Abstract:
An apparatus for reducing earth station interference in a receiver antenna from non-GSO and terrestrial sources is disclosed. The apparatus comprises an absorber coupled to a receiver antenna feed assembly disposed between the non-GSO or terrestrial source and the feed assembly. Embodiments are disclosed in which the absorber is strategically placed where it minimally affects the receiver antenna mainlobe performance, while reducing interference from non-GSO and terrestrial sources.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following co-pending and commonly assigned patent application, which application is incorporated by reference herein: 
     Application Ser. No. 09/480,089, entitled “METHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES,” filed on Jan. 10, 2000, by Paul R. Anderson, attorney&#39;s docket number PD-990074. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods receiving broadcast signals, and in particular to a system and method for receiving satellite broadcasts while reducing interference from terrestrial sources or from satellite sources such as nongeostationary fixed satellite service networks. 
     2. Description of the Related Art 
     It has been proposed to cooperatively share the current Broadcasting-Satellite Service (BSS) frequency bands to allow additional programming material to be transmitted to BSS users or subscribers using the same frequency bands as currently used by BSS satellites. This may be implemented through the use of non-geostationary orbit (GSO) and/or terrestrially-based transmitters to transmit the additional programming. Such systems typically rely on spatial diversity to minimize the probability of interference. This usually requires a BSS satellite ground antenna having highly directional, monocular sensitivity characteristics in order to realize low interference levels. 
     Unfortunately, existing BSS antennae do not exhibit a highly directional sensitivity characteristic. Instead, as described in application Ser. No. 09/480,089, entitled “METHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES,” which application is hereby incorporated by reference, existing BSS antennae exhibit a sensitivity characteristic that includes substantial sensitivity in a rearward direction. They also exhibit a sensitivity characteristics in the sideward and upward directions. This sensitivity can result in substantial interference between transmissions from BSS satellites and transmissions from non-GSO or terrestrial sources. 
     U.S. Pat. No. 3,430,244, issued to H. E. Barlett et al. discloses a transmitting reflector antenna. The transmitting antenna includes a solid dielectric guiding structure imposed between the feed and the reflector. The dielectric surface acts as a lens to direct the radiation emanating from the feed at the reflector surface. Because the incident angle of the electromagnetic energy from the phase center of the horn to the lens is at a small angle, the electromagnetic energy is largely reflected. If not for the lens, the electromagnetic energy would emanate from the phase center of the horn and continue beyond and behind the reflector surface, thus creating spillover. While this design reduces spillover, this design requires use of an expensive dielectric structure extending from the horn to the reflector surface, thus complicating installation, and requires a modified reflector surface in order to direct the rays where required. The design can also result in significant phase distortion. 
     U.S. Pat. No. 3,176,301 issued to R. S. Wellons et al. discloses an antenna design having multiple feeds. A cylindrical metallic shield is placed on the periphery of the reflector and a second cylindrical metallic shield is placed surrounding the feeds to reduce spillover. While this design can reduce spillover, the metallic surface permits reflections within the shield itself, potentially compromising the spillover reduction, and permitting distortion of the received signal. The reflections within the metallic shield are also made worse because the shield itself is distant from each of the horns. Further, the metallic shield is not easily attached to the assembly of horns. 
     U.S. Pat. No. 3,706,999, issued to Tocquec et al. discloses a Cassegraninan antenna with a design that is said to reduce spillover energy. However, exising BSS antennae are simple offset reflector designs and cannot be easily modified in accordance with the disclosed Cassegranian design. 
     U.S. Pat. No. 4,263,599, issued to Bielli et al. discloses a parabolic reflector antenna having a reflector periphery lined with absorbent material to reduce spillover. While design reduces spillover, it requires the use of a substantial amount of absorbent material. 
     U.S. Pat. No. 4,380,014, issued to Howard, U.S. Pat. No. 4,803,495, issued to Monser et al., U.S. Pat. No. 5,905,474 issued to Nagi et al., and U.S. Pat. No. 5,959,590 issued to Sanford et al. each disclose designs which reduce spillover. However, in each case, the design disclosed is not one that can be obtained with simple modification of existing BSS antennae. 
     What is needed is an inexpensive, but effective way to modify the sensitivity characteristic of existing BSS antennae to reduce the interference from non-GSO and terrestrial broadcast sources. The present invention satisfies this need. 
     SUMMARY OF THE INVENTION 
     To address the requirements described above, the present invention discloses an antenna for receiving electromagnetic energy from a first transmitter and substantially rejecting electromagnetic energy from a second transmitter spatially diverse from the first transmitter. The antenna comprises a reflector having a reflecting surface for reflecting and focusing the electromagnetic energy from the first transmitter to at least one focal point; a feed assembly for receiving the reflected electromagnetic energy, the feed assembly having a sensitive axis facing the reflecting surface wherein the feed assembly and the reflector together define a spillover region bounded by a feed assembly beamwidth extending from the sensitive axis at least partially beyond the reflector surface; and an electromagnetic energy absorber, attached to the feed assembly and disposed at least partially between the spillover region and the feed assembly. The present invention is also described by a method of receiving electromagnetic energy from a first transmitter and substantially rejecting electromagnetic energy from a second transmitter spatially diverse from the first transmitter. The method comprises the steps of receiving electromagnetic energy from the first transmitter reflected by a reflector surface in a feed assembly, the feed assembly and reflective surface together defining a spillover region defined by a feed assembly beamwidth extending from a feed assembly sensitive axis at least partially beyond the reflector surface; and absorbing the electromagnetic energy from the second transmitter with an absorber coupled to the feed assembly and disposed at least partially between the spillover region and the feed assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 is a diagram showing one embodiment of a satellite receive antenna; 
     FIGS. 2 a-b  is a diagrams showing a sensitivity characteristic of a representative satellite receive antenna; 
     FIG. 3 is a diagram depicting a top view of the satellite receive antenna spillover lobe geometry; 
     FIG. 4 is a diagram of one embodiment of the present invention in which the absorber is placed within the feed assembly horn; 
     FIGS. 5A-5D are diagrams presenting cross sections of a plurality of embodiments of the present invention; 
     FIG. 6 is a diagram illustrating another embodiment of the present invention wherein the absorber is disposed only where required to prevent interference from a stationary transmitter; 
     FIG. 7 is a diagram showing typical physical dimensions for a feed assembly; 
     FIG. 8 is a diagram illustrating an approach to reduce the effect of spillover sidelobes; 
     FIG. 9 is a diagram illustrating a further embodiment of the present invention; 
     FIG. 10 is a diagram illustrating an embodiment utilizing a feed horn extension and absorbers coupled to the reflector; and 
     FIG. 11 is a diagram presenting illustrative operations that can be used to practice one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
     FIG. 1 is a diagram of one embodiment of satellite receive antenna  100  configured to receive transmissions from BSS satellites. The satellite receive antenna  100  includes a reflector  102 , which reflects and focuses the energy from the satellite transmitter  110  on a means for receiving the signal from the BSS satellite (e.g. a feed  104  such as a low noise block converter (LNB)) disposed at an angle (in one embodiment, 22.5 degrees)  106  from the centerline  108  of the reflector  102 . This angle positions the LNB  104  out of the way to minimize attenuation of the incoming signal along the antenna centerline or boresight. In one embodiment, the reflector  102  may be parabolic with a slightly ovoid shape to account for the offset in LNB  104  position. 
     The polar sensitivity characteristic of the satellite receive antenna  100  is a function of a number of interrelated physical and electrical antenna characteristics. These characteristics include, among other things, the sensitivity characteristics and physical location of the LNB  104  relative to the reflector  102 , and the shape of the surface of the reflector  102 . 
     For example, the LNB  104  may be disposed closer to the surface of the reflector  102 , but the focus of the parabolic reflector  102  (and hence its external surface contour) must be changed to account for this modified LNB  104  location. Further, the beamwidth along the sensitive axis of the LNB  104  must be modified to achieve the desired antenna sensitivity. Similarly, the LNB  104  may be placed farther away from the reflector  102 , and other antenna  100  parameters must be modified to reflect this difference. 
     To maximize the antenna sensitivity along its centerline  108 , it is desirable that the beamwidth of the sensitive axis of the LNB  104  be wide enough to accept signals from as much of the reflector  102  surface as possible, including the outer periphery. At the same time, if the beamwidth of the LNB  104  is too wide (exceeding the periphery of the reflector  102 ), spillover signals from a non-GSO satellite  112  or a terrestrial transmitter  114  from behind the reflector  102  can be received by the LNB  104 . In such cases, the sensitivity characteristic of the antenna  100  will include sidelobes in the posterior (rear) side of the antenna  100  having a significant sensitivity. 
     FIGS. 2A and 2B are diagrams depicting the sensitivity characteristic of a representative satellite receive antenna  100 . FIG. 2A depicts an azimuthal slice of the antenna characteristic, while FIG. 2B shows a slice along the elevation direction at a zero azimuth angle. 
     FIG. 2A discloses an azimuthal sensitivity characteristic including an anteriorly-disposed main lobe  202  substantially aligned along a primary sensitive axis  204 , and a plurality of sidelobes  210 A,  210 B,  206 A, and  206 B. Nulls such as null  212 A and null  212 B are disposed between the sidelobes  210 A,  210 B,  206 A, and  206 B. Nulls  212 A and  212 B are disposed substantially along null axes  214 A and  214 B. Posterior sidelobes  206 A and  206 B are substantially along secondary sensitive axes  208 A and  208 B, respectively. As described above, the posterior sidelobes  206 A and  206 B are the result of satellite receive antenna design compromises, resulting, among other things, in spillover from the rear of the reflector  102  to the feed or LNB  104 . 
     FIG. 2B discloses an elevation sensitivity characteristic including the main lobe  202 , sidelobes  216 A and  216 B substantially along sidelobe axes  218 A and  218 B. Nulls  222 A and  222 B are disposed along null axes  222 A and  222 B, respectively, between the main lobe  202  and the sidelobes  216 A and  216 B, as well as between other sidelobes not illustrated. The depictions of the mainlobe  202  and sidelobes in FIGS. 2A and 2B above are intended to be representative depictions of the polar sensitivity characteristic of a satellite receive antenna  100  by which the present invention may be practiced. The present invention could be practiced with antennae having sensitivity characteristics with different lobes and null patterns with suitable modification. 
     FIG. 3 is a diagram showing the satellite receive antenna spillover lobe geometry. The source of the satellite receive antenna spillover lobes  206 A and  206 B is the relationship between the beamwidth  304  of the LNB  104  about the LNB sensitive axis  306 , the diameter of the reflector  102 , and the distance of the LNB  104  from the reflector  102 . When the beamwidth  304  of the LNB  104  about the LNB  104  sensitive axis  306  exceeds the diameter of the reflector  102 , electromagnetic energy from behind the reflector  102  can be sensed by the LNB  104 . This allows the satellite receive antenna  100  to have a gain characteristic with significant posterior lobes  206 A and  206 B. As shown in FIG. 2, the peak of the posterior side lobe (or spillover lobe  206 ) is at an angle 180°-S degrees from the satellite receive antenna  100  boresight  108 , where S represents the angle (in degrees) between the rear-facing portion of the antenna centerline  206  and the peak of the posterior side lobe  206  in direction  302 . The geometry of the reflector  102 , feed assembly  104  and the the beamwidth  304  of the feed assembly  104  define a spillover region  308 . 
     FIG. 4 is a diagram illustrating one embodiment of the present invention in which an electromagnetic energy absorber  402  is placed within the feed assembly horn. The dimensions of the absorber  402  are determined from the relative geometry of the reflector  102 , the feed horn  404 , the phase center  406  of the horn  404 , and the beamwidth  304  of the feed horn assembly. The dimensions of the absorber  402  are selected so that electromagnetic energy following path  408  (from the intended transmitter (e.g. the satellite  110 ) to the reflector  102  and reflected towards the feed assembly  104  by the reflective surface  410 ) is not adversely attenuated or absorbed by the absorber  402  to a significant degree, while electromagnetic energy following path  412  (spillover) is attenuated by the absorber  402 . 
     FIG. 5A is a diagram presenting a cross section of another embodiment of the present invention. In the illustrated embodiment, the absorber  402  is disposed on an inner surface  502  of the horn  404 . The absorber  402  can be sized so that the dimension d 1  proximate the outer periphery  504  of the horn  404  and the dimension in the inner horn d 2  are equal, or different. The insertion of the absorber  402  can change boundary conditions and the sidelobe and mainlobe patterns of the antenna  100 , but by judicious selection of dimensions d 1  and d 2 , spillover may be substantially attenuated while allowing the mainlobe to remain effectively unaltered. The absorber  402  need not extend from the outer periphery  504  of the horn  404  to the inner horn. Instead, the length l of the absorber  402  can also be selected to effect a compromise between spillover suppression and mainlobe performance. Unlike dielectric materials which are either transparent or reflective to electromagnetic energy depending on the incident angle of the energy on the surfaces of the dielectric, the absorber  402  illustrated above is substantially opaque at all incident angles. 
     FIG. 5B is a diagram of another embodiment of the present invention in which the absorber  402  is disposed on the feed horn  404  aperture. In this embodiment the absorber  402  is disposed circumferentially on an outer periphery  504  and parallel to the sensitive axis of the feed horn  404 . The length l and the thickness t of the absorber  402  can be selected to maximize spillover suppression while minimizing the effect on mainlobe performance. Further, the absorber structure shown in FIG. 5B can be used in combination with the absorber  402  shown in FIG.  5 A. 
     FIG. 5C is a diagram of another embodiment of the present invention. In this embodiment, the absorber  402  is disposed on an outer periphery  504  of the feedhorn  404 , however, the absorber is disposed perpendicular to the sensitive axis of the feed horn assembly  104 . The dimensions of the absorber  402  (length and thickness) can also be selected to maximize spillover suppression while minimizing any effects on mainlobe performance. 
     FIG. 5D is a diagram of another embodiment of the present invention. Typically, the feed horn  404  of the present invention is protected by a electromagnetic energy-transparent cap  508 . The absorber  402  can be integrated with or attached to the cap  508 . In this embodiment, the absorber  402  can be an electromagnetic absorbing paint or an absorbent material. This embodiment has the advantage of not exposing the absorbent material to the atmosphere or the sun (typically, the cap is optically opaque). In an alternative embodiment, the cap  508  remains electromagnetically transparent, but a second cap having the absorber  402  is attached over the cap  508 . This cap can be installed as a part of a retrofit kit for the consumer. 
     It is noted that in embodiments wherein the absorber  402  is asymmetrically disposed (more or less absorbent material on different parts of the cap  508 ), it may be advantageous to include a reference on the cap so that the absorbent material is oriented properly relative to the reflector  102  and the sources of interfering electromagnetic energy. This reference allows the user to place the cap  508  on the feed horn  404  with the proper rotation angle about the sensitive axis  306 . 
     FIG. 6 is a diagram of another embodiment of the present invention wherein the absorber  402  is disposed only between a second (and potentially interfering) transmitter and the feed assembly. This embodiment is particularly useful in situations where spillover is only an issue for substantially stationary transmitters. For example, if spillover allows terrestrially located transmitters to interfere with the reception of electromagnetic energy from a BSS transmitter, the absorbent material need only be placed between these terrestrially located transmitters and the feed horn assembly, and not on the entire feed horn assembly. This embodiment is also particularly useful with reflective antennae that are of an offset feed design, such as those used to receive BSS satellite broadcasts, since the spillover pattern for such antennae are asymmetric (the asymmetric nature of the spillover pattern for such antennae are fully discussed in application Ser. No. 09/480,089, entitled “METHOD AND APPARATUS FOR MITIGATING INTERFERENCE FROM TERRESTRIAL BROADCASTS SHARING THE SAME CHANNEL WITH SATELLITE BROADCASTS USING AN ANTENNA WITH POSTERIOR SIDELOBES.”) Although the absorber  402  illustrated in FIG. 6 includes a first portion  402 A and a second portion  402 B, more portions, or only a single portion may be employed. Further, the shape of the absorber portions  402 A and  402 B may be modified to account for the transmitting characteristics of the second (and interfering transmitter), and thus, each portion may have different dimensions and be located on different portions of the feed horn  404 . Note also that while FIG. 6 illustrates an embodiment where the absorber  402  is placed inside the feed horn  404 , this need not be the case. The absorber  402  may be placed exterior to the feed horn  404 , as illustrated in FIGS. 5B and 5C, for example. 
     It is noted that adding the absorber  402  will alter the boundary conditions of the radiation pattern of the antenna  100 . Further, the foregoing designs need not completely attenuate the spillover electromagnetic energy. Instead, substantial absorption of the spillover energy (enough to prevent interference), can be obtained while retaining effective mainlobe performance. In the foregoing examples, the absorber  402  can be fashioned from a bulk absorber or from electromagnetic energy absorbing paint. There are a wide variety of commercially available X-band/Ku-band absorbers for such purpose. 
     The foregoing designs will reduce the sensitivity of the antenna  100 . A simple estimate of the percentage of power that will be lost from the radiated beam can be performed. 
     FIG. 7 is a diagram showing typical physical dimensions of feed assembly (or LNB)  104 . From the approximate dimensions of the circular waveguide  702 , the mode in the guide is TE 11 , since this is the only TE mode that is not cut off at 12.5 GHz. The radial and azimuthal electric and magnetic fields in a 1.7 centimeter waveguide can be used to calculate the Poynting vector to provide an estimate of the power flowing in the waveguide. For example, see  Microwave Engineering, Passive Circuits,  by Rizzi, pages 233 et seq., which are hereby incorporated by reference. The field components for TE 11  mode in cylindrical coordinates, can be derived as follows:          E   r     =       -     2            E   0            λ   c       λ   g            (       λ   c       2      π                 r       )            J   1          (       2      π                 r       λ   c       )          sin                 φ                   sin        (       ω                 t     -     β                 z       )                   E   φ     =       -     2            E   0            λ   c       λ   g              J   1   ′          (       2      π                 r       λ   c       )          cos                 φ                   sin        (       ω                 t     -     β                 z       )                   H   z     =       2          H   0            J   1          (       2      π                 r       λ   c       )          cos                 φ                   cos        (       ω                 t     -     β                 z       )                   H   r     =     -       E   φ       Z   TE                   H   φ     =       E   r       Z   TE                              
     where 
     λ c =1.706D; 
     D is the diameter of the circular waveguide; 
     ω is the frequency (radians/sec) of the electromagnetic energy, 
     t is time (sec); 
     r is the radial variable in cylindrical coordinates; 
     φ is the angular variable in cylindrical coordinates; 
     z in the axial variable in cylindrical coordinates; 
     J 1  is the first order Bessel Function of the First Kind; 
     J 1 ′ is the first derivative of J 1 ; 
     E 0  is a scalar whose value depends on the power transmitted through the circular waveguide; 
     E φ  is the electric field in the azimuthal direction; 
     E r  is the electric field in the radial direction, 
     β is equal to (ω 2 μ∈−k c   2 ) ½ ; 
     k c  is equal to 2π/λ c ; 
     μ is the permeability of the air-filled cylindrical waveguide, and is equal to the permeability of free space, 4π×10 −7  Henry/m; 
     ∈ is the permittivity of the air-filled cylindrical waveguide, and is equal to the permittivity of free space, 8.85×10 −12  Farad/m; 
     H 0  is equal to E o /Z TE ; 
     Z TE  is the impedance of the TE 11  mode in the cylindrical waveguide; 
     H r  is the magnetic field intensity in the radial direction; 
     H φ  is the magnetic field intensity in the azimuthal direction; 
     H z  is the magnetic field intensity in the axial direction; 
     λ g =λ 0 [1−(λ 0 /λ c ) 2 ] −0.5 ; 
     λ 0  is the free space electromagnetic wavelength at the frequency of interest; and radial, axial and azimuthal directions are as defined for a cylindrical coordinate system. 
     Forming the cross product of E and H yields the z-component of the Poynting vector, which has a value of            E   r   2     +     E   φ   2         Z   TE                            
     in the z direction (i.e., out of the waveguide). 
     Using the equations above, the Poynting vector can be simplified to 
     
       
           f (φ, r )=α└( k   c   r ) −2 ( J   1 ( k   c   r )) 2  sin 2 (φ)+( J   1 ′( k   c   r )) 2  cos 2 (φ)┘, 
       
     
     where 
     
       
           k   c =2π/λ c   
       
     
     and 
     α is a constant that does not depend on r or φ. 
     Integrating the expression for power flux density over the unblocked aperture (in terms of coordinates r and φ) allows the power flux across different portions of the waveguide aperture to be estimated. 
     For a waveguide diameter of 1.7 cm, approximately 11% of the power would be affected by a ring of absorbing material 0.1 cm wide around the outer edge of the waveguide aperture. Interestingly, the reduction in the cross-sectional area of the waveguide (from a diameter of 1.7 to 1.6 cm) is also about 11%. 
     While the foregoing computations involve the waveguide aperture (which is more easily solved, as expressions for the electric and magnetic fields are easily derived), the foregoing can be extended by scaling the sizes of the ring of absorbing material and the horn aperture. This implies that the ring of absorber could be at least a few millimeters wide along the outer edge of the horn. 
     Another simple scaling approach can be used in which the reduction in area of the horn aperture as seen by a ray entering the horn through the spillover sidelobe is used to estimate the reduction in the mainlobe sensitivity. For an angle of 60 degrees, the horn aperture area is          Area   1     =         π        [       (     diameter   2     )     2     ]            [     cos        (   φ   )       ]               2                              
     without the absorber ring, and          Area   2     =         π        [       (       diameter   -   0.6     2     )     2     ]            [     cos        (   φ   )       ]               2                              
     with the absorber ring, where φ is the angle between the feed assembly sensitive axis  306  and the direction of the ray (see for example, FIG.  8  and accompanying text below). With diameter=5 centimeters and φ=60 degrees, Area 1 =4.9 cm 2  and Area 2 =3.8 cm 2 . This is an area reduction of about 22%. 
     Another approach can be used to reduce the effect of the spillover sidelobes. FIG. 9 is an illustration of the deployment of an absorber  402  that can be used to ameliorate the spillover energy of the antenna. Using the dimensions for the example shown in FIGS. 7 and 8, A=45 degrees and B=36.7 degrees. For this case, an absorber with a length of about 0.9 cm will block the spillover sidelobe from the center of the waveguide aperture. This configuration both reduces the spillover sidelobe while also minimally perturbing the antenna&#39;s main lobe radiation pattern. The spillover sidelobe is not reduced to zero, but a useful reduction in spillover sidelobe power is expected. Note that the length of the absorber  402  can be increased or decreased, depending on the precise geometry for the reflector and feed. 
     FIG. 10 is a diagram illustrating another embodiment of the present invention. In this embodiment, elements  1002 A and/or  1002 B, which are substantially opaque to the electromagnetic energy are affixed to the reflector  102 . Elements  1002 A and/or  1002 B can comprise material that either absorbs or reflects electromagnetic energy. Element(s)  1002 A/ 1002 B can be placed around the entire periphery of the reflector  102 , or only in locations where required to block electromagnetic energy from the second (and interfering) transmitter. Elements  1002 A/ 1002 B can be placed at a variety of desired angles θ, including an angle which essentially extends the aperture of the antenna by extending the edge of the reflector  102 . In one embodiment of the present invention, element  1002  is configured to allow attachment to the reflector, and can be bent to the proper angle as desired. This embodiment allows a technician or a customer to install the element  1002  and modify it as required to minimize spillover yet maintain mainlobe performance. 
     Those skilled in the art will recognize many modifications may be made to this configuration without departing from the scope of the present invention. For example, those skilled in the art will recognize that any combination of the above components, or any number of different components, peripherals, and other devices, may be used with the present invention. 
     FIG. 11 is a flow chart presenting illustrative process steps that can be used to practice one embodiment of the present invention. In block  1102 , electromagnetic energy is received from a first transmitter  110 . The electromagnetic energy has been reflected by the reflector surface  410  to a feed assembly  104 . The feed assembly  104  and the reflector surface  410  together define a spillover region  308  bounded by the beamwidth  304  extending from a feed assembly sensitive axis  306  to at least partially beyond the reflector surface  410 . In block  1104 , the electromagnetic energy is absorbed with an absorber  402  coupled at least partially between the spillover region  308  and the feed assembly  104 . 
     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. For example, while the foregoing has been described with respect to an antenna having a reflector and a single feed assembly, the present invention may be practiced in embodiments using multiple feed assemblies. 
     It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.