [0014]    More particularly, an exemplary zoom contour beam antenna system comprises a shaped subreflector, a shaped main reflector. a feed horn, a main reflector displacement mechanism coupled to the shaped main reflector, and a subreflector displacement mechanism coupled to the shaped subreflector. The main reflector displacement mechanism repositions the main reflector, and the subreflector displacement mechanism repositions the subreflector. Thus, the subreflector displacement mechanism adjusts the position of the subreflector relative to the feed horn, and the main reflector displacement mechanism adjusts the position of the main reflector relative to the subreflector in order to zoom the beam produced by the antenna system. the repositioning of the various components is done in accordance with a predetermined displacement equation.  
           [0015]    Thus, in accordance with the teachings of the present invention, a Gregorian reflector antenna may be used in circumstances where the desired antenna radiation pattern(s) is (are) required to be broadened (changed). Both the main reflector and the subreflector are attached to mechanical devices (displacement mechanisms) such that the reflectors can be displaced. The displacements of the two reflectors are not independent, and are related according to the displacement equation. The present invention is not limited to systems having a single pencil beam antenna, and may be used in circumstances involving multiple beam (multiple feed horn) antennas. The present invention may also be employed with a contoured beam antenna. In such a contoured beam antenna application, both the main reflector and subreflector are shaped to obtain a desired (for example, CONUS) radiation pattern. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    The various features and advantages of the present invention may be more readily understood with reference to the following detailed description taken in conjunction with the accompanying drawings wherein like reference numerals designate like structural elements, and in which:  
         [0017]    [0017]FIG. 1 illustrates a first embodiment of a variable beamwidth antenna system in accordance with the principles of the present invention;  
         [0018]    [0018]FIG. 2 illustrates a reduced-to-practice embodiment of the variable beamwidth antenna system shown in FIG. 1;  
         [0019]    [0019]FIG. 3 illustrates design parameters of exemplary variable beamwidth antenna system when the antenna is in focused condition;  
         [0020]    [0020]FIG. 4 illustrates the antenna radiation pattern of the antenna shown in FIG. 3;  
         [0021]    [0021]FIG. 5 illustrates the broadened radiation pattern of the antenna shown in FIG. 3 after displacement of the main reflector and the feed horn displaced in accordance with the principle of this invention;  
         [0022]    [0022]FIG. 6 illustrates a second embodiment of a variable beamwidth antenna system in accordance with the principles of the present invention;  
         [0023]    [0023]FIG. 7 illustrates a third embodiment of a variable beamwidth antenna system in accordance with the principles of the present invention;  
         [0024]    [0024]FIG. 8 illustrates a satellite in an elliptical orbit;  
         [0025]    [0025]FIGS. 9, 10 and  11  show typical examples of the view of CONUS from different satellite positions in a typical elliptical orbit;  
         [0026]    [0026]FIG. 12 illustrates a zoom contour beam antenna system in accordance with the principles of the present invention;  
         [0027]    [0027]FIG. 13 illustrates movement of displacement mechanisms employed in the system shown in FIG. 12;  
         [0028]    [0028]FIG. 14 illustrates an exemplary zoom contour beam antenna with exemplary design parameters; and  
         [0029]    [0029]FIGS. 15, 16 and  17  show examples of radiation patterns corresponding to the examples shown in FIGS. 9, 10 and  11  showing various zoomable beams that are produced by the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]    Referring to the drawing figures, FIG. 1 illustrates a first embodiment of a variable beamwidth antenna system  10  in accordance with the principles of the present invention. The variable beamwidth antenna system  10  comprises a main reflector  11 , a subreflector  12 , a feed horn  13 , a feed horn displacement mechanism  14 , and a main reflector displacement mechanism  15 . The function of the feed horn displacement mechanism  14  is to reposition the feed horn  13 , and the function of the main reflector displacement mechanism  15  is to reposition the main reflector  11 .  
         [0031]    The subreflector  12  is a sector of an ellipsoidal surface, whose two foci are at O′ and O. The main reflector is a sector of paraboloidal surface. When the antenna is in the focused position, i.e., the case where neither the main reflector  11  nor the feed horn  13  is displaced, the focal point of the main reflector  11  is located at O′, and the feed horn  13  is located at O, as shown in FIG. 1. Point A in FIG. 1 is the intersection point of the axis of the feed horn  13  and the surface of the subreflector  12 . Point B is the intersection of the surface of the main reflector  11  and line AO′. The distance OA is “c” in Equation (1) below, and the distance AO′ is “d” in Equation (1).  
             y   =         d   2        x         c   2     -     x        (     c   +   d     )                   (   1   )                               
 
         [0032]    where “x” is the distance of RF feed horn displacement, and “y” is the distance of the main reflector displacement. Under the focused position, the antenna system  10  provides the narrowest radiation pattern.  
         [0033]    [0033]FIG. 2 illustrates the action of a reduced-to-practice embodiment of the variable beamwidth antenna system  10 . In order to broaden the beamwidth, two mechanical motions are required. First, the feed displacement mechanism  14  must push (or reposition) the feed horn  13  closer to the subreflector  12 . 13   a  in FIG. 2 is the new feed horn position. Second, the reflector displacement mechanism  15  must pull (or reposition) the main reflector  11  farther away from the subreflector  12 . The new position of the main reflector is identified as main reflector  11   a  in FIG. 2. The feed horn displacement “x” and the main reflector displacement “y” are not two independent variables. They are related by Equation (1).  
         [0034]    A numerical example will now be illustrated. An exemplary variable beamwidth antenna system  10  with exemplary design parameters is shown in FIG. 3. For this antenna geometry, c=38.138 inches and d=36.826 inches.  
         [0035]    [0035]FIG. 4 shows radiation contours of the exemplary system  10  when the system is in focus, i.e., when x=y=0. The 3 dB beamwidth of the beam shown in FIG. 4 is approximately 0.7 degrees.  
         [0036]    [0036]FIG. 5 shows the radiation contours of the exemplary system  10  when x=11.0 inches and y=23.68 inches (from Equation (1)). The 3 dB beamwidth of the beam shown in FIG. 4 is broadened to 1.45 degrees from 0.71 degrees.  
         [0037]    Due to optical aberration, Equation (1) is an approximate expression for the displacement of the main reflector  11   a  of the variable beamwidth antenna system  10 . For practical applications, a fine-tuning of the location of the main reflector  11   a  may be required.  
         [0038]    It is also to be understood that the RF feed horn  13  can be made stationary instead of the subreflector  12  as shown in FIG. 6. More particularly, FIG. 6 illustrates a second embodiment of a variable beamwidth antenna system  10   a  in accordance with the principle of the present invention.  
         [0039]    In the second embodiment of the variable beamwidth antenna system  10   a , the subreflector displacement mechanism  16  displaces the subreflector  12  to a proper location and the main reflector displacement mechanism  15  displaces the main reflector  11  to a proper location while keeping the RF feed horn stationary. This system  10   a  in FIG. 6 is equivalent to the variable beamwidth antenna system  10  described above with reference to FIG. 2.  
         [0040]    Referring now to FIG. 7, it illustrates a third embodiment of a variable beamwidth antenna system  10   b  in accordance with the principles of the present invention. In the third embodiment of the variable beamwidth antenna system  10   b  shown in FIG. 6, there are plurality of RF feed horns  13   b  used instead of a single RF feed horn  13 . As a result, there will be multiple variable beams produced by the system  10   b.    
         [0041]    [0041]FIG. 8 illustrates a satellite  30  in an elliptical orbit around the Earth  31 . The satellite  30  comprises a zoom contour beam antenna system  40  in accordance with the principles of the present invention.  
         [0042]    [0042]FIGS. 9, 10 and  11  show typical examples of the view of the coverage area (CONUS) from different positions of the satellite  30  in a typical elliptical orbit.  
         [0043]    Referring now to FIG. 12, it illustrates an exemplary zoom contour beam antenna system  40  in accordance with the principles of the present invention. The zoom contour beam antenna system  40  comprises a shaped main reflector  11 , a shaped subreflector  12 , a feed horn  13 , a subreflector displacement mechanism  14  and a main reflector displacement mechanism  15 . The arrangement of the components of the zoom contour beam antenna system  40  are generally as was described with reference to the variable beamwidth antenna systems described previously. However, the main reflector displacement mechanism  15  is coupled to the shaped main reflector  11 , and the subreflector displacement mechanism  14  is coupled to the shaped subreflector  12 .  
         [0044]    The function of the main reflector displacement mechanism  15  is to reposition the main reflector  11 , and the function of the subreflector displacement mechanism  14  is to reposition the subreflector  12 . Thus, the subreflector displacement mechanism  14  adjusts the position of the subreflector  12  relative to the feed horn  13 , and the main reflector displacement mechanism  15  adjusts the position of the main reflector  11  relative to the subreflector  12 .  
         [0045]    In order to zoom the contours, the shaped subreflector  12  must be moved (along the line AO) closer to the feed horn  13  by “x” and the shaped main reflector  11  must be pulled away (along the line AO′) from the shaped subreflector  12  by “y” as shown in FIG. 13. More particularly, FIG. 13 illustrates movement of the shaped main reflector and shaped subreflector  11 ,  12  by the displacement mechanisms  14 ,  15  employed in the system  40  shown in FIG. 12. The displacements “x” and “y” are related by the displacement equation:  
         y   =         d   2        x         c   2     -     x        (     c   +   d     )             ,                         
 
         [0046]    where “c” is the distance AO and “d” is the distance AO′ in FIG. 12.  
         [0047]    A numerical example will now be illustrated with reference to FIG. 14. An exemplary zoom contour beam antenna system  40  having exemplary design parameters is shown in FIG. 14. The geometry of the exemplary antenna system  40  is, c=68.626 inches and d=66.319 inches. The zoom characteristics of the exemplary antenna system  40  are summarized in Table 1.  
         [0048]    [0048]FIGS. 15, 16 and  17  show examples of radiation patterns (corresponding to the examples shown in FIGS. 9, 10 and  11 ) showing various zoomable beams (radiation patterns) that are produced by the present invention and which are referenced in Table 1. The term “EOC” means “edge of coverage”.  
                                                     TABLE 1                       Subreflector   Main reflector       EOC           displacement   displacement   Radiation   gain   Zoom       x (inches)   y (inches)   pattern   (dBi)   factor                                0.00   0.000       29.6   1.0       8.00   9.693       27.2   1.3       13.10   19.586       25.6   1.6                  
 
         [0049]    Due to optical aberration, equation (1) is an approximate expression for reflector displacements. For practical applications, a fine-tuning of the locations of the reflectors may be required.  
         [0050]    There are three RF components in the zoom contour beam antenna system  40 . These are the shaped main reflector  11 , the shaped subreflector  12  and feed horn  13 . It is to be understood that any one component may be chosen to be stationary.  
         [0051]    Thus, in accordance with the principles of the present invention, a Gregorian reflector antenna system may be used when the desired antenna radiation pattern(s) is (are) required to be broadened (changed). The main reflector  11  and the subreflector  12  are attached to mechanical devices (the displacement mechanisms,  14 ,  15 ) that allow the reflectors  11 ,  12  to be displaced. The displacements of the two reflectors  11 ,  12  are related by the displacement equation.  
         [0052]    The present invention is not limited to systems having a single pencil beam antenna, and may be used in circumstances involving multiple beam (multiple feed horn) antennas, such as is illustrated in FIG. 7. The present invention may also be employed with a contoured beam antenna. In such a contoured beam antenna application, both the main reflector  11  and subreflector  12  are shaped to obtain a desired (for example, CONUS) radiation pattern.  
         [0053]    Thus, improved variable beamwidth antenna systems have been disclosed. It is to be understood that the described embodiments are merely illustrative of some of the many specific embodiments that represent applications of the principles of the present invention. Clearly, numerous and other arrangements can be readily devised by those skilled in the art without departing from the scope of the invention.