Abstract:
A method of reducing blockage in a reflector antenna includes disposing a feed mechanism in front of a first reflector and disposing a second reflector in front of the feed mechanism. The second reflector permits energy to pass that would otherwise have been blocked from being received or transmitted by the first reflector. A reflector antenna is also formed in accordance with this method. Another method of reducing blockage in a reflector antenna includes disposing a first feed mechanism in front of a first reflector and disposing a second antenna in front of the first feed mechanism. The first feed mechanism blocks energy from being received or transmitted by the first reflector. The second antenna receives or transmits energy blocked by the first feed mechanism. A reflector antenna is also formed in accordance with this method.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/540,137, filed Jan. 29, 2004, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to reflector antennas, and more particularly relates to a method and apparatus that reduce the effects of collector surface blockage in reflector antennas while increasing antenna gain and efficiency.  
         [0004]     2. Description of the Prior Art  
         [0005]     Parabolic antennas have been used for many years as an inexpensive fixed beam antenna in both transmit and receive applications.  FIG. 1  shows an example of a center-feed parabolic antenna  10 , which has been in common use in backyards as a satellite reception antenna.  
         [0006]     Center-feed parabolic antennas  10  work very well in such applications. However, when sidelobe reduction is either desired or required, performance of this type of reflector antenna is limited by blockage of its collector surface  12  by its antenna feed structure  14 . This blockage causes discontinuities in the illumination of the parabolic collector surface  12 , which are manifested by an increase in undesirable sidelobe levels.  FIG. 2  shows an antenna pattern of a parabolic antenna, such as the antenna  10  shown in  FIG. 1 , in which the sidelobe levels  16  have been increased due to blockage by its feed structure  14 .  
         [0007]     Therefore, there is an obvious need for a method of reducing the effects of collector surface blockage by feed structures and/or subreflectors in all types of reflector antennas.  
       OBJECTS AND SUMMARY OF THE INVENTION  
       [0008]     It is an object of the present invention to provide a method and apparatus for achieving substantially ideal performance characteristics from a reflector antenna.  
         [0009]     It is another object of the present invention to provide a method and apparatus for increasing antenna gain and efficiency, as well as reducing sidelobe levels of a reflector antenna.  
         [0010]     It is yet another object of the present invention to provide a method and apparatus for eliminating the effects of collector surface blockage by a feed mechanism or subreflector in a reflector antenna.  
         [0011]     A method of reducing blockage in a reflector antenna in accordance with one form of the present invention, which incorporates some of the preferred features, includes disposing at least a portion of a feed mechanism in front of a first reflector and disposing at least a portion of a second reflector in front of the feed mechanism. The feed mechanism is adapted to receive or transmit energy. At least a portion of the second reflector is adapted to permit energy to pass therethrough. The energy passing through the second reflector would otherwise have been blocked from being received or transmitted by the first reflector.  
         [0012]     A reflector antenna formed in accordance with another form of the present invention, which incorporates some of the preferred features, includes a first reflector, a feed mechanism, and a second reflector. At least a portion of the feed mechanism is disposed in front of the first reflector and adapted to receive or transmit energy. At least a portion of the second reflector is disposed in front of the feed mechanism. At least a portion of the second reflector is adapted to permit energy to pass therethrough, which would otherwise have been blocked from being received or transmitted by the first reflector.  
         [0013]     A method of reducing blockage in a reflector antenna in accordance with yet another form of the present invention, which incorporates some of the preferred features, includes disposing at least a portion of a first feed mechanism in front of a first reflector and disposing at least a portion of a second antenna in front of the first feed mechanism. At least a portion of the first feed mechanism blocks energy from being received or transmitted by the first reflector. The first feed mechanism is adapted to receive or transmit energy. The second antenna is adapted to receive or transmit at least a portion of the energy blocked by the first feed mechanism.  
         [0014]     A reflector antenna formed in accordance with still another form of the present invention, which incorporates some of the preferred features, includes a first reflector, a first feed mechanism, and a second antenna. The first reflector is adapted to receive or transmit energy. At least a portion of the first feed mechanism is disposed in front of the first reflector. At least a portion of the first feed mechanism blocks energy from being received or transmitted by the first reflector. The first feed mechanism is adapted to receive or transmit energy. At least a portion of the second antenna is disposed in front of the first feed mechanism, and is adapted to receive or transmit at least a portion of the energy blocked by the first feed mechanism.  
         [0015]     These and other objects, features, and advantages of this invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is conventional parabolic satellite television antenna, which includes a prime focus feed mechanism.  
         [0017]      FIG. 2  is an antenna pattern of a parabolic antenna, which exhibits blockage of its collector surface by its feed mechanism.  
         [0018]      FIG. 3  is an ideal antenna pattern of a parabolic antenna formed in accordance with the present invention, in which blockage of its collector surface by its feed mechanism has been substantially eliminated.  
         [0019]      FIG. 4   a  is a conventional parabolic satellite television antenna, which incorporates a cassegrain geometry.  
         [0020]      FIG. 4   b  is a conventional parabolic satellite television antenna, which incorporates a gregorian geometry.  
         [0021]      FIG. 5  is a plot of antenna field strength as a function of antenna aperture for a conventional reflector antenna that exhibits blockage of its collector surface by its feed mechanism.  
         [0022]      FIG. 6   a  is an embodiment of the present invention in which energy is allowed to pass through a leaky subreflector to mitigate the effects of collector surface blockage.  
         [0023]      FIG. 6   b  is a plot of antenna field strength as a function of antenna aperture, in which a shadow caused by feed mechanism blockage has been filled in by leaked energy in accordance with the present invention.  
         [0024]      FIG. 7  is a pictorial representation of a conventional parabolic collector surface having a prime focus feed mechanism, which shows a shadow caused by blockage from the feed mechanism.  
         [0025]      FIG. 8  is a pictorial representation of a secondary antenna used to reduce feed mechanism blockage in accordance with the present invention.  
         [0026]      FIG. 9   a  is a first embodiment of the present invention applied to a parabolic collector surface.  
         [0027]      FIG. 9   b  is the first embodiment of the present invention applied to a FLAPS collector.  
         [0028]      FIG. 9   c  is the first embodiment of the present invention applied to a generic collector surface.  
         [0029]      FIG. 10   a  is a second embodiment of the present invention applied to a parabolic collector surface.  
         [0030]      FIG. 10   b  is the second embodiment of the present invention applied to a FLAPS collector.  
         [0031]      FIG. 10   c  is the second embodiment of the present invention applied to a generic collector surface.  
         [0032]      FIG. 10   d  is the second embodiment of the present invention applied to a parabolic collector surface and a prime focus feed mechanism.  
         [0033]      FIG. 10   e  is a second embodiment of the present invention applied to a FLAPS collector and a prime focus feed mechanism.  
         [0034]      FIG. 10   f  is the second embodiment of the present invention applied to a generic collector surface and a prime focus feed mechanism.  
         [0035]      FIG. 11  is an antenna pattern obtained from an experimental implementation of the second embodiment of the present invention, such as that shown in  FIGS. 10   a - c , in comparison with an antenna pattern exhibiting blockage of the collector surface. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]      FIG. 3  shows an ideal antenna pattern, which is one goal of a reflector antenna formed in accordance with the present invention, in which blockage of a collector or reflector surface is substantially eliminated. The present invention provides approaches that counteracts the effects of reflector surface blockage, thereby increasing antenna gain and efficiency, as well as reducing sidelobe levels  18 , as shown in  FIG. 3 .  
         [0037]     There are essentially two types of center-feed mechanisms commonly used with reflector antennas.  FIG. 1  shows a prime focus feed mechanism  14 . This type of feed mechanism is preferably placed at a focal point of a parabolic reflector surface  12 . Although the prime focus feed mechanism  14  has a straightforward design, the resulting antenna  10  exhibits inherent disadvantages in terms of increased size, and the need to route signal cables to the feed mechanism  14 , both of which affect blockage and cause increased sidelobe levels  16 , as shown in the plot of  FIG. 2 .  
         [0038]      FIGS. 4   a  and  4   b  show two additional types of antenna feed mechanism.  FIG. 4   a  incorporates a cassegrain geometry and  FIG. 4   b  incorporates a gregorian geometry. These types of feed mechanism are preferably placed near the center of the reflector surface and directed substantially upwards to illuminate a subreflector. The subreflector functions to collect energy reflected by the main reflector surface. These types of feed can provide additional form factor options that may result in decreased blockage of the reflector surface. For instance, by reducing the size of the subreflector and/or increasing the size of the main reflector, the percentage of the main reflector that is blocked in relation to the subreflector can effectively be decreased.  
         [0039]     As shown in  FIG. 4   a , an incoming ray  20  from an incoming plane wavefront  22  is preferably directed back towards a vertex  24  of a reflector surface  26  when reflected by a subreflector  28 , which is represented by a hyperboloid H. The cassegrain system shown in  FIG. 4   a  focuses by collocating one focus of the hyperboloid subreflector  28  with a focus of the reflector surface  26  at focus F 1 . Specifically, the incoming plane wave  22  is reflected from the parabolic reflector  26  and then from the subreflector  28  to finally be focused at focus F 2 , with is another focus of the hyperboloid subreflector  28 . A feed mechanism is preferably located at focus F 2  to receive the incident energy.  
         [0040]     The gregorian feed geometry shown in  FIG. 4   b  includes a subreflector  30  represented by ellipsoid E, which has a near focus F 1  collocated with a focus of a main reflector  32  represented by a paraboloid P. A feed is preferably located at F 2 , which is another focus of the ellipsoid subreflector  30 , to receive the energy from the subreflector  30 .  
         [0041]     For the types of feed mechanism shown in  FIGS. 1, 4   a , and  4   b , sidelobe performance of the center-feed reflector antenna is limited by a so-called “shadow”. This shadow is caused by the feed mechanism, in the prime focus feed antenna shown in  FIG. 1 , or the subreflector, in the cassegrain and gregorian geometry-based antennas shown in  FIGS. 4   a  and  4   b . The present invention substantially eliminates the effects of this shadow.  
         [0042]      FIG. 5  shows a plot of antenna field strength as a function of antenna aperture. This plot illustrates the effects of blockage on the illumination of a reflector surface and provides a basis for the method and apparatus formed in accordance with the present invention. A central portion  34  of the plot represents the attenuation in field strength caused by the shadow.  
         [0043]      FIG. 6   a  shows one solution in accordance with the present invention to the effects of blockage shown in  FIG. 5 , in which a leaky subreflector  50  enables a portion of the blocked energy to pass as leaked energy  51 .  FIG. 6   b  is a plot of antenna field strength as a function of aperture for the reflector antenna shown in  FIG. 6   a . The central portion  34  of the plot, which represents attenuation by the shadow, has been filled in by the leaked energy  51  shown in  FIG. 6   a , which is represented by a dotted line  36  in  FIG. 6   b , in accordance with the present invention.  
         [0044]     Thus, the resulting plot  40  in  FIG. 6   b  incorporates the contribution of leaked energy represented by the dotted line  36 , while the plot in  FIG. 5  is shown by a dashed line  38  in  FIG. 6   b . By comparing plot  40  with dashed line  38  it becomes clear that the reflector antenna formed in accordance with the present invention substantially increases field strength and significantly decreases the sidelobe levels of conventional reflector antennas. One of the goals of the present invention is to achieve the ideal performance represented in  FIGS. 3 and 6   b  by eliminating the effects of collector surface blockage.  
         [0045]      FIG. 7  is a pictorial representation of a collector surface  42  and a prime focus feed mechanism  44 . Blockage by the feed mechanism  44  is shown as a shadow  46  on a central portion of the collector surface  42 . The method and apparatus formed in accordance with the present invention essentially collect energy, which is represented by the shadow  46 , and electrically add this energy to signals actually captured by the feed mechanism  44 , thereby eliminating the effects of the shadow  46 .  
         [0046]      FIG. 8  is a pictorial representation of a secondary antenna  48  that is preferably used to reduce or eliminate the effects of the shadow on the collector surface  42  caused by feed mechanism blockage. The secondary antenna  48  is preferably placed in front of the feed mechanism  44  and has an electronic size substantially the same as the shadow  46  on the collector surface  46 .  
         [0047]     Two preferred embodiments of the present invention will now be described. Both of these embodiments may be implemented with a parabolic collector surface ( FIGS. 9   a ,  10   a ,  10   d ), a Flat Parabolic Surface (FLAPS) collector ( FIGS. 9   b ,  10   b ,  10   e ) disclosed in U.S. Pat. No. 4,905,014 to Gonzalez et al. and No. 6,198,457 to Walker et al., which are incorporated herein by reference, or any other collector surface known in the art ( FIGS. 9   c ,  10   c ,  10   f ).  
         [0048]     A first embodiment shown in  FIGS. 9   a ,  9   b , and  9   c , which is preferably applied to cassegrain or gregorian geometry-based reflector antennas, utilizes energy that passes through an electrically porous or leaky subreflector  50  in order to mitigate the shadow caused by the feed mechanism  52  on a main reflector  54 .  FIG. 9   a  shows the first embodiment applied to a parabolic collector surface  54 ,  FIG. 9   b  shows the first embodiment applied to a FLAPS collector surface  56 , and  FIG. 9   c  shows the first embodiment applied to a collector surface  58  known in the art. Energy  60  that flows through the leaky subreflector  50 , may be directionally adjusted, as well as adjusted in phase and amplitude so that it may be appropriately combined with energy  64  from the main collector surface  54 ,  56 ,  58 , thereby eliminating the effects of collector surface blockage.  
         [0049]     Direction, amplitude, and phase adjustments are preferably implemented by a lens  62 , shaped aperture, or any structure known in the art  66 , such as a dielectric coating, as shown in  FIGS. 9   a ,  9   b , and  9   c , respectively. The shape of the lens  62 , aperture, or structure  66  may limit use of the first embodiment to applications within a preferred bandwidth, such as 10% of the full bandwidth of the antenna system.  
         [0050]     As described above, the shadow  46  shown in  FIGS. 7 and 8  appears on the main collector surface  42  due to blockage by the feed mechanism  44 . This shadow  44  is also manifested as a smaller secondary shadow (not shown) on the subreflector of a cassegrain or gregorian geometry-based reflector antenna. The second embodiment of the present invention preferably provides a signal that substantially eliminates the effects of the secondary shadow on the subreflector of cassegrain or gregorian geometry antennas, which thereby eliminates the corresponding shadow on the main collector surface.  
         [0051]      FIG. 10   a  shows the second embodiment of the present invention applied to a parabolic collector surface  54 ,  FIG. 10   b  shows the second applied to a FLAPS collector surface  56 , and  FIG. 10   c  shows the second embodiment applied to a collector surface  58  known in the art. In the second embodiment, the leaky subreflector of the first embodiment shown in  FIGS. 9   a ,  9   b , and  9   c  is preferably replaced by a solid subreflector  68 . As described above, a smaller secondary shadow (not shown) is manifested in the center of the solid subreflector  68  that corresponds to the shadow on the main collector surface  54 ,  56 ,  58 .  
         [0052]     The second embodiment preferably collects energy  74  using an auxiliary antenna  70 ,  72  in substantially the same way shown in  FIG. 8  with respect to the prime focus feed mechanism and provides this energy  74  so that it may be combined with energy  64  collected from the main reflector surface  64 . Thus, the second embodiment minimizes the effects of the shadow due to feed or subreflector blockage. As described with respect to the first embodiment, the collected energy  74  may be adjusted in direction, amplitude, and phase so that it can be appropriately combined with energy  64  from the main collector surface  54 ,  56 ,  58 . These adjustments are preferably performed in the second embodiment by a lens antenna or horn antenna  70  shown in  FIGS. 10   a  and  10   b  or an alternative structure  72  known in the art shown in  FIG. 10   c.    
         [0053]     In the second embodiment, a hole  76  is preferably cut in the subreflector  68  where the blockage shadow is located. The energy from the secondary antenna  70 ,  72  is preferably routed to a secondary feed mechanism  78  placed in the hole  76  in the subreflector  68 .  
         [0054]     Placing the secondary feed mechanism  78  where the shadow is located on the subreflector  68  substantially meets the requirements of having the signals in the proper geometrical location, but it does not account for proper phasing or amplitude between the signal injected at the secondary feed mechanism  78  and the signal from the primary feed mechanism  52 .  
         [0055]     Proper phasing between these signals is preferably accomplished by introducing an electrical delay or delay element  80 ,  82  between the primary feed mechanism  52  and the secondary feed mechanism  78 . This electrical delay  80 ,  82  is preferably implemented by coupling the secondary antenna  70 ,  72  to the secondary feed mechanism  78  through a coaxial cable having a length in accordance with the desired delay. Direction, amplitude, and phase adjustments may also be implemented in the delay element  82  by means known in the art.  
         [0056]     If the delay  80 ,  82  introduced is correct to within modulo 360°, that is, the energy  74  from the secondary antenna  70 ,  72  and the energy  64  from the main collector surface  54 ,  56 ,  58  differ in phase, if at all, by a multiple of 2π radians, then the second embodiment preferably exhibits a bandwidth performance that is substantially the same as that of the first embodiment. However, if the delay  80 ,  82  introduced corresponds to that of the path length between the main reflector shadow and the subreflector  68 , and this is not modulo 360°, then the bandwidth of the second embodiment would be limited by the particular microwave components used to implement the antenna.  
         [0057]      FIGS. 10   d ,  10   e ,  10   f  provide greater detail than that shown in  FIG. 8  regarding the second embodiment of the present invention applied to a reflector antenna having a prime focus feed mechanism.  FIGS. 10   d ,  10   e , and  10   f  are substantially similar and correspond  FIGS. 10   a ,  10   b ,  10   c , except that the subreflector  68  in  FIGS. 10   a ,  10   b ,  10   c  has been replaced with a prime focus feed mechanism  84  or an alternative feed mechanism  86  known in the art in  FIGS. 10   d ,  10   e , and  10   f . The feed mechanism  84 ,  86  is preferably operatively coupled to the secondary antenna  70 ,  72  through a coaxial cable or delay element  80 ,  82 .  
         [0058]     A solid line  84  in  FIG. 11  represents an antenna pattern obtained from an experimental implementation of the second embodiment of the present invention shown in  FIG. 10   b . A dotted line  86  in  FIG. 11  represents an antenna pattern exhibiting blockage by the feed mechanism. Clearly, the level of the sidelobes shown by the dotted line  86  is substantially higher than that shown by the solid line  84 . Thus, in accordance with one goal, the method and apparatus formed in accordance with the present invention effectively reduce sidelobe levels.  
         [0059]     It is to be noted that references herein to receive and/or transmit functions apply to either and/or both of these functions, which are intended to be within the scope of the present invention in accordance with the reciprocity theorem as it relates to antenna design.  
         [0060]     Therefore, the method and apparatus formed in accordance with the present invention achieve substantially ideal performance characteristics from a reflector antenna by increasing antenna gain and efficiency, as well as reducing sidelobe levels. The method and apparatus formed in accordance with the present invention also substantially eliminate the effects of collector surface blockage by a feed mechanism or subreflector in reflector antennas.  
         [0061]     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawing, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be applied therein by one skilled in the art without departing from the scope or spirit of the invention.