Patent Publication Number: US-2011057849-A1

Title: Dynamic polarization adjustment for a ground station antenna

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to a ground station antenna for exchanging electromagnetic signals with a satellite and, more particularly, to a ground station antenna, for a mobile platform, with improved dynamic polarization alignment with the satellite&#39;s transponder. 
       FIGS. 1A and 1B  shows a typical parabolic dish antenna  10  for communicating with a communication satellite such as a Fixed Service Satellite (FSS). Antenna  10  includes a parabolic dish  12  and a Low Noise Block downconverter Feed horn (LNBF)  14  supported by supports  16  at the focus of dish  12 . Dish  12  is mounted on a mount  18 .  FIG. 1A  is a perspective view of antenna  10 .  FIG. 1B  is a frontal view of dish  12  and LNBF  14 . LNBF  14  includes a Low Noise Block (LNB) with two orthogonal receive dipoles  20  shown in  FIG. 1B  in phantom. Each dipole receives K u -band signals from the FSS at which antenna  10  is aimed. 
     An FSS is a geostationary satellite whose transponders transmit and receive linearly polarized radio waves in the K u  band. One transponder of a transponder pair transmits and receives horizontally polarized waves. The other transponder of the transponder pair transmits and receives vertically polarized waves. LNB dipoles  20  are intended for receiving signals in respective allocated frequency segments from respective transceivers of the FSS: the horizontal dipole antenna  20  is for receiving signals from the transponder that transmits horizontally polarized waves and the vertical dipole antenna  20  is for receiving signals from the transponder that transmits vertically polarized waves. If the FSS is at the same longitude as a stationary antenna  10 , then when dish  12  is aimed at the FSS by appropriate adjustment of mount  18  in azimuth and elevation, the horizontal LNB dipole  20  is aligned with the horizontal polarization direction of the FSS and the vertical LNB dipole  20  is aligned with the vertical polarization of the FSS. If the FSS is not at the same longitude as a stationary antenna  10  then the polarization directions of the FSS are tilted with respect to LNB dipoles  20  and dish  12  must be rotated, as indicated by an arrow  22  in  FIG. 1B , to align LNB dipoles  20  with the polarization directions of the FSS. 
     If antenna  10  is stationary, then dish  12  only needs to be rotated once and then fixed in place on mount  18 . If antenna  10  is mounted on a moving platform such as a truck, a boat, an aircraft or some other vehicle, the orientation of dish  12  must be adjusted continuously to keep dish  12  pointed at the FSS and to keep LNB dipoles  20  aligned with the polarization directions of the FSS. Even if antenna  10  is stationary, if antenna  10  communicates with a satellite that is not in a geosynchronous obit, dish  12  must be adjusted continuously to keep dish  12  pointed at the satellite and to keep LNB dipoles  20  aligned with the satellite&#39;s polarization directions. Hsiung, in U.S. Pat. No. 6,377,211, teaches an antenna aiming apparatus for keeping an antenna that is mounted on a moving vehicle properly aligned with a satellite in a non-geosynchronous orbit. U.S. Pat. No. 6,377,211 is incorporated by reference for all purposes as if fully set forth herein. 
     Heretofore, dish  12  has been rotated as a whole, in the directions indicated by arrow  22 , to keep LNB dipoles  20  aligned with the polarization directions of the satellite with which antenna  10  communicates. It would be highly advantageous to be able to keep LNB dipoles  20  aligned with the polarization directions of the satellite without having to rotate dish  12  as a whole. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a ground station antenna including: (a) a low noise block having at least one dipole; (b) a feed horn; (c) waveguide, between the low-noise block and the feed horn, operative to transform a polarization of an electromagnetic wave propagating between the low-noise block and a communications satellite; and (d) a mechanism for rotating at least a portion of the waveguide, relative to the low-noise block, so as to match the polarization to a respective dipole of the low-noise block and also to a satellite antenna on the communication satellite. 
     According to the present invention there is provided a ground station antenna including: (a) a main waveguide; (b) two mutually orthogonal distal branch waveguides at a distal end of the main waveguide; (c) a coupling mechanism for coupling two orthogonal antenna dipoles to a proximal end of the main waveguide; and (d) a transformation mechanism operative to transform a polarization of an electromagnetic wave propagating between the distal branch waveguides and a communications satellite via the antenna dipoles so as to match the polarization to one of the distal branch waveguides and also to a satellite antenna on the communication satellite. 
     According to the present invention there is provided a method of rotating an input direction of polarization of a linearly polarized transverse wave to an output direction, including the steps of (a) transforming the transverse wave into a circularly polarized transverse wave; and (b) transforming the circularly polarized transverse wave into a linearly polarized transverse wave whose direction of polarization is the output direction. 
     One ground station antenna of the present invention includes a low noise block with one dipole or with two orthogonal dipoles, a feed horn and a waveguide. The waveguide is between the low-noise block and the feed horn. The waveguide is operative to transform the polarization of an electromagnetic wave that propagates between the low-noise block and a communication satellite. Examples of such transformations include rotating the plane of polarization of a linearly polarized wave, transforming a circularly polarized wave to a linearly polarized wave, and transforming a linearly polarized wave to a circularly polarized wave. The ground station antenna also includes a rotation mechanism for rotating at least a portion of the waveguide, relative to the low-noise block, so as to match the polarization of the electromagnetic wave to a respective dipole of the low-noise block and also to a satellite antenna on the communication satellite. 
     Preferably, the waveguide includes two polarizers. A “polarizer” is a phase shifter that receives a linearly polarized signal as input and converts it to a circularly polarized output signal, or vice versa. The rotation mechanism rotates a first one of the polarizers while a second one of the polarizers remains fixed relative to the dipole(s) of the low-noise blocks. 
     In one preferred embodiment of the ground station antenna, each polarizer includes a single respective dielectric slab. Most preferably, the dielectric slabs are quarter-wavelength slabs (relative to the wavelength of the electromagnetic wave). Also most preferably, the second dielectric slab is fixed at a 45-degree angle relative to the dipole(s) of the low-noise block. 
     In another preferred embodiment of the ground station antenna, the polarizers are quad ridge polarizers, most preferably quarter-wavelength (relative to the wavelength of the electromagnetic wave) quad ridge polarizers. Also most preferably, the second quad ridge polarizer is fixed at a 45-degree angle relative to the dipole(s) of the low-noise block. 
     Another ground station antenna of the present invention includes a main waveguide, two mutually orthogonal distal branch waveguides at a distal end of the main waveguide, a coupling mechanism for coupling two orthogonal antenna dipoles to a proximal end of the main waveguide, and a transformation mechanism. The transformation mechanism is operative to transform the polarization, of an electromagnetic wave, that propagates between the distal branch waveguides and a communication satellite via the antenna dipoles, so as to match the polarization of the electromagnetic wave to one of the distal branch waveguides and also to a satellite antenna on the communication satellite. 
     In one embodiment of the ground station antenna, the transformation mechanism includes two pairs of electrically-conducting dipoles mounted rotatably about the longitudinal axis of the main waveguide at the proximal end of the main waveguide, with both pairs of dipoles being oriented perpendicular to each other and to the longitudinal axis of the main waveguide. Preferably, the coupling mechanism couples each antenna dipole to one dipole of a respective dipole pair. 
     In another embodiment of the ground station antenna, the coupling mechanism includes two mutually orthogonal proximal branch waveguides at the proximal end of the main waveguide. Each proximal branch waveguide is coupled to a respective one of the antenna dipoles. The transformation mechanism includes two polarizers that are integral to the main waveguide (meaning that the polarizers are part of the main waveguide), with the first polarizer being rotatable relative to the branch waveguides wile the second polarizer remains fixed relative to the branch waveguides. Preferably, the second polarizer is fixed at a 45-degree angle (around the longitudinal axis of the main waveguide) relative to the distal branch waveguides. Also preferably, the polarizers are quad ridge polarizers, most preferably quarter-wavelength (relative to the wavelength of the electromagnetic wave) quad ridge polarizers. 
     A method of the present invention, for rotating the input direction of polarization of a linearly polarized transverse wave to an output direction, includes the step of transforming the transverse wave into a circularly polarized transverse wave and then transforming the circularly polarized transverse wave into a linearly polarized transverse wave whose direction of polarization is the desired output direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments are herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIGS. 1A and 1B  show a prior art parabolic dish antenna; 
         FIGS. 2A-2D  illustrate a LNBF of the present invention; 
         FIG. 3  illustrates the tapering of the dielectric slab of the polarizer of  FIG. 2C  or  2 D; 
         FIG. 4  is a simplified block diagram of a mechanism for pointing a moving ground station antenna at a geostationary satellite; 
         FIG. 5  is a transparent perspective view of a quad ridge polarizer; 
         FIG. 6  is a cross sectional view of the quad ridge polarizer of  FIG. 5  along line  5 - 5  and perpendicular to the longitudinal axis; 
         FIG. 7  is a plot of XPD vs. frequency for the dual slab polarizer of  FIG. 3  vs. the quad ridge polarizer of  FIGS. 5 and 6 . 
         FIGS. 8 and 9  illustrate embodiments of the present invention in which the mechanism for untangling the mixed linear polarization is downstream from fixed dipoles in reception and upstream from fixed dipoles in transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principles and operation of a ground station antenna according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Although the present invention is described herein are in terms of a parabolic dish antenna such as antenna  10 , the present invention is applicable to antennas of other shapes, e.g. flat antennas. Similarly, although the present invention is described herein in terms of radio-frequency signals in the Ku band, those skilled in the art will appreciate that the present invention also is applicable to other radio-frequency bands, such as the L band (1 GHz to 2 GHz), the S band (2 GHz to 3 GHz), the C band (4 GHz to 6 GHz), the X band (7 GHz to 9 GHz) and the Ka band (17 GHz to 20 GHz). Furthermore, although the present invention is described herein in terms of communication with a geostationary satellite, it will be appreciated that the present invention also is applicable to communication with satellites that are not in geosynchronous orbits. 
     Returning now to the drawings,  FIGS. 2A-2D  illustrate two embodiments 30 and 31 of a LNBF of the present invention.  FIG. 2A  is a side view of LNBF  30  showing that LNBF  30  includes, in series, a feed horn  48 , a waveguide  50  and a LNB  35 .  FIG. 2B  is a side view of LNBF  31  showing that LNBF  31  includes, in series, feed horn  48 , waveguide  50  and an Orthogonal Mode Transducer (OMT)  36 . Waveguide  50  includes a rotating polarizer  32  and a fixed polarizer  34 .  FIG. 2C , a cross section of LNBF  30  through section A-A, shows that rotating polarizer  32  of LNBF  30  includes a quarter-wavelength dielectric slab  42 .  FIG. 2D , a cross section of LNBF  30  through section B-B, shows that fixed polarizer  34  of LNBF  30  includes a quarter-wavelength dielectric slab  44 . Also shown in phantom in  FIG. 2D  are the orientations of the horizontal dipole  38  and the vertical dipole  40  of LNB  35 . Slab  44  is fixed at a 45-degree angle to both horizontal dipole  38  and vertical dipole  40 . 
     In general, a single quarter-wavelength dielectric slab that is placed at a 45-degree angle to a linearly polarized electromagnetic wave, transverse to the direction of propagation of the linearly polarized electromagnetic wave, transforms the linearly polarized electromagnetic wave to a circularly polarized electromagnetic wave. Appropriate rotation of just rotating polarizer  32 , as indicated by an arrow  46  in  FIG. 2C , suffices to keep LNB dipoles  38  and  40  aligned with the polarization directions of the satellite with which an antenna that includes LNBF  30  communicates. Specifically, rotating polarizer  32  is rotated to place slab  42  at a 45-degree angle to the polarization directions of the satellite. Rotating polarizer  32  transforms the linearly polarized signal from the satellite to a circularly polarized signal, and fixed polarizer  34  transforms the circularly polarized signal to a linearly polarized signal that is aligned correctly with the appropriate LNB dipole  38  or  40 . 
     Without loss of generality, an incoming, linearly polarized electromagnetic signal may be represented as: 
         H   IN (ω t )= x  cos(ω t )+ y  cos(ω t )
 
     where x and y are unit vectors perpendicular and parallel to a quarter-wavelength dielectric slab and ω is the angular frequency of the signal. The quarter-wavelength slab delays the phase angle of the parallel component of the signal by 90 degrees but does not shift the perpendicular component of the signal. As a result, the outgoing signal on the other side of the quarter-wavelength slab is: 
         H   OUT (ω t )= x  cos(ω t )+ y  sin(ω t )
 
     which is a pure circularly polarized signal. At a frequency ±Δf with respect to the central frequency f 0  of the signal, a phase shift error 
       Δφ=(±Δ f/f   0 )(π/2)
 
     is introduced that causes cross-polarization as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
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     The first term in brackets on the right hand side represents the right hand circularly polarized wave. The second term in brackets on the right hand side represents the linear unbalanced wave, which is half right hand circularly polarized and half left hand circularly polarized. Taking the time averaged power of these terms gives, for the first (circular) term: 
     
       
         
           
             
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     and for the second (linear) term at 50% power: 
       &lt;[ y√ 2 cos(ω t )sin(Δφ/2)] 2 &gt;=[sin(Δφ/2)] 2  
 
     Therefore, the cross polarization for a single slab in units of dB is 
         XPD= 20 log [sin(Δφ/2)]
 
     and the cross polarization for both slabs  42  and  44  is 
         XPD= 20 log [sin(Δφ/2)]+3 dB
 
     To minimize reflections in waveguide  50 , slabs  42  and  44  should be tapered in the direction of propagation, as shown in  FIG. 3 . The lengths A and B should satisfy 2A+B≈0.25λ/√∈, where λ is the wavelength of the electromagnetic signal in free space and ∈ is the dielectric constant of the dielectric material of slabs  42  and  44 . Length C is tuned for optimal matching of the propagating wave through waveguide  50 . Typical values of A, B and C for a Ku-band LNBF  30  are 2 mm, 4 mm and 4 mm, respectively. The dielectric material of slabs  42  and  44  should be of low loss tangent at the operating frequency, e.g. Plexiglas™ (polymethyl methacrylate). 
       FIG. 4 , which is adapted from FIG. 2 of Hsiung, is a simplified block diagram of a mechanism for pointing a parabolic dish antenna, that includes LNBF  30  and that is mounted on a moving vehicle, at a geostationary earth satellite while rotating polarizer  32  to keep LNB dipoles  38  and  40  aligned with the polarization directions of the satellite. A Global Positioning System (GPS) receiver  110  mounted on the vehicle receives signals from GPS satellites in a known manner and produces signals that represent vehicle position, the current time (coordinated Universal Time or UPC) and a one-pulse-per-second timing pulse, all of which are applied to a Digital Signal Processor (DSP)  112 . The vehicle position information includes latitude, longitude and altitude. A vehicle speed sensor  114  produces signals representing the speed of the vehicle, which are applied to DSP  112 . DSP  112  also receives signals representing vehicles roll, inclination (pitch) and azimuth angle (yaw) from (an) appropriate sensor(s)  116  mounted on the vehicle. One such sensor is the Crossbow Model HDX-AHRS, available from Crossbow Technology, Inc. of San Jose Calif., that senses roll, inclination and azimuth angle, and that includes a three-axis magnetometer to make a true measurement of magnetic heading. The azimuth information may be in the form of signals representing vehicle yaw relative to magnetic north; magnetic correction then can be performed in DSP  112  based on the location information from GPS receiver  110  together with stored magnetic declination data. GPS receiver  110 , orientation sensor(s)  116  and speed sensor  114  provide DSP  112  with data at an update rate faster than once per second, thereby allowing the antenna pointing system to have a near-real-time response. 
     The location of the satellite also is stored in DSP  112 . DSP  112  processes the sensor signals relative to the location of the satellite to produce antenna drive or control signals, which are applied to the drive motors of the parabolic dish antenna, including a motor for rotating polarizer  32 , to keep LNBF  30  pointed at the satellite and to rotate polarizer  32  to keep LNB dipoles  38  and  40  aligned with the polarization directions of the satellite. 
     If the communication satellite with which LNBF  30  communicates is a satellite, such as a Direct Broadcast Satellite (DBS), that use circularly polarized signals, then slab  42  is kept fixed at a 90-degree angle to slab  44 . The input signal may be represented as: 
         H   IN (ω t )= x  cos(ω t )= y  sin(ω t )
 
     A quarter-wavelength slab placed parallel to the x direction converts the circularly polarized signal to a linearly polarized signal with the vector components 
         H   OUT (ω t )= x  cos(ω t )± y  cos(ω t )=( x±y )cos(ω t )
 
     i.e., with a linear polarization direction at a 45-degree angle to the slab. Accordingly, if slab  42  is aligned 45-degrees or 135-degrees relative to slab  44 , the polarization of the intermediate generated linearly polarized signal, as detected by LNB dipole  38  or  40 , stays fixed and complies with the incoming left hand circular polarized signal or right hand circular polarized signal. 
     The embodiments described above have an acceptable XPD of at least 30 dB in the Ku transmission band (13.75 GHz to 14.5 GHz) or at least 20 dB in the Ku reception band (10.9 GHz to 12.75 GHz) but not in both bands simultaneously. Therefore, LNBF  30  is suitable for a reception-only system, such as a Television Receive Only (TVRO) system. To obtain acceptable XPD performance throughout the whole Ku band (10.9 GHz to 14.5 GHz), yielding a XPD of 40 dB in transmission and 20 dB in reception, the dual quad ridge design of Vezmar, U.S. Pat. No. 6,097,264 is used. U.S. Pat. No. 6,097,264 is incorporated by reference for all purposes as if fully set forth herein. 
       FIGS. 5 and 6 , that are adapted from U.S. Pat. No. 6,097,264, show a broad band quad ridge polarizing waveguide  200 . The waveguide has width a, height b, and length L. Preferably the height and width of the waveguide are equal. However this is not essential and the waveguide may have a rectangular or even a curved cross section. Waveguide  200  has four wall regions, such as walls  212 ,  214 ,  216 , and  218 , each having a respective axial ridge  220 ,  222 ,  224 ,  226 . The addition of a second pair of opposing ridges results in a lower cutoff frequency of the waveguide and increased frequency at which higher order modes can occur, therefore providing a device which operates over a relatively broad range of frequencies. The second pair of ridges have similar phase vs. frequency characteristics as the first pair. This allows for non-divergent phase characteristics over a relatively large bandwidth. Preferably, opposing ridges  220 ,  224  and  222 ,  226  are in alignment with each other. More preferably, each of the ridges is positioned equally distant from the two adjacent wall regions and run down the center of the wall on which it is located, as shown in the cross-section of  FIG. 6 . Most preferably, opposing ridges  20 ,  24  and  22 ,  26  are symmetric to each other and ridge pair  20 ,  24  has a different geometry than ridge pair  22 ,  26 . 
     The first pair of opposing ridges  220 ,  224  each have a height h 1  inward from the respective walls  212 ,  216 , a width w 1 , and a length L 1 . The height, width, and length of these ridges determines the phase shift of signal component E 1 . Similarly, the second pair of opposing ridges  222 ,  226  each have a height h 2  inward from respective walls  214 ,  218 , a width w 2 , and a length L 2 . The dimensions of ridges  222 ,  226  determine the phase shift of the other signal component, E 2 . The design of single and dual axial ridges is well known to those of skill in the art. See, e.g., W. Hoefer and M. Burton, Closed-Form Expressions for the Parameters of Finned and Ridged Waveguides,  IEEE Transactions on Microwave Theory and Techniques , Vol. MTT-30, No. 12, pp. 2190-2194, December 1982. Similar techniques may be utilized to select the proper dimensions for the additional ridges provided in the quad ridge configuration of  FIGS. 5 and 6 . 
     Advantageously, the variability in the height, width, and length of the four ridges allows sufficient freedom of design to achieve the two different phase velocities as required for broad band performance. The difference in phase between signal components E 1  and E 2  is designed to provide a circularly polarized output signal within the frequency range of interest. A wide bandwidth can be achieved if the phase characteristics of the orthogonal signal components E 1  and E 2  entering the waveguide  200  are approximately 90 degrees apart and have the same curvature over a wide frequency range. An exact match in curvature is achieved when both pairs of ridges are identical. However, this situation would not introduce the necessary phase difference between the components. 
     The dimensions of the ridges may be chosen to provide similar phase characteristics with close to a 90 degree phase difference over a wide frequency range. One configuration for achieving this result is for the first pair of ridges  220 ,  224  to have a relatively large width w 1  and height h 1 , but a small length L 1 , while the second pair of ridges  222 ,  226  have a comparatively narrow width w 2 , small height h 2 , but a long length L 2 . In other words, w 1  is greater than w 2 , h 1  is greater than h 2 , and L 1  is less than L 2 . Generally, the ridge width is not as critical a dimension as the length and height while in general, a relatively large height corresponds to a relatively small length. So in an alternate configuration, w 1  is equal to or even less than w 2  while h 1  is greater than h 2 , and L 1  is less than L 2 . 
     Preferably, the ends of the ridges are also stepped, as illustrated in  FIG. 5 . Stepping the ridges reduces the mismatch in impedance which results when there is an abrupt transition from a smooth to ridged waveguide wall by providing a gradual impedance transformation between the ridged portion of the waveguide and the input and output waveguide portions, which may be rectangular, square, or even curved. The design of stepped ridges is well known to those skilled in the art. See, e.g., S. Hopfer, The Design of Ridged Waveguides,  IRE Transactions on Microwave Theory and Techniques , Vol. MTT-3 pp. 20-29, October 1955. 
     A quad ridge polarizer may be manufactured as an integral die cast device. Advantageously, a quad ridge polarizer is inexpensively and accurately manufactured as an integrally molded component using die cast fabrication techniques and without the need to integrate dielectric materials with metallic materials. Preferably, the waveguide is made of aluminum or zinc, depending on its size. Other conventional materials such as copper also may be used. 
       FIG. 7  is a plot of XPD in dB vs. frequency in GHz for the dual slab polarizer of  FIG. 3  vs. for the quad ridge polarizer of  FIGS. 5 and 6 . 
     The superior XPD of a quad ridge polarizer allows a LNBF that uses such polarizers as polarizers  32  and  34  to be used for simultaneous reception and transmission. LNBF  31  of  FIG. 2B  is such a LNBF. OMT  36  couples a LNB (not shown) for reception and a Block Up-Converter (BUC) (not shown) for transmission to waveguide  50 . The LNB and the BUC are coupled to OMT  36  at branch waveguides similar to distal branch waveguides  82  of  FIG. 9  below. OMT  36  provides for simultaneous reception in one linear polarization and transmission in the other polarization by providing 35 dB decoupling (isolation) between transmission and reception. In addition, the LNB includes a transmission reject filter for an additional 35 dB isolation at minimum for undisturbed reception by the LNB in the receive band during transmission via the transmit band in the same LNBF  31 . 
     In the embodiments of  FIG. 2 , polarizers  32  and  34  are upstream from the LNB dipole(s) in reception and downstream from the transmission dipole of the BUC in transmission.  FIGS. 8 and 9  illustrate embodiments in which the mechanism for untangling the mixed linear polarizations is downstream from the orthogonal dipoles of a fixed (not rotating to align the dipoles with the satellite transponders) antenna in reception and upstream from the antenna dipoles in transmission. 
       FIG. 8A  is a perspective view of the distal portion of a main waveguide  60  that has a circular cross-section and a longitudinal axis  64 . At the distal end of main waveguide  60  are two branch waveguides  62 .  FIG. 8B  is an end-on view of the proximal end of main waveguide  60 , showing that at the proximal end of main waveguide  60  there is a frame  70  that is rotatable about axis  64 . Mounted in frame  70  are two electrically-conducting dipole pairs  66  and  68 . Each dipole pair  66  or  68  is coupled, via a respective coax adapter  72  and a respective coaxial cable  74 , to a respective one of the antenna dipoles. 
     In the frame of reference of the communication satellite, the received or transmitted signals are: 
     Horizontal signal: H x (t)=H 0 (t) 
     Vertical signal: V y (t)=V 0 (t) 
     Because the antenna is, in general, tilted with respect to the frame of reference of the satellite at an angle θ, the signals to/from the LNB are 
         H   x1 ( t )= H   x ( t )cos(θ)+ V   y ( t )sin(θ)
 
         V   y1 ( t )=− H   x ( t )sin(θ)+ V   y ( t )cos(θ)
 
     Following back-rotation by dipole pairs  66  and  68 , the signals from/to branch waveguides  62  are: 
     
       
         
           
             
               
                 
                   
                     
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     Frame  70  is rotated by antenna controller  120 , similarly to how polarizer  32  is rotated by antenna controller  120 , to maintain frame  70  at the angle θ that ensures that each branch waveguide  62  is dedicated to the correct signal. 
     The embodiment of  FIGS. 8A and 8B  suffers from the disadvantage that coaxial cables  74  can easily become entangled with each other. The embodiment illustrated in  FIG. 9  overcomes this disadvantage. The embodiment illustrated in  FIG. 9  includes a main waveguide  80  of circular cross-section. At the distal end of main waveguide  80  are two distal branch waveguides  82 . One branch waveguide is coupled to an LNB (not shown) for reception; the other branch waveguide is coupled to a BUC (not shown) for transmission. At the proximal end of main waveguide  80  are two proximal branch waveguides  84 . Each proximal branch waveguide is coupled to a respective antenna dipole by a coaxial cable (not shown). Between distal branch waveguides  82  and proximal branch waveguides  84  are two quarter-wave quad ridge polarizers: a rotating polarizer  86  and a fixed polarizer  88 . Fixed polarizer  88  is fixed at a 45-degree angle to distal branch waveguides  82 , just as slab  44  of fixed polarizer  34  is fixed at a 45-degree angle to LNB dipoles  38  and  40 . 
     The signals from/to distal branch waveguides  82  to/from proximal branch waveguides  84  are: 
           H   ( t )= H[x  cos(θ)+ y  sin(θ)] sin(ω t )
 
           V   ( t )= V[−x  sin(θ)+ y  cos(θ)] sin(ω t )
 
     where θ is the misalignment angle between the LNB polarization direction and the satellite&#39;s frame of reference. Antenna controller  120  dynamically adjusts the alignment angle of rotating polarizer  86  to be θ+45 degrees resulting in two circular wave components having the signals  H (t) and  V (t) respectively in the following form: 
           H   ( t )= H[x  cos(ω t )+ y  sin(ω t )]
 
           V   ( t )= V[−x  sin(ω t )+ y  cos(ω t )]
 
     which are clockwise and counterclockwise circularly polarized waves. Polarizer  88 , that is fixed at 45 degrees relative to distal branch waveguides  82 , transforms the circular polarized waves back to linear polarized waves that match the receive and transmit ports. Which one of distal branch waveguides  82  is to be coupled to the LNB and which is to be coupled to the BUC is determined by which of the two possible 45-degree orientations, relative to distal branch waveguides  82 , that polarizer  88  is fixed in. 
     It will be appreciated that the satellite communication technology described herein is an instantiation of an innovative method for rotating the plane of polarization of a linearly polarized transverse wave such as a linearly polarized electromagnetic wave (or, for that matter, any other linearly polarized transverse wave, e.g. a linearly polarized shear wave. The linearly polarized transverse wave is transformed into a circularly polarized wave. Then, the circularly polarized wave is transformed into a linearly polarized transverse wave whose plane of polarization is rotated as desired. Heretofore, polarizer slabs such as slabs  42  and  44  of  FIGS. 2C and 2D , and quad ridge polarizers such as those taught by Vezmar, have been used to convert circularly polarized signals to and from linearly polarized signals, but have not been used to rotate the polarization planes of linearly polarized signals. 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made. Therefore, the claimed invention as recited in the claims that follow is not limited to the embodiments described herein.