Abstract:
A cup waveguide antenna with integrated polarizer and OMT for simultaneously communicating left and right hand circularly polarized electromagnetic waves is adjustable to obtain efficient propagation and reception of electromagnetic waves. The antenna includes a circular waveguide having an orthomode transducer utilizing first and second pins longitudinally spaced apart and oriented orthogonally with respect to each other. Six radially-oriented adjustable polarizer screws extend from the exterior to the interior of the waveguide. A septum intermediate the first and second pins is aligned with the first pin. Adjustment of the polarizer screws enables maximized propagation of and/or response to left hand circularly polarized electromagnetic waves by the first pin while simultaneously enabling maximized propagation of and/or response to right hand circularly polarized electromagnetic waves by the second pin.

Description:
The invention described herein was made by employees and by employees of a contractor of the United States Government, and may be manufactured and used by the government for government purposes without the payment of any royalties therein and therefor. 
    
    
     FIELD OF THE INVENTION 
     The invention is in the field of short backfire antennas with circular cylindrical waveguides capable of simultaneously propagating and receiving left and right hand circularly polarized electromagnetic waves. 
     BACKGROUND OF THE INVENTION 
     The Tracking and Data Relay Satellite System (TDRSS) is a constellation of geosynchronous satellites which are the primary source of space-to-ground voice, data and telemetry for the Space Shuttle. The satellites also provide communications with the International Space Station and scientific spacecraft in low-Earth orbit such as the Hubble Space Telescope. Integral to the design of the TDRSS class of satellites is an architecture that includes a multiple access (MA), S-band, phased array antenna. Among its capabilities, the MA system receives and relays data simultaneously from multiple lower data-rate users and transmits commands to a single user. 
     An enhanced MA array antenna element was proposed which has simultaneous circular polarization capability and increased beamwidth. If developed, simultaneous circular polarization capability (left hand circular polarization (LHCP) and right hand circular polarization (RHCP)) will be required. 
     The proposed design specifications for the enhanced MA antenna elements are set forth below. Two bandwidth requirements, for example, narrowband and wideband are included in the specification. The wideband specification includes both the system transmit and receive bands. 
     TDRSS enhanced MA antenna element specifications. 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 Narrowband frequency (GHz) 
                 2.2-2.3 
               
               
                 Wideband frequency (GHz) 
                 2.03-2.3 
               
               
                 Peak directivity (dBi) 
                 15 
               
               
                 Directivity at 20 degree cone 
                 ≧11 
               
               
                 (dBi) 
                   
               
               
                 Axial ratio (dB) 
                 &gt;−5 dB 
               
               
                 Polarization 
                 Simultaneous LHCP and RHCP 
               
               
                 Return loss (dB) 
                 ≦−20 
               
               
                 Isolation (dB) 
                 ≦−10 
               
               
                   
               
             
          
         
       
     
     Short backfire antennas are widely used for mobile satellite communications, tracking, telemetry and wireless local network applications due to their compact structure and excellent radiation characteristics. Typically these antennas consist of half-wavelength dipole excitation elements for linear polarization or crossed half-wavelength dipole elements for circular polarization. To achieve simultaneous dual circular polarization using the related art would require integrating a network of hybrid switching components which introduces significant losses as well as disadvantages as to cost reliability, etc.). 
     Helix antennas naturally provide circular polarization. However, achieving dual circular polarization requires placing two helix antennas with opposite helical windings side by side, or a dual feeding arrangement. Placing helical antennas in proximity to each other can be problematic in the sense that coupling of the electromagnetic waves of one antenna to the other can occur absent a separation structure which would add weight to the assembly. 
     An article entitled “Compact Coaxial-Fed CP Polarizer,” by B. Subbarao and V. F. Fusco, IEEE Antennas and Wireless Propagation Letters, Vol. 3, 2004, states: “ . . . we use a circular waveguide with metal post inserts . . . to obtain a CP wave from an LP input, a 90° phase shift must be induced in one of the orthogonal components E∥ or E⊥, of the linearly polarized wave E which is applied at 45° to the post arrangement . . . . This phase shift is obtained by introducing slightly different phase constants for E∥ or E⊥. These are introduced by metal rods of equal size and spacing positioned diametrically across the aperture of the waveguide section. An equivalent circuit for a simplified version of this type of arrangement given in [ ] suggests that the inductance of these posts, together with their capacitive coupling, is providing the E∥ component with an impedance matched high-pass equivalent circuit thus advancing the phase of this component relative to its orthogonal component which propagates at normal waveguide phase velocity. By judicious design E∥, E⊥ components can be made to have equal amplitudes, hence if the length of the differential phase delay is made to be 90°, the exit signal will be a circularly polarized wave.” 
     An article entitled “Short Backfire Antenna With Conical Back Reflector And Double Small Front Reflectors by A. A. Ahmed, Journal of Islamic Studies, (9:2, 49-52, 1996 discloses “a conical back reflector and double plane small front reflectors fed through an open-ended circular waveguide excited with the dominant TE 11  mode.” and which “shows a relatively high gain (17.2 dB).” Another article entitled “Experimental Measurements Of The Short Backfire Antenna” by L. R. Dod, October 1966, NASA Goddard Space Flight Center, Greenbelt Md., Technical Manual X-525-66-490 states on page 3 thereof that: “[t]he short backfire antenna is a medium gain antenna (10-15 dB.) with low side and back radiation. The antenna can be cross-polarized for orthogonal linear or circular polarization . . . . The addition of a λ/4 rim on the large reflector is necessary for low back radiation . . . . The short backfire may also serve advantageously as an array element.” 
     Polarization of an electromagnetic wave is defined as the orientation of the electric field vector. In a transverse electromagnetic (TEM) wave, the electric field vector is perpendicular to the direction of travel and it is also perpendicular to the magnetic field vector. Linear polarization is commonly referred to as vertical or horizontal polarization depending on the orientation of the emitter with respect to some local frame of reference. If there are two orthogonal emitters and if they are out of phase then an elliptical pattern is traced by the tip of the electric field vector as a function of time on a fixed plane through which the combined electromagnetic wave passes. A special case of the elliptical polarization is circular polarization where the orthogonal components are equal in magnitude and 90° out of phase. 
     The present invention discloses a short backfire antenna in combination with a cylindrical waveguide which includes an orthomode transducer (OMT), septum and adjustable impedance screws (polarizers) enabling simultaneous propagation and/or reception of two oppositely oriented circularly polarized electromagnetic waves. None of the foregoing references disclose this unique assembly of features and functions. 
     SUMMARY OF THE INVENTION 
     The cup cylindrical waveguide antenna includes a short backfire antenna. The antenna further includes a dual reflector system circular disk subreflector and a circular cup. A cylindrical waveguide structure is utilized for antenna excitation. Dual, simultaneous, circular polarization is achieved using a compact 6-post polarizer integrated into the cylindrical waveguide. The cylindrical (circular) waveguide also includes an orthomode transducer with coaxial ports and pins to achieve simultaneous dual polarization. This design technique allows a compact circular waveguide, orthomode transducer and polarizer to be implemented in approximately 11 inches at S-band, substantially less space than a commercially available model measuring approximately 32 inches at the same frequency. Scaling of the cup cylindrical waveguide antenna for use at other frequencies is within the scope of the invention. 
     Narrowband Cup Waveguide Antenna 
     The narrowband frequency bandwidth specification is 2.2-2.3 GHz. The cup waveguide is a type of short backfire antenna (SBA). Short Backfire Antennas (SBAs) are dual reflector systems widely utilized for mobile satellite communications, tracking, telemetry, and wireless local area network (WLAN) applications due to their compact structure and excellent radiation characteristics. SBAs typically use a dipole or cross-dipole exciter, circular disk subreflector, and a circular cup. Similarly, the cup waveguide antenna is a dual reflector system with circular disk subreflector and circular cup. However, unlike conventional SBAs it uses a circular waveguide exciter. To achieve circular polarization, a compact 6-post polarizer is integrated into the circular waveguide somewhat similar to that described in the article entitled “Compact-Coaxial Fed CP Polarizer” identified herein above. The circular waveguide also includes an orthomode transducer (OMT) with coaxial ports to achieve simultaneous dual polarization. The overall length of the OMT and polarizer is about 11″ compared to approximately 32″ for a commercially available model. 
     The aforementioned subreflector is held in place within the cup using an EPS (expandable polystyrene) cylinder anchored inside the excitation waveguide. 
     Wideband Cup Waveguide Antenna 
     The wideband frequency bandwidth specification is 2.03-2.3 GHz. To accommodate the larger bandwidth, the narrowband cup waveguide design was modified to include a larger excitation circular waveguide diameter. In addition, the antenna includes a conical cup and two subreflectors. Other bandwidth driven changes to the design include an increase of cup diameter to about 12.15 inches to meet the gain specification, and the addition of a tuning screw to the OMT to maintain the return loss specification. Return loss is another way of expressing impedance mismatch. It is a logarithmic ratio measured in dB that compares the power reflected by the antenna to the power fed into the antenna. To achieve circular polarization a compact 6-post polarizer was used. In this case six polarizer screws were used to test adjustable insertion distances into the compact polarizing section of the waveguide. Once the insertion distances were determined, they were replaced with non-adjustable posts. A single adjustable tuning screw was added diametrically across from and longitudinally near the second port and second pin location. 
     The invention disclosed herein represents a significant savings in mass and size as compared to existing technology. Simulations for the antennas described herein used the three-dimensional electromagnetic software entitled Microwave Studio. Compared to a helix antenna, the design and fabrication of the instant invention is somewhat more complex since the polarizer and OMT require several additional components. Assembly was fairly straightforward, but the antenna required fine tuning, which was complicated by the additional variables of the polarizer screw depths, coaxial port pin lengths, and the subreflector height above the circular waveguide. 
     It is an object of the present invention to provide an antenna which includes a cylindrical waveguide having a pair of longitudinally spaced orthogonal ports, each of the ports includes a pin, having a septum intermediate to the pins, and, having an adjustable impedance matching mechanism. 
     It is a further object of the invention to provide an antenna wherein the adjustable impedance matching mechanism is a screw. 
     It is a further object of the invention to provide an antenna having a cup and a subreflector, the cup is affixed to the waveguide, the cup includes a reflector, and, the subreflector is separated apart from the reflector. 
     It is an object of the present invention to provide a short backfire antenna in combination with a cylindrical waveguide having impedance transforming structures enabling the propagation and reception of simultaneous right and left hand circular polarized electromagnetic waves in the range of 2.03 to 2.3 GHz. 
     It is an object of the present invention to provide a corrugated horn in combination with a cylindrical waveguide having polarization transforming structure enabling the propagation and reception of simultaneous right and left hand circular polarized electromagnetic waves. 
     It is an object of the present invention to provide a corrugated horn in combination with a cylindrical waveguide wherein the waveguide includes a septum aligned with one of the pins of one of the orthogonal ports. 
     It is an object of the present invention to provide an antenna having a waveguide which includes six adjustable polarizer screws. 
     It is an object of the present invention to provide an antenna which is short in length and light weight which meets the specification set forth above. 
     It is an object of the present invention to provide an antenna for communicating left and right hand circularly polarized electromagnetic waves utilizing a waveguide which includes an exterior and an interior, an orthomode transducer including first and second pins longitudinally spaced apart and oriented orthogonally with respect to each other, six radially-oriented adjustable polarizing screws extending from the exterior to the interior of the waveguide, a septum intermediate to the first and second pins aligned with the first pin, adjustment of the screws enables maximized propagation of left hand circularly polarized electromagnetic waves by the first pin and/or enables maximized response to left hand circularly polarized waves by the first pin; and, adjustment of the screws enables maximized propagation of a right hand circularly polarized electromagnetic waves by the second pin and/or enables maximized response to a right hand circularly polarized electromagnetic waves. 
     It is an object of the invention to provide three posts or screws diametrically across the aperture of the waveguide from three other posts or screws. 
     It is an object of the invention to provide additional posts numbering greater than six in a diametrical relationship. 
     These and other objects of the invention will be best understood when reference is made to the Brief Description of the Drawings, the Description of the Invention and the Claims which follow hereinbelow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of tracking and data relay satellite (TDRS). 
         FIG. 2  is a left front perspective view of narrowband cup waveguide antenna. 
         FIG. 2A  is front view of the narrowband cup waveguide antenna. 
         FIG. 2B  is a left side view of narrowband cup waveguide antenna. 
         FIG. 2C  is a right side view of the narrowband cup waveguide antenna. 
         FIG. 2D  is a partial cross-sectional view of the narrowband cup waveguide antenna taken along the lines  2 D- 2 D of  FIGS. 2 and 2A . 
         FIG. 2E  is a partial cross-sectional view of the narrowband cup waveguide taken along the lines  2 E- 2 E of  FIGS. 2 and 2A . 
         FIG. 2F  is a top view of the narrowband cup waveguide antenna. 
         FIG. 2G  is a right rear perspective view of the narrowband cup waveguide antenna. 
         FIG. 2H  is a cross-sectional view of the wideband conical cup waveguide antenna. 
         FIG. 2I  is a cross-sectional view of the corrugated horn waveguide antenna. 
         FIG. 2W  is a cross-sectional view of the waveguide taken along the lines  2 W- 2 W of  FIG. 2B . 
         FIG. 3A  is a graph of return loss versus frequency for port  1  of the narrowband cup waveguide antenna. 
         FIG. 3B  is a graph of return loss versus frequency for port  2  of the narrowband cup waveguide antenna. 
         FIG. 3C  is a graph of isolation versus frequency for ports  1  and  2  of the narrowband cup waveguide antenna. 
         FIG. 3D  is a graph of port  1  co- and cross-polarization versus Azimuth angle for the narrowband cup waveguide antenna at 2.25 GHz and phi of 90 degrees. 
         FIG. 3E  is a graph of port  2  co- and cross-polarization versus Azimuth angle for the narrowband cup waveguide antenna at 2.25 GHz and phi of 90 degrees. 
         FIG. 3F  is a graph of port  1  axial ratio versus Azimuth angle for the narrowband cup waveguide antenna at 2.25 GHz. and phi of 90 degrees 
         FIG. 3G  is a graph of port  2  axial ratio versus Azimuth angle for the narrowband cup waveguide antenna at 2.25 GHz. and phi of 90 degrees 
         FIG. 4A  is a graph of port  1  return loss versus frequency for the wideband cup waveguide antenna. 
         FIG. 4B  is a graph of port  2  return loss versus frequency for the wideband cup waveguide antenna. 
         FIG. 4C  is a graph of ports  1  and  2  isolation versus frequency for the wideband cup waveguide antenna. 
         FIG. 4D  is a graph of port  1  co- and cross-polarization versus Azimuth angle for the wideband cup waveguide antenna at 2.07175 GHz and phi of 0 degrees. 
         FIG. 4E  is a graph of port  2  co- and cross-polarization versus Azimuth angle for the wideband cup waveguide antenna at 2.07175 GHz and phi of 0 degrees. 
         FIG. 4F  is a graph of port  1  co- and cross-polarization versus Azimuth angle for the wideband cup waveguide antenna at 2.25 GHz and phi of 0 degrees. 
         FIG. 4G  is a graph of port  2  co- and cross-polarization versus Azimuth angle for the wideband cup waveguide antenna at 2.25 GHz and phi of 0 degrees. 
         FIG. 4H  is a graph of port  1  axial ratio versus Azimuth angle for the wideband cup waveguide antenna at 2.07175 GHz and phi of 0 degrees. 
         FIG. 4I  is a graph of port  2  axial ratio versus Azimuth angle for the wideband cup waveguide antenna at 2.07175 GHz and phi of 0 degrees. 
         FIG. 4J  is a graph of port  1  axial ratio versus Azimuth angle for the wideband cup waveguide antenna at 2.25 GHz and phi of 0 degrees. 
         FIG. 4K  is a graph of port  2  axial ratio versus Azimuth angle for the wideband cup waveguide antenna at 2.25 GHz and phi of 0 degrees. 
         FIG. 5A  is a graph of port  1  return loss versus frequency of the corrugated horn waveguide antenna. 
         FIG. 5B  is a graph of port  2  return loss versus frequency of the corrugated horn waveguide antenna. 
         FIG. 5C  is a graph of ports  1  and  2  isolation versus frequency for the corrugated horn waveguide antenna. 
         FIG. 5D  is a graph of port  1  co- and cross-polarization for the corrugated horn waveguide antenna at 2.25 GHz and phi of 0 degrees. 
         FIG. 5E  is a graph of port  2  co- and cross-polarization for the corrugated horn waveguide antenna at 2.25 GHz and phi of 0 degrees. 
         FIG. 5F  is a graph of port  1  axial ratio versus Azimuth angle for the corrugated horn waveguide antenna at 2.25 GHz and phi of 0 degrees. 
         FIG. 5G  is a graph of port  2  axial ratio versus Azimuth angle for the corrugated horn waveguide antenna at 2.25 GHz and phi of 0 degrees. 
     
    
    
     The drawings will be best understood when reference is made to the Description of the Invention and the Claims which follow hereinbelow. 
     DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic view  100  of the tracking and data relay satellite (TDRS). Reference numeral  101  is an array of 32 antenna elements which are used for communication. These antenna elements are the subject of the invention. 
       FIG. 2  is a left front perspective view  200  of the narrowband cup waveguide antenna.  FIG. 2A  is a front view  200 A of the narrowband cup waveguide antenna illustrating a portion of the cylindrical waveguide  201 , narrowband cup  202 , screws  220  affixing the narrowband cup to the waveguide, subreflector  206  supported by the EPS (expanded polystyrene)  205  and the reflector  207  of the cup. Aluminum or other light weight metal is used in the construction of all of the antenna components except for the SMA (SubMiniature version A) coaxial connectors  209  are used as an interface for coaxial cable type coupling mechanisms. SMA connectors typically have a 50Ω impedance. Mounting block  210  is secured to the exterior of the cylindrical waveguide  201  with adhesive or some other mounting mechanism. 
     The narrowband cup  202  has a 10.585 inch diameter (about at 2.25 GHz) and has a rim height of approximately 5.421 inches. The cup reflector  207  is preferably polished Aluminum and the subreflector  206  is mounted approximately 3.873 inches away from the reflector  207 . The diameter of the subreflector  206  is approximately 1.807 inches. Overall length of the narrowband cup and the cylindrical waveguide  201  is approximately 15.17 inches. Subreflector  206  is supported by EPS (Expandable Styrene) which is inserted and secured within the approximate 3.614 inch inner diameter of the cylindrical waveguide  201 . The outer diameter of the cylindrical waveguide is approximately 3.850 inches. Subreflector  206  may be adhesively affixed to the Expandable Styrene or it may be embedded therein. 
     Still referring to  FIG. 2 , support  203  and clamp  204  are illustrated by way of example as one possible method for securing the antenna to a satellite. Cylindrical waveguide end cap  208  includes a highly polished inner portion  208 S which acts as a back short to electromagnetic waves within the waveguide. See  FIG. 2W  which is a cross-sectional view  200 W of the waveguide taken along the lines  2 W- 2 W of  FIG. 2B . Cylindrical end cap  208  is affixed to the cylindrical waveguide  201  with an interference fit or some type of mechanical affixation such as adhesive, set screws, threads, welding, etc. 
       FIG. 2W  is a cross-sectional view  200 W of the cylindrical waveguide taken along the lines  2 W- 2 W of  FIG. 2B .  FIG. 2W  shows a cross-section of the polarizer and orthomode transducer (OMT) illustrating the orientation of the coaxial ports, polarizer screws, and septum plate (which acts as a back short to the first port first pin  226 . 
     Septum  211  is approximately 0.0625 inches thick and is adhesively or mechanically secured in a receiving slot in the waveguide. Referring to  FIG. 2W , the septum  211  extends across the exterior diameter of the waveguide and is flush therewith such that no part of the septum protrudes out of the waveguide. Septum  211  acts as a back short for first pin  226  which is the center conductor of the coaxial port  1  connector  221 . 
     Still referring to the  FIG. 2W , the first port of the waveguide includes a first pin  226  which extends radially 1.21 inches into waveguide  201 . Sometimes herein the structure identified as the first pin  226  may be referred to as the first port. First pin  226  has a diameter of 0.036 inches and is aligned along the centerline of the septum  211 . As previously indicated the septum is 0.0625 inches thick and is thicker than the 0.036 inch diameter of the first pin  226 . A portion of the dielectric  225  of the SMA connector  221  may or may not extend into the waveguide  201  through mounting block  222 . The first pin  226  is located distally with respect to the back short  208 S. Second pin  224  extends radially 1.19 inches into waveguide  201 . Sometimes herein the structure referred to as the second pin  224  may be referred to as the second port. A portion of dielectric  223  of the SMA connector  209  may or may not extend in the waveguide  201  through mounting block  210 . Mounting blocks  210  and  222  are secured (by adhesive or other means of affixing metal blocks to cylindrical devices) to the exterior of the waveguide and may include threads therein for interengagement with the SMA connectors. 
     The dimensions in inches of the narrowband cup waveguide antenna, polarizer and orthomode transducer are summarized below. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Cylindrical waveguide 201 inner diameter 
                 3.614 
               
               
                   
                 Cylindrical waveguide 201 outer diameter 
                 3.850 
               
               
                   
                 Septum 211 plate thickness 
                 0.0625 
               
               
                   
                 Coax port 1 first pin 226 depth into waveguide 
                 1.21 
               
               
                   
                 Coax port 2 second pin 224 depth into 
                 1.19 
               
               
                   
                 waveguide 
                   
               
               
                   
                 Polarizer screw 212, 213, 214, 216, 217, 218 
                 0.80 
               
               
                   
                 depth into waveguide 
                   
               
               
                   
                 Polarizer screw 212, 213, 214, 216, 217, 218 
                 0.375 
               
               
                   
                 diameter 
                   
               
               
                   
                 First 226 and second 224 coax port pin diameter 
                 0.036 
               
               
                   
                   
               
             
          
         
       
     
     Still referring to the  FIG. 2W , polarizer screws  214 ,  218  are nominally 0.375 inches in diameter and extend radially from the exterior of the waveguide  201  into the interior of the waveguide. Nominally, the insertion depth of screws  214 ,  218  into the waveguide is approximately 0.80 inches. There are two additional polarizer screws behind each of polarizer screws  214 ,  218  which are not illustrated in  FIG. 2W  for clarity. The additional screws would be viewed in  FIG. 2W  if at least one of the polarizer screws is adjusted to a different depth. The polarizer screws are made of an electrically conductive material which interacts with the electromagnetic waves in the cylindrical waveguide. Lock nuts  214 A,  218 A secure the adjustable screws  214 ,  218  to the desired depth. Each additional polarizer screw hidden behind screws  214  and  218  have respective lock nuts also not shown and hidden by lock nuts  214 A and  218 A in  FIG. 2W . Threads in mounting blocks  215 ,  219  and the waveguide  201  interengage the corresponding threads on the adjustable screws  214 ,  218 . Mounting blocks  215 ,  219  are secured to the waveguide with adhesive or with mechanical structure not shown. 
     The cylindrical waveguide  201  is used in conjunction with the short backfire antenna. The short backfire antenna includes waveguide cup  202 , reflector  207 , waveguide  201  protruding into the waveguide cup  202  and the subreflector  206  supported by the EPS form the narrowband cup waveguide antenna. 
     Still referring to  FIG. 2W , first pin  226  and second pin  224  are diametrically the same size and are oriented at 90° with respect to each other. First pin  226  propagates linearly polarized electromagnetic waves which are transformed by the polarizer screws  212 - 214  and  216 - 218  into left hand circularly polarized waves. First pin  226  also receives linearly polarized electromagnetic waves transformed from incident left hand circularly polarized electromagnetic waves by the polarizer screws  212 - 214  and  216 - 218 . 
     Second pin  224  propagates linearly polarized electromagnetic waves which are transformed by the polarizer screws into right hand circularly polarized waves. Second pin  224  receives linearly polarized electromagnetic waves transformed from incident right hand circularly polarized electromagnetic waves which are transformed by the polarizer. 
     Screws  212 ,  213  and  214  are located at an angle of 45° counterclockwise from second pin  224 . Screws  216 ,  217  and  218  are located at an angle of 45° clockwise from first pin  226 . Screws  212 - 214  extend radially inwardly into the waveguide aperture and are located diametrically opposite screws  216 - 218  which also extend radially inwardly into the waveguide aperture. 
       FIG. 2W  shows a cross-sectional view of the polarizer and OMT. The polarizer screw interspacing and depth into the waveguide were varied to optimize axial ratio. Then, the position of the septum plate and length of the port  1  coaxial pin ( 226 ) were varied to optimize the port  1  return loss. The position of the back waveguide short and the length of the port  2  coaxial pin ( 224 ) were varied to optimize the port  2  return loss. 
       FIG. 2B  is a left side view  200 B of narrowband cup waveguide antenna. Mounting ring  202 A is secured to waveguide  201  by set screw  202 B. Port  1  coaxial pin  1   226  is not shown in  FIG. 2B .  FIG. 2B  illustrates polarizer screws  212 - 214  and  216 - 218  inserted at various depths into the waveguide. 
       FIG. 2C  is a right side view  200 C of the narrowband cup waveguide antenna illustrating the coaxial connector  221  affixed to the mounting block  222 . Polarizing screws  216 - 218  are illustrated with various insertion depths.  FIG. 2D  is a partial cross-sectional view  200 D of the narrowband cup waveguide antenna taken along the lines  2 D- 2 D of  FIGS. 2 and 2A  and illustrates the polarizer screws  212 - 214 ,  216 - 218  and OMT (first pin  226  and second pin  224 ). Mounting ring  202 A is illustrated as are screws  220  which affix the narrowband cup to the waveguide. The narrowband cup  202  includes a rim, reflector  207  and subreflector  206 , and waveguide positioned to form a backfire antenna coupled to the cylindrical waveguide  201  to form the narrow band cup waveguide antenna.  FIG. 2D  illustrates the adjustable polarizer screws protruding through the wall of the cylindrical waveguide  201 . First pin  226  and second pin  224  are orthogonally arranged and longitudinally spaced. First pin  226  (first port) creates a left hand circularly polarized electromagnetic wave and second pin  224  (second port) creates a right hand circularly polarized electromagnetic wave. Septum  211  is thicker than first pin  226  and is aligned therewith to form a back short with respect to first pin  226 . Septum  211  as viewed in  FIG. 2D  resides intermediate to the first  226  and the second pin  224 . 
     The open end of the waveguide  201  resides within the narrowband cup and is 2.4 inches from the centerline of the first polarizing screw  218 . The centerline of the second polarizer screw  217  is 0.920 inches from the centerline of the first polarizer screw  218 . The centerline of the third polarizer screw  216  is 0.920 inches from the centerline of the second polarizer screw  217 . First pin  226  resides 1.5 inches from the centerline of the third polarizer screw  216 . The leading edge of septum  211  is spaced 1.6 inches from the centerline of the first pin  226  and is radially aligned with the first pin  226 . First pin  226  has a diameter of 0.036 inches and the septum  211  is 0.0625 inches thick and 1.0 inch in longitudinal extent. Second pin  224  is oriented at a right angle to septum  211  and first pin  226  and is located 1 inch from the trailing edge of septum  211 . The inner surface  208 S of the end cap (not labeled in  FIG. 2D ) is spaced 1.7 inches from second port pin  224 . 
       FIG. 2E  is a partial cross-sectional view  200 E of narrowband cup waveguide taken along the lines  2 E- 2 E of  FIGS. 2 and 2A  and illustrates the polarizer and OMT similarly to  FIG. 2D .  FIG. 2F  is a top view  200 F of the narrowband cup waveguide antenna. 
       FIG. 2G  is a right rear perspective view  200 G of the narrowband cup waveguide antenna which illustrates three polarizing screws  212 - 214  located 180° from the other three polarizing screws  216 - 218  with all of the screws radially extending into and through the circular waveguide  201 . 
       FIG. 3A  is a graph  300 A of measured and simulated return loss versus frequency for port  1  of the narrowband cup waveguide antenna.  FIG. 3B  is a graph  300 B of measured and simulated return loss versus frequency for port  2  of the narrowband cup waveguide antenna.  FIGS. 3A  and B show the measured and simulated return loss for ports  1  and  2  demonstrating both ports are within specification, to with, less than −20 dB at the center frequency of 2.25 GHz.  FIG. 3C  is a graph  300 C of measured and simulated isolation versus frequency for ports  1  and  2  of the narrowband cup waveguide and indicates excellent agreement between measured and calculated data. 
       FIG. 3D  is a graph  300 D of waveguide port  1  co- and cross-polarization versus Azimuth angle for the narrowband cup waveguide antenna at 2.25 GHz. Excellent agreement was also obtained between measured and simulated farfield patterns. For example,  FIGS. 3D-E  and  3 D-G show the co- and cross-polarization levels, and the axial ratios, respectively, for ports  1  and  2  at the center frequency in compliance with the design specifications. Axial ratio is used to describe the relationship between the magnitudes of the two orthogonal, linearly polarized electric field components in a circularly polarized wave. In a purely circularly polarized wave both electric field components have equal magnitude and the axial ratio will be unity. Axial ratio is an expression of the quality of the circular polarization. The axial ratio when expressed in units of dB is equal to 10 times the logarithm (base 10) of the axial ratio (ratio of the orthogonal electric field magnitudes). In addition, the measured far-field patterns show good agreement with simulation, and are within specification across the operating frequency band. 
       FIG. 3E  is a graph  300 E of port  2  co- and cross-polarization versus Azimuth angle for the narrowband cup waveguide antenna at 2.25 GHz.  FIG. 3F  is a graph  300 F of port  1  axial ratio versus Azimuth angle for the narrowband cup waveguide antenna at 2.25 GHz indicating axial ratios of less than 5 dB at all angles and indicating measured axial ratios of less than 1 dB from about −15 to +15 degrees. Similarly,  FIG. 3G  is a graph  300 G of port  2  axial ratio versus Azimuth angle for the narrowband cup waveguide antenna at 2.25 GHz indicating axial ratios of less than 5 dB at all angles and indicating measured axial ratios of less than about 1.5 dB from about 15 to +15 degrees. 
       FIG. 2H  is a cross-sectional view  200 H of the wideband conical cup waveguide antenna. Dimensions (in inches) of the wideband cup waveguide antenna, polarizer, and OMT are given below. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Cylindrical waveguide 227 inner diameter 
                 3.670 
               
               
                   
                 Cylindrical waveguide 227 outer diameter 
                 4.200 
               
               
                   
                 Septum plate thickness, 235 
                 0.0625 
               
               
                   
                 Coax port pin 1 (237) and 2 (234) diameter 
                 0.036 
               
               
                   
                 Polarizer screw/post diameter, 228, 229, 239 
                 0.375 
               
               
                   
                 and three additional screws/posts not illustrated 
                   
               
               
                   
                 Tuning screw 238 diameter 
                 0.164 
               
               
                   
                   
               
             
          
         
       
     
     Fabrication of the wideband cup cylindrical waveguide  227  was similar to the narrowband cup waveguide with the exception of the added tuning screw  238  in the OMT, the use of posts  228 ,  229  and  230  (plus three not illustrated) rather than screws for the polarizer section and the conical cup  240 ,  241  which was fabricated using computer numerical control (CNC) machining. 
     Tuning was performed by isolating sections of the assembly as follows. First, the cup  240  and subreflectors  245 ,  246  were removed. The polarizer posts  228 ,  229 ,  230  and three other posts arranged diametrically across the waveguide aperture were removed and their mounting holes were temporarily closed off flush to the inner surface of the waveguide using screws. Port pins  236 ,  234  were then tuned by comparing measured data with the simulation for the same configuration. The screws plugging the post holes were then removed and the machined to length polarizer posts  228 ,  229 ,  230  (and the three opposite posts) were simply put in place in there respective mounting holes through the waveguide wall. Return loss and isolation were measured and checked against simulated results. This was done to ensure that the assembly was achieving the expected performance at each level of assembly. Once good agreement was achieved for the return loss and isolation with all of the polarizer posts in place, the cup and subreflectors which form the backfire antenna were added to the assembly and the final S-parameter, radiation pattern and gain measurements were taken. The measured radiation patterns showed excellent agreement with simulation, and satisfy the specifications across the frequency bandwidth of 2.03-2.3 GHz. 
     The overall length of the wideband cup waveguide antenna is approximately 16.231 inches and the cup diameter is approximately 12.150 inches. The tuning screw  238  is approximately 2.41 inches from the end plate  231  and it is locked in place with a nut  239 . The wall thickness of the circular waveguide used in the wideband application is 0.265 inches thick and includes threads therein for the interengagement with threads on the tuning screw  238 . 
     Subreflector  246  is approximately 2.186 inches in diameter and subreflector  245  is approximately 2.548 inches in diameter. Both subreflectors are supported by EPS  244 . Subreflector  246  is the datum line and is referenced as zero inches into the antenna when reference is made from right to left viewing  FIG. 2H . Subreflector  245  is spaced apart from subreflector  246  approximately 0.471 inches. The upper lip or beginning of the cylindrical waveguide is approximately 2.831 inches leftwardly from subreflector  246 . Cup  240  begins to gradually curve approximately 3.376 inches from subreflector  246  until it meets conical section  241  of the cup which is affixed to the mounting ring  242 . The wideband cup  240  includes a conical or frustum-conical section  241  which is tapered and is secured with screws to the mounting ring  242  approximately 4.741 inches from the subreflector  246 . Cylindrical waveguide  227  extends approximately 1.91 inches into the waveguide cup. 
     The first  230  and third  228  polarizer posts can be referred to as the outside polarizer posts and they protrude radially inwardly into the cylindrical waveguide approximately 0.710 inches. The middle or second polarizer post  229  protrudes radially into the cylindrical waveguide approximately 0.860 inches. The polarizer posts are secured with adhesive or some other type of mechanical affixation. The first polarizer post  230  resides 5.331 inches from subreflector  246 , the second polarizer post  229  resides approximately 7.031 inches from subreflector  246  and the third polarizer post  228  resides approximately 8.731 inches from subreflector  246 . 
     Still referring to  FIG. 2H , first pin  237  may or may not include a short sheath of dielectric material  236  therearound as previously described in connection with the narrowband cup waveguide antenna described above in  FIGS. 2-2G . First pin  237  resides 10.631 inches from the subreflector  246 . Septum plate  235  is one inch in longitudinal extent, 0.0625 inches thick, and resides at its beginning or leading edge 12.256 inches from subreflector  246 . Septum plate  235  acts as a back short to first pin  237  and is aligned therewith. Adjusting screw  238  is 0.164 inches in diameter and resides 13.821 inches from subreflector  246  and primarily tunes in a vernier fashion second pin  234 . First and second pins  237 ,  234  are common with the center conductors of coaxial cables and are secured with an SMA connector (shown for port  2  only as  233  and mounting block arrangement  232 ) as described above. End cap  231  is cylindrical and is secured to cylindrical waveguide  227  using a force fit. 
       FIG. 4A  is a graph  400 A of port  1  return loss versus frequency for the wideband cup waveguide antenna.  FIG. 4B  is a graph  400 B of port  2  return loss versus frequency for the wideband cup waveguide antenna.  FIGS. 4A and 4B  compare the measured and simulated return loss, respectively, and the agreement is very good with the port  2  return loss just slightly exceeding the specified goal of −20 dB at about 2.3 GHz.  FIG. 4C  is a graph  400 C of ports  1  and  2  isolation versus frequency for the wideband cup waveguide antenna. 
       FIG. 4D  is a graph  400 D of port  1  co- and cross-polarization versus Azimuth angle for the wideband cup waveguide antenna at 2.07175 GHz.  FIG. 4E  is a graph  400 E of port  2  co- and cross-polarization versus Azimuth angle for the wideband cup waveguide antenna at 2.07175 GHz. 
       FIG. 4F  is a graph  400 F of port  1  co- and cross-polarization versus Azimuth angle for the wideband cup waveguide antenna at 2.25 GHz.  FIG. 4G  is a graph  400 G of port  2  co- and cross-polarization versus Azimuth angle for the wideband cup waveguide antenna at 2.25 GHz. 
       FIG. 4H  is a graph  400 H of port  1  axial ratio versus Azimuth angle for the wideband cup waveguide antenna at 2.07175 GHz.  FIG. 4I  is a graph  400 I of port  2  axial ratio versus Azimuth angle for the wideband cup waveguide antenna at 2.07175 GHz.  FIG. 4J  is a graph  400 J of port  1  axial ratio versus Azimuth angle for the wideband cup waveguide antenna at 2.25 GHz.  FIG. 4K  is a graph of port  2  axial ratio versus Azimuth angle for the wideband cup waveguide antenna at 2.25 GHz. 
       FIG. 2I  is a cross-sectional view  200 I of corrugated horn  260  waveguide antenna. Stepped corrugations  261  are arranged on the inner circumference as illustrated in  FIG. 2I . The corrugated horn antenna was designed using a method of moments code for rotationally symmetric feeds. The OMT and polarizer dimensions are similar with some variation to that described above for the narrowband cup waveguide. Compare with  FIGS. 2-2G  wherein slightly different pin depths and slightly different nominal polarizer screw depths are used. It should be kept in mind that the polarizer screw depths stated in connection with the narrowband cup waveguide antenna, the wideband cup waveguide antenna and the corrugated horn antenna are nominal and will in fact vary when tuned. See  FIG. 2I  where screw  217  is illustrated as being inserted relatively less than screws  216  and  218 . 
     Dimensions (inches) of the corrugated horn antenna  260 , polarizer  212 - 214  and  216 - 218 , and OMT are given below. 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Cylindrical waveguide inner diameter, 201 
                 3.614 
               
               
                   
                 Cylindrical waveguide outer diameter, 201 
                 3.801 
               
               
                   
                 Septum thickness, 211 
                 0.0625 
               
               
                   
                 Coax port pin 1 depth into waveguide, 226 
                 1.175 
               
               
                   
                 Coax port pin 2 depth into waveguide, 224 
                 1.175 
               
               
                   
                 Polarizer screw depth into waveguide 
                 0.75 
               
               
                   
                 Polarizer screw diameter, 212, 213, 214, 216, 
                 0.375 
               
               
                   
                 217, 218 
                   
               
               
                   
                 Coax center pin diameter, 226, 224 
                 0.036 
               
               
                   
                   
               
             
          
         
       
     
     Flange  262  of horn  260  is affixed by screws  248  to mounting ring  247  which in turn is affixed to waveguide  201 . 
     The fabrication complexity of the corrugated horn waveguide antenna is somewhat more complex than the narrowband and wideband cup waveguide antennas because of the machining of the horn corrugations. However, assembly was straightforward requiring only a flange connection between the horn and the OMT/polarizer. Tuning was also straightforward requiring only minor adjustments to the polarizer screws and the coaxial pins. 
       FIG. 5A  is a graph  500 A of port  1  return loss versus frequency of the corrugated horn waveguide antenna.  FIG. 5B  is a graph  500 B of port  2  return loss versus frequency of the corrugated horn waveguide antenna. 
       FIG. 5C  is a graph  500 C ports  1  and  2  measured and simulated isolation versus frequency for the corrugated horn waveguide antenna. The results easily meet the specifications for both ports with the return loss being less than −20 dB at both ports for the frequency of interest, to with, 2.2-2.3 GHz. Further, the isolation for both ports is less than −10 dB. 
       FIG. 5D  is a graph  500 D of port  1  measured and simulated co- and cross-polarization versus Azimuth angle for the corrugated horn waveguide antenna at 2.25 GHz.  FIG. 5E  is a graph  500 E of port  2  measured and simulated co- and cross-polarization versus Azimuth angle for the corrugated horn waveguide antenna at 2.25 GHz. 
       FIG. 5F  is a graph  500 F of port  1  measured and simulated axial ratio versus Azimuth angle for the corrugated horn waveguide antenna at 2.25 GHz.  FIG. 5G  is a graph  500 G of port  2  measured and simulated axial ratio versus Azimuth angle for the corrugated horn waveguide antenna at 2.25 GHz.  FIGS. 5F and 5G  show the measured and simulated axial ratios, which again show very good agreement. The graphs show data at the center frequency. However, the corrugated horn waveguide antenna met the specifications for directivity and axial ratio across the bandwidth of 2.2-2.3 GHz. 
     LIST OF REFERENCE NUMERALS 
     
         
           100 —schematic view of tracking and data relay satellite (TDRS) 
           101 —32 element multiple access antenna 
           200 —narrowband cup waveguide antenna 
           200 A—left front perspective view of narrowband cup waveguide antenna 
           200 B—left side view of narrowband cup waveguide antenna 
           200 C—right side view of narrowband cup waveguide antenna 
           200 D—partial cross-sectional view of narrowband cup waveguide antenna taken along the lines  2 D- 2 D of  FIGS. 2 and 2A   
           200 E—partial cross-sectional view of narrowband cup waveguide antenna taken along the lines  2 E- 2 E of  FIGS. 2 and 2A   
           200 F—top view of the narrowband cup waveguide antenna 
           200 G—right rear perspective view of the narrowband cup waveguide antenna 
           200 H—cross-sectional view of wideband conical cup waveguide antenna 
           200 I—cross-sectional view of corrugated horn waveguide antenna 
           200 W—cross-sectional view of the waveguide taken along the lines  2 W- 2 W of  FIG. 2B   
           201 —waveguide 
           202 —narrowband cup 
           202 A,  247 —mounting ring 
           202 B—set screw of mounting ring 
           203 —support 
           204 —strap of support 
           205 ,  244 —EPS (expandable polystyrene) support 
           206 ,  245 ,  246 —subreflector 
           207 —cup reflector 
           208 —waveguide end cap 
           208 S—inner portion of waveguide end cap 
           209 —coaxial connector pin/port  2   
           210 —mount for coaxial connector pin/port  2   
           211 —septum plate 
           212 ,  213 ,  214 ,  216 ,  217 ,  218 —adjustable threaded post 
           212 A,  213 A,  214 A,  216 A,  217 A,  218 A—lock nuts for threaded posts 
           215 ,  219 —mounting block for screws/posts 
           220 ,  248 —waveguide to cup/horn screws 
           221 —coaxial connector for pin/port  1   
           222 —mount for coaxial connector pin/port  1   
           223 —dielectric sheath on coax pin/port  2   
           224 —coax port  2  pin/probe 
           225 —dielectric sheath on coax pin/port  1   
           226 —coax port  1  pin/probe 
           227 —waveguide for wideband 
           228 ,  230 —relatively shorter posts 
           229 —relatively longer post 
           231 —end plate 
           232 —mounting block for port  2   
           233 —coaxial connector for port  2   
           234 —coax port  2  pin/probe 
           235 —septum 
           236 —dielectric sheath for port/pin  1   
           237 —coax port  1  pin/probe 
           238 —tuning screw for port  2   
           239 —lock nut 
           240 —wideband cup 
           241 —frusto-conical portion of wideband cup 
           242 —mounting ring 
           243 —screws affixing wideband cup to ring 
           244 —EPS 
           245 —sub reflector 
           246 —sub reflector 
           247 —mounting ring 
           248 —screws 
           260 —corrugated horn 
           261 —corrugation 
           262 —flange 
           300 A—narrowband cup waveguide antenna port  1  graph of return loss versus frequency 
           300 B—narrowband cup waveguide antenna port  2  graph of return loss versus frequency 
           300 C—narrowband cup waveguide antenna ports  1  and  2  graph of isolation versus frequency 
           300 D—narrowband cup waveguide antenna port  1  co and cross-polarization versus Azimuth angle at 2.25 GHz 
           300 E—narrowband cup waveguide antenna port  2  co- and cross-polarization versus Azimuth angle at 2.25 GHz 
           300 F—narrowband cup waveguide antenna port  1  axial ratio versus Azimuth angle at 2.25 GHz. 
           300 G—narrowband cup waveguide antenna port  2  axial ratio versus Azimuth angle at 2.25 GHz 
           400 A—wideband cup waveguide antenna port  1  graph of return loss versus frequency 
           400 B—wideband cup waveguide antenna port  2  graph of return loss versus frequency 
           400 C—wideband cup waveguide antenna ports  1  and  2  graph of isolation versus frequency 
           400 D—wideband cup waveguide antenna port  1  co- and cross-polarization versus Azimuth angle at 2.07175 GHz 
           400 E—wideband cup waveguide antenna port  2  co- and cross-polarization versus Azimuth angle at 2.07175 GHz 
           400 F—wideband cup waveguide antenna port  1  co- and cross-polarization versus Azimuth angle at 2.25 GHz 
           400 G—wideband cup waveguide antenna port  2  co- and cross-polarization versus Azimuth angle at 2.25 GHz 
           400 H—wideband cup waveguide antenna port  1  axial ratio versus Azimuth angle at 2.07175 GHz 
           400 I—wideband cup waveguide antenna port  2  axial ratio versus Azimuth angle at 2.07175 GHz 
           400 J—wideband cup waveguide antenna port  1  axial ratio versus Azimuth angle at 2.25 GHz 
           400 K—wideband cup waveguide antenna port  2  axial ratio versus Azimuth angle at 2.25 GHz 
           500 A—corrugated horn waveguide antenna port  1  graph of return loss versus frequency 
           500 B—corrugated horn waveguide antenna port  2  graph of return loss versus frequency 
           500 C—corrugated horn waveguide antenna ports  1  and  2  graph of isolation versus frequency 
           500 D—corrugated horn waveguide antenna port  1  co- and cross-polarization versus Azimuth angle at 2.25 GHz 
           500 E—corrugated horn waveguide antenna port  2  co- and cross-polarization versus Azimuth angle at 2.25 GHz 
           500 F—corrugated horn waveguide antenna port  1  axial ratio versus Azimuth angle 
           500 G—corrugated horn waveguide antenna port  2  axial ratio versus Azimuth angle 
       
    
     Those skilled in the art will readily recognize that the invention has been set forth by way of example only and that many changes may be made to the invention without departing from the spirit and scope of the claims which follow hereinbelow.