Abstract:
A multi-pattern antenna for providing a plurality of antenna patterns at different frequencies or polarizations from a single reflector body eliminates the need for multiple reflector antennas on a single spacecraft. The reflector antenna comprises a reflector body and an illumination source. The illumination source illuminates the reflector with a plurality of RF signals each of a preselected frequency or polarization. The reflector comprises a plurality of zones with each zone reflecting preselected RF signals. A plurality of antenna patterns are generated from the reflected RF signals. Each zone is sized to a preselected shape such that the antenna patterns have a desired shape or beamwidth characteristic.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of reflector antennas, and more particularly, to a reflector antenna which includes frequency selective or polarization sensitive zones to provide a plurality of antenna patterns having different polarizations or frequencies from a single reflector. 
     2. Description of the Prior Art 
     Reflector antennas are frequently used on spacecraft to provide multiple uplink and downlink communication links between the spacecraft and the ground. The downlinks operate at one frequency, typically around 20 GHz, and the uplinks operate at a second higher frequency, typically around 30 or 44 GHz. It is typically desirable for a single spacecraft to have multiple uplink and downlink antennas where each antenna provides a separate antenna pattern covering a predetermined coverage zone on the earth. It is also typically desirable to provide both an uplink and downlink antenna pattern having the same beamwidth so that users can both receive and transmit to the same spacecraft. For example, a single spacecraft may have one uplink antenna which provides a 3°×6° antenna beam at 30 GHz for uplink communications from the continental United States (CONUS), and, one downlink antenna at a frequency of 20 GHz which provides a 30°×6° beam for downlink communications to CONUS. The method typically used to provide multiple uplink and downlink antenna patterns from a single spacecraft is to provide separate reflectors for each uplink and downlink antenna. This requires a large amount of space on a spacecraft, is expensive and extracts a weight penalty. 
     One method attempted to save weight is to couple one uplink and one downlink antenna together in a single reflector body. To do so, an illumination source is configured to illuminate the reflector body with two RF signals, one having a frequency of 20 GHz and the other having a frequency of 30 GHz. The reflector is typically fabricated of a composite or honeycombed material coated with a reflective material, typically aluminum, which is reflective to RF signals of all frequencies. The disadvantage with this system is that it is difficult to provide antenna patterns having predetermined beamwidths at different frequencies from the typical reflector. The beamwidth of an antenna beam is inversely proportional to the size of the reflector and the frequency of illumination. From the same sized reflector, the uplink antenna pattern at 30 GHz would have a smaller beamwidth than the downlink antenna pattern at 20 GHz thereby covering a smaller coverage zone than the downlink antenna pattern. To address this problem, conventional reflector antennas have used specially designed feed horns configured to under illuminate the reflector at 30 GHz, the higher frequency, thereby generating an antenna pattern at 30 GHz having a wider beamwidth. This is inefficient and often difficult to do since feed horns are extremely sensitive to tolerance and bandwidth limitations. 
     A need exists to have a single reflector which provides a plurality of antenna patterns each having a predetermined beamwidth allowing a single spacecraft to carry the weight and expense of only one reflector while having the ability to provide multiple uplink and downlink antenna patterns. 
     SUMMARY OF THE INVENTION 
     The aforementioned need in the prior art is satisfied by this invention, which provides a reflector antenna having frequency selective or polarization sensitive zones to provide a plurality of antenna patterns from a single reflector body. A reflector antenna, in accord with the invention, comprises a single concave reflector body having a plurality of zones with each zone configured as a frequency selective or polarization sensitive zone. The zones can be partially, completely or not overlapping. An illumination source is configured to illuminate the reflector body with a plurality of RF signals with each zone reflecting one or more of the RF signals. The reflector body generates a plurality of antenna patterns from the reflected RF signals with the shape &amp; beamwidth of the antenna patterns being determined by the shape and dimensions of each zone. The shape and dimensions of each zone is thus preselected to provide an antenna pattern having a desired shape and beamwidth. 
     For the preferred embodiment of the invention, the reflector body has two concentric zones comprised of an inner zone and an outer zone encompassing the inner zone. The two zones are illuminated with the RF signals having frequencies of approximately 20 GHz and 30 GHz. The inner zone is comprised of a material which is reflective to RF signals of all frequencies, and, the outer zone is comprised of a material which reflects RF signals of a 20 GHz frequency and passes RF signals having a frequency of 30 GHz. The 30 GHz signal is reflected only by the inner zone and is not reflected by the second zone. Antenna patterns are generated at 20 and 30 GHz from the 20 and 30 GHz reflected signals respectively with the size and shape of only the inner zone determining the shape and beamwidth of the 30 GHz antenna pattern and the shape and beamwidth of both zones determining the shape and beamwidth of the 20 GHz antenna pattern. The dimensions of the inner and first zone are preselected to generate 20 and 30 GHz antenna patterns having approximately equal shapes and beamwidth. 
    
    
     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   a  is a top plane view of a reflector body in accordance with one embodiment of the invention; 
     FIG.  1   b  is a side plane view of a reflector antenna having the reflector body shown in FIG.  1   a;    
     FIG.  1   c  shows antenna patterns generated by the reflector antenna shown in FIG.  1   b;    
     FIG.  2   a  is a top plane view of a reflector body in accordance with a second embodiment of the invention; 
     FIG.  2   b  is a side plane view of a reflector antenna having the reflector body shown in FIG.  2   a;    
     FIG.  2   c  shows antenna patterns generated by the reflector antenna shown in FIG.  2   b;    
     FIG.  3   a  is a top plane view of circular loop frequency selective elements in accordance with a third embodiment of the invention; 
     FIGS.  3   b  and  3   c  are top plane views of nested circular loop frequency selective elements in accordance with a fourth embodiment of the invention; 
     FIG.  4   a  is a top plane view of a reflector body in accordance with a fifth embodiment of the invention; 
     FIG.  4   b  is a side plane view of a reflector antenna having the reflector body shown in FIG.  4   a;    
     FIGS.  4   c  and  4   d  show the x and y axis principle plane antenna patterns respectively generated by the reflector antenna shown in FIG.  4   b.    
     FIG.  5   a  is a top plane view of a reflector body in accordance with a sixth embodiment of the invention; 
     FIG.  5   b  is a side plane view of a reflector antenna having the reflector body shown in FIG.  5   a;    
     FIG.  5   c  shows antenna patterns generated by the reflector antenna shown in FIG.  5   b;    
     FIG.  6   a  is a top plane view of a reflector body in accordance with a seventh embodiment of the invention; 
     FIG.  6   b  is a side plane view of a reflector antenna having the reflector body shown in FIG.  6   a;    
     FIG.  6   c  shows antenna patterns generated by the reflector antenna shown in FIG.  6   b;    
     FIG.  7   a  is a side plane view of a reflector body in accordance with a eighth embodiment of the invention; 
     FIG.  7   b  is a side plane view of a reflector antenna having the reflector body shown in FIG.  7   a ; and, 
     FIG.  7   c  shows antenna patterns generated by the reflector antenna shown in FIG.  7   b.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS.  1   a - 1   c,  a reflector antenna  10  for providing multiple antenna patterns  12 - 16  is illustrated. The reflector antenna  10  can be configured as a prime focus feed reflector, an offset reflector, a cassegrain reflector or the like. The reflector antenna  10  includes a reflector body  18  and an illumination source  20 . The reflector body  18  is comprised of a plurality of zones  22 - 26  with each zone  22 - 26  configured to be a frequency selective or polarization sensitive zone. The illumination source  20  is configured to illuminate the reflector body  18  with a plurality of RF signals depicted by the lines marked  28 - 32  with each RF signal  28 - 32  being of a preselected frequency or polarization. Each zone  22 - 26  is configured to selectively reflect, pass or absorb selected RF signals  28 - 32  having preselected frequencies or polarizations. Antenna patterns  12 - 16  are generated from each reflected RF signal  34 - 38  with the characteristics of each antenna pattern  12 - 16 , including the shape and beamwidth, being determined by the shape and dimensions of the zones  22 - 28 . The size and shape of each zone  22 - 28  is preselected so that antenna patterns  12 - 16  are generated having desired shapes and beamwidths. By configuring a single reflector body  18  to comprise one or more frequency selective or polarization sensitive zones  22 - 26 , a plurality of antenna patterns  12 - 16 , each being of a preselected shape and beamwidth, can be generated from a single reflector antenna  10 . 
     For one embodiment of the invention shown in FIGS.  2   a - 2   c,  the reflector body  40  is comprised of three concentric zones  42 - 46 . The first zone  42  is configured to reflect RF signals having frequencies of f 1 -f 3 ; the second zone  44  is configured to reflect RF signals having frequencies f 2  and f 3  and pass RF signals having a frequency of f 1 . The third zone  46  is configured to reflect RF signals having frequencies of f 3  and pass RF signals having frequencies of f 1  and f 2 . The illumination source  48  is configured to generate three RF signals depicted by the lines marked  50 - 54  where each RF signal  50 - 54  is of a different frequency f 1 -f 3  respectively. 
     The first RF signal  50  is incident on the reflector body  40  with the portion of the first RF signal  50  which is incident upon the first zone  42  being reflected by the first zone  42 . However, the portion of the first RF signal  50  which is incident on the second  44  and third  46  zones is not reflected and pass through the second  44  and third  46  zones. Thus, only the first zone  42  reflects the first RF signal  50  to provide a first reflected signal  56  which will form a first antenna pattern  58  having characteristics including shape and beamwidth which are substantially determined by the shape and dimensions of only the first zone  42 . The shape and dimensions of the first zone  42  is thus preselected to provide a first antenna pattern  58  having predetermined pattern characteristics such as shape and beamwidth. 
     The first zone  42  is preferably formed of a light weight core  60  fabricated from a material such as Graphite, Kevlar™, Nomex™, aluminum honeycomb, or the like which are all commercially available materials with Kevlar™ being fabricated by Hexcel Corporation located in Huntington Beach, Calif. and Nomex™ being fabricated by Hexcel Corporation located in Huntington Beach, Calif. A highly reflective coating  62  such as aluminum is typically applied to the top surface  64  of the light weight core  60  preferably by a vapor deposition or sputtering process to provide a surface which is highly reflective to RF signals  50 - 54  of a plurality of frequencies. A more detailed description of processes such as vapor deposition or sputtering used to apply materials can be found in Microelectronic Processing and Device Design, by Roy A Colclaser, 1980. 
     The second RF signal  52  is incident on the reflector body  40  with the portion of the second RF signal  52  which is incident upon the first  42  and second  44  zones being reflected  66  by the first  42  and second  44  zones. However, the portion of the second RF signal  52  which is incident on the third  46  zone is not reflected and passes through the third  46  zone. Thus, only the first  42  and second  44  zones reflect the second RF signal  52  to provide a second reflected signal  66  which will form a second antenna pattern  68  having characteristics which are substantially determined by the shape and dimensions of both the first  42  and second  44  zones combined. 
     The third RF signal  54  is incident on the reflector body  40  and is reflected  70  by the all three zones  50 - 54 . A third antenna pattern  72  is generated from the third reflected RF signal  70  with characteristics associated with the dimensions of all three zones  42 - 46  combined. 
     Each frequency selective zone  44  &amp;  46  is typically comprised of a patterned metallic top layer  74  or  76  over a dielectric core  78  or  80  respectively. The dielectric cores  78  and  80  are fabricated of materials such as Kevlar™, Nomex™, Ceramic Foam, Rohacell foam™ or the like which are commercially available materials known in the art to pass RF signals with Rohacell foam™ being fabricated by Richmond Corporation located in Norwalk, Calif. For simplicity in manufacturing, all three cores  60 ,  78  and  80  are typically fabricated of the same materials. To produce the patterned metallic top layers  74  and  76 , a metallic top layer is first applied to the dielectric cores  78  and  80  using a vapor depositing or sputtering process and portions of the metallic top layer are removed by an etching technique thereby forming the patterned metallic top layers  78  and  80 . A more detailed discussion of vapor depositing, sputtering and etching processes can be found in the reference cited above. Alternatively, the patterned top layers  74  and  76  can be formed on separate sheets of material and then bonded to the cores  78  and  80  respectively. The patterned layers  74  and  76  typically include crosses, squares, circles, “Y&#39;s” or the like with the exact design and dimensions of the patterned top layers  74  and  76  being determined by experimental data coupled with design equations and computer analysis tools such as those found in the book Frequency Selective Surface and Grid Array, by T. K. Wu, published by John Wiley and Sons, Inc. The design and dimensions of the first patterned top layer  74  covering the second core  78  is selected to reflect RF signals having frequencies f 2  and f 3  and pass RF signals having a frequency of f 1 , whereas, the patterned top layer  76  covering the third core  80  is selected to reflect RF signals having a frequency of f 3  and pass RF signals having frequencies f 1  &amp; f 2 . 
     For example, referring to FIGS.  2   a,    2   b,  and  3   a,    3   b  and  3   c,  the first patterned metallic top layer  74  could consist of a plurality of singular circular loops  81  each of which having a diameter of D 1  and a width of W 1 . Alternatively, the first patterned metallic top layer  74  could consist of a plurality of nested circular loops  82  where each nested circular loop  82  is comprised of an inner loop  83  and an outer loop  84 . Each inner loop  83  has a diameter D 2  and a width W 2 , and, each outer loop  84  has a diameter D 3  and width W 3  where D 2 &lt;D 3  and W 2 &lt;W 3 . Both the singular circular loops  81  and the nested circular loops  82  will pass RF signals having a frequency of 44 GHz and reflect RF signals having frequencies of 29 and 30 GHz. Nested circular loops  82  are preferred for embodiments which pass and reflect RF signals which are closely spaced in frequency. 
     The second metallic top layer  76  could also consist of a plurality of nested circular loops  85  where each nested circular loop  85  is comprised of an inner loop  86  and an outer loop  87 . Each inner loop  86  has a diameter D 4  and a width W 4 , and, each outer loop  87  has a diameter D 5  and width W 5  where D 4 &lt;D 5  and W 4 &lt;W 5 . These nested circular loops  85  will pass RF signals having frequencies of 30 and 44 GHz but will reflect RF signals having a frequency of 20 GHz. 
     Alternatively, frequency selective zones  44  &amp;  46  can be fabricated from RF absorbing materials which absorb RF signals of preselected frequencies and reflect RF signals of other preselected frequencies. One such material is a carbon loaded urethane material manufactured by The Lockheed-Martin Corporation located in Sunnyvale Calif. 
     For the embodiment of the invention shown in FIGS.  4   a - 4   d,  the reflector antenna  86  is comprised of an offset reflector body  88  having four zones  90 - 96  with each zone  90 - 96  configured to pass or reflect RF signals, depicted by the lines marked  98 - 104  of preselected frequencies f 1 -f 4 . The illumination source  106  is comprised of four feed horns  108 - 114  with each feed horn  108 - 114  generating one of the RF signals  98 - 104  respectively. The first zone  90  is configured to be reflective to RF signals of all frequencies such that all four RF signals  98 - 104  are reflected  116 - 122  by the first zone  90 . The second zone  92  is configured to be reflective to RF signals  100 - 104  having frequencies of f 2 -f 4  and pass RF signals  98  having a frequency of f 1  such that the second  100  through fourth  104  RF signals are reflected  118 - 122  by the second zone  92  and the first RF signal  98  passes through the second zone  92 . The third zone  94  is configured to be reflective to RF signals  102  and  104  having frequencies of f 3  &amp; f 4  and pass RF signals  98  and  100  having frequencies of f 1  &amp; f 2  such that the third  102  and fourth  104  RF signals are reflected  120  and  122  by the third zone  94  and the first  98  and second  100  RF signals pass through the third zone  94 . The fourth zone  96  is configured to reflect an RF signal  104  having a frequency of f 4  and pass RF signals  98 - 102  having frequencies of f 1 -f 3  such that the fourth  104  RF signal is reflected  122  by all from zones  90 - 96 . 
     The dimensions of each zone  90 - 96  determines the characteristics of the antenna patterns  124 - 130  generated therefrom. FIGS.  4   c  and  4   d  shows the principal plane cuts of the antenna patterns generated by the antenna  86  in the x and y planes (FIG.  4   a ) respectively. The first  90  and third  94  zones are configured in elliptical shapes, and, the second  92  and fourth  96  zones are configured in circular shapes. Thus, the antenna patterns  130  and  126  generated from the first  116  and third  120  reflected signals will have elliptical pattern shapes and the antenna patterns  128  and  124  generated from the second  118  and fourth  122  reflected signals will have circular pattern shapes. This embodiment of the invention generates four antenna patterns  124 - 130  from a single reflector antenna  86  with each antenna pattern having a predetermined shape and being of a different frequency f 1 -f 4  respectively. 
     Referring to FIGS.  5   a - 5   c,  for a second embodiment of the invention, the first zone  132  reflects all RF signals, the second zone  134  is a polarization sensitive zone; and, the third zone  136  is both a frequency selective and polarization sensitive zone. 
     Polarization sensitive zones will pass RF signals having one sense of polarization and reflect orthogonally polarized signals. For example, a polarization sensitive zone will either pass horizontally polarized RF signals and reflect vertically polarized RF signals or pass vertically polarized RF signals and reflect horizontally polarized RF signals. Like the frequency selective zones described in the embodiments above, polarization sensitive zone are typically comprised of a patterned metallic top layer over a dielectric core. For horizontally or vertically polarized RF signals, the patterned top layer typically includes metallic parallel lines oriented such that an RF signal having one sense of polarization is passed through and an orthogonally polarized RF signal is reflected. Using polarization sensitive zones enables two oppositely polarized RF signals operating at the same frequency to be coupled in a single reflector with each reflected RF signal providing a separate antenna pattern having a desired shape and beamwidth. 
     For example, the first zone  132  is configured to reflect all RF signals. The second zone  134  is configured as a polarization sensitive zone  134  designed to reflect all vertically polarized RF signals regardless of the frequency. The third zone  136  is configured to be both a frequency selective and polarization sensitive zone  136  which is designed to reflect only vertically polarized RF signals having a frequency of f 2 . 
     The reflector  138  is illuminated by three RF signals, depicted by the lines marked  140 - 144 . The first RF signal  140  is at a first frequency f 1  and is horizontally polarized. This RF signal  140  will be reflected  146  by the first zone  132  but will pass through the second  134  and third  136  zones. A horizontally polarized antenna pattern  152 , having a frequency of f 1 , and having characteristics determined by the dimensions of the first zone  132  will be generated from the first reflected signal  146 . 
     The second RF signal  142  is also at a frequency of f 1  but is vertically polarized. This second RF signal  142  will be reflected  148  by both the first  132  and second  134  zones but will pass through the third zone  136 . A vertically polarized antenna pattern  154 , having a frequency of f 1 , and having characteristics determined by the characteristics of both the first  132  and second  134  zones will be generated from the second reflected signal  148 . 
     The third RF signal  144  is also vertically polarized but is at a different frequency f 2 . The third zone  136  is both a frequency selective and a polarization sensitive zone  136  configured to pass all horizontally polarized RF signals regardless of frequency but reflect vertically polarized RF signals of a frequency f 2 . The third RF signal  144  will be reflected  150  by all three zones  132 - 136 . A vertically polarized antenna pattern  156 , having a frequency of f 2 , and having characteristics determined by the characteristics of the entire reflector  138  will be generated from the third reflected signal  150 . 
     For the embodiment of the invention shown in FIGS.  6   a - 6   c,  the reflector antenna  158  generates two antenna patterns  160  and  162  each having approximately the same shape and beamwidth with the first antenna pattern  160  being at a frequency of approximately 20 GHz and the second antenna pattern  162  being at a frequency of approximately 30 GHz. The reflector antenna  158  includes an illumination source  164  and a reflector body  166 . The illumination source  164  is configured to illuminate the reflector body  166  with two RF signals, depicted by the lines marked  168  and  170 . The first  168  and second  170  RF signals have frequencies of 20 &amp;    30   GHz respectively. The first zone  172  of the reflector body  166  is configured to be reflective to RF signals having frequencies of 20 and 30 GHz and the second zone  174  is a frequency selective zone  174  which is configured to be reflective to RF signals having a frequency of 20 GHz and pass RF signals having a frequency of 30 GHz signal. The first  172  and second  174  zones of the reflector body  166  are dimensioned to generate antenna patterns  160  and  162  having equal beamwidths at frequencies of 20 and 30 GHz respectively. Since the beamwidth of an antenna pattern  160  and  162  is inversely proportional to both the frequency and the diameter d 1  or d 2  of the reflective zones  172  and  174 , generating the antenna pattern  160  and  162  respectively, to generate antenna patterns at both 20 and 30 GHz which have the same beamwidth, the diameter d 1  of the first zone  172  should be approximately two thirds the diameter d 2  of the second zone  174 . 
     Referring to FIGS.  7   a - 7   c,  the present invention is not limited to antenna reflectors having concentric zones but may be implemented with a reflector body  176  having a plurality of zones  178 - 184  located within the reflector body  176 , with each zone  178 - 184  being of a preselected shape and dimension. For this embodiment, the illumination source  186  is configured to generate three RF signals, depicted by the lines marked  188 - 192 . The first and second zones  178  and  180  are configured to reflect the first RF signal  188  generating a first antenna pattern  194  therefrom whereas the third  182  and fourth  184  zones are configured to pass the first RF signal  188 . The second  180  and third  182  zones are configured to reflect the second RF signal  190  generating a second antenna pattern  196  therefrom whereas the first  178  and fourth  184  zones are configured to pass the second RF signal  190 . All four zones  178 - 184  are configured to reflect the third RF signal  192  and generate a third antenna pattern  198  therefrom. 
     The portions of the first  188  and second  190  RF signals which pass through zones  178 - 184  of the reflector body  176  can create problems in other electronic components (not shown) being in a close proximity to the reflector body  176 . RF absorbing material  200  can be attached to the bottom side  202  of the reflector body  176  and absorb the passed through RF signals  188 - 190 . 
     It is typically desirable for the antenna patterns  196 - 198  generated from a reflector body  176  to have low sidelobe levels  204 - 208 . To do so, a ring of resistive material  210 , such as R-card™ manufactured by Southwall Technologies Corporation located in Palo Alto, Calif. can be coupled to the reflector body  176 . Analysis has shown that the sidelobe levels  204 - 208  of an antenna pattern  194 - 198  generated by a reflector body  176  is decreased when resistive material  210  is coupled to the edge of a reflector body  176 . 
     The present invention utilizes a preselected plurality of frequency selective and/or polarization sensitive zones to provide multiple antenna patterns from a single reflector antenna. By configuring each zone to a preselected shape and dimension, the present invention generates a plurality of antenna patterns from a single reflector body with each antenna pattern having a desired shape and beamwidth. In this manner, a single reflector can replace multiple reflector antennas saving weight, cost and real estate. 
     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.