Abstract:
A selectable waveguide transitions between two positions to at least two independent signals by their polarization or frequency. This waveguide consists of dissimilar wave sections coupled to an antenna feed and is mechanically actuated for signal selection switching to route signals to output ports and respective probes that are polarization sensitive. The waveguide sections offer high polarization purity so that signal components remain separated to avoid mutual interference and low insertion loss to maintain system efficiency. Selectable waveguide can be extended for multiple polarization and frequency operation. Frequency selective surfaces and tapers establish pass bands and attenuation levels for frequency selection. The selectable waveguide is suitable for both frequency and polarization selectively in antenna system. Selectable waveguides can be cascaded and mechanically actuated for creating selectable waveguides arrangements enabling a wide variety of frequency and polarization selectively operating through a single apparatus.

Description:
STATEMENT OF GOVERNMENT INTEREST  
       [0001] The invention was made with Government support under contract No. F04701-93-C-0094 by the Department of the Air Force. The Government has certain rights in the invention. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates to the field of antenna systems used in satellite communications where orthogonal polarizations are employed to increase system capacity.  
         BACKGROUND OF THE INVENTION  
         [0003]    The demands for satellite communication capacity have resulted in the implementation of several different techniques. One technique is to extend satellite capacity using orthogonal polarization states to send two independent signals to the same coverage region thereby doubling the information that can be delivered to that region. This technique is referred to as polarization reuse. The success of this technique depends in part on the ability to maintain the separation of the two signals to avoid mutual interference that degrades communication performance. The required signal separation in turn imposes requirements on the polarization purity of the signals.  
           [0004]    Polarization reuse is very commonly used on commercial satellites operating at the C band (4-6 GHz) and Ku band (11-14 GHz) frequencies. The required separation between signals used in these systems depends on the power differences in the signal levels and the susceptibility of the reception to co-channel interference. A typical requirement for the polarization purity needed for signal separation is to limit the reception of the undesired signal to a level that is 27 dB lower, that is, {fraction (1/500)} of the power, than the desired signal component. The degree of polarization purity needed to satisfy this requirement is significantly more stringent than the polarization purity required to insure minimal signal loss caused by polarization mismatch.  
           [0005]    Different satellite systems, however, are not consistent in the polarization states used. Some systems use orthogonal linear polarization states while other systems use orthogonal circular polarization states. Within a given satellite system, antenna systems for a single polarization state have been developed. However, if antenna systems are developed for use with several different satellite systems, the antenna system requires the capability to select the polarization state depending on the satellite system being used. Clearly, antenna systems capable of operating with different satellite systems afford advantages in flexibility and potential cost effectiveness. However, such antenna designs have to be fully compatible with the requirements for each satellite system. In view of the various polarization signaling methods, antenna systems designed for inter-program compatibility must be capable of processing dual polarization signals with either linear or circular polarization states and must meet system requirements for polarization purity.  
           [0006]    The design requirements to achieve the requisite polarization purity must address the antenna, its feed system, and the ports for each polarization. These design requirements must be maintained over the entire bandwidth spanned by the satellite systems. The antenna, for example, must be designed with a high degree of symmetry so that cross polarized components are not generated that would degrade polarization purity. Similarly, the feed system must be designed to produce rotationally symmetric illumination of the antenna system and attention must be paid to the excitation of higher order modes that produce cross polarized components that degrade polarization purity. The terminals of the feed system must be constructed with precision to avoid polarization coupling, and any combining circuitry used to transform polarization states must satisfy stringent matching requirements to avoid the generation of cross polarized components that degrade polarization purity. The satisfaction of the overall system requirements for polarization purity is limited by the aggregate of the imperfections in the antenna, feed system, terminals and transforming circuitry.  
           [0007]    One fundamental limitation in the development of designs that permit selection of the polarization state results from the inherent imperfections when hybrid combining circuitry is used to transform polarization states. The conventional approach to this problem is to combine one of the polarization states with hybrid circuitry to obtain the other polarization state. The limitation of this approach lies with the inherent imperfections of the hybrid. Quadrature hybrids needed to convert the linearly polarized state to the circular polarized state can maintain a ninety degree phase shift but the amplitude response is unequal over the bandwidth. This amplitude imbalance results in coupling between the two polarization states resulting in co-channel interference. When linearly polarized components are transformed to circularly polarized components, for example, the circular components are obtained from the addition of equal levels of each linearly polarized component with a ninety degree phase shift between the components. Such combining is typically implemented using a quadrature hybrid. Practical hybrids provide the appropriate ninety degree phase shift but exhibit the problem of an imbalance when combining the amplitudes that then varies over the required bandwidth. This amplitude combining imbalance is a limiting factor in achieving the polarization isolation needed to maintain signal separation. A similar limitation exists with one hundred and eighty degree hybrids used to combine circularly polarized components to obtain linearly polarized components. One problem with one hundred and eighty degree hybrids is the resulting phase imbalance. A second problem is the insertion loss inherent when using combining circuitry results. Such insertion loss degrades system sensitivity. The insertion loss reduces transmitted power delivered to the antenna and also limits the power handling because the thermal energy resulting from the insertion loss must be dissipated. The insertion loss in receiving antennas not only reduces the received signal strength but also increases the total system temperature, a factor that is extremely important when modern low noise receivers are used.  
           [0008]    A means of switching is also required to select between the polarization states. Three distinct switch technologies exist. Diode switch devices can switch very rapidly but are relatively lossy and limited in their power handling capability. Ferrite switching technology has somewhat less loss, slower switching time, and greater power handling capability and very low loss, but with disadvantageous slow switching times. The low loss and power handling capabilities are desired in this polarization reuse applications and rapid switching may not present a problem. Thus, waveguide switch technology is preferred in this polarization reuse application having low loss and high power handling capabilities, but with slow switching times. Conventional waveguide switch has a single dominant waveguide mode. A dominant waveguide mode may be TE 01  or TE 10  for square polarized signals and orthogonally disposed TE 11  for circular polarized signals. Tapers and frequency selective surfaces have long been used for frequency isolation. The most familiar waveguide switch uses rotating waveguide bends to route the signals between four ports. The conventional waveguide switch has two selectable position settings for aligning two curved waveguide section bends symmetrical about a rotating axis. The curved selectable waveguide section does not use reflecting surfaces, but rather circular or rectangular cross section waveguide sections. This dual position arrangement is analogous to a double-pole double throw switch. This configuration is commonly referred to as a baseball switch, because the waveguide bends resemble the stitching on a baseball. However, this switch technology is not capable of switching orthogonally polarized signals because the bends inherently result in coupling between the linear and circular polarized signals. These and other disadvantages are solved or reduced using this invention.  
         SUMMARY OF THE INVENTION  
         [0009]    An object of the invention is the capability to receive and/or transmit dual orthogonally polarized signals with selection between linear and circular states.  
           [0010]    Another object of the invention is to achieve a high degree of polarization purity over a wide bandwidth to avoid co-channel interference of one signal to another.  
           [0011]    Yet another object of the invention is to achieve a low loss design to increase system efficiency in antenna systems.  
           [0012]    A further object of the present invention is to provide the means of transmitting and/or receiving two orthogonally polarized antenna signals with a high degree of polarization purity and with low loss and the capability to select either linearly or circularly polarized polarization states.  
           [0013]    Yet a further object of the present invention is to provide the capability for a dual polarized, selectable polarization state waveguide capable of operation for multiple frequency bands.  
           [0014]    The present invention is directed towards a waveguide switch having a plurality of switch positions for communicating a signal between at least one input port and a respective plurality of output ports through a respective plurality of dissimilar waveguide sections. In the preferred form, the waveguide switch has two output ports respectively connected to the input port through a straight waveguide section and a bent waveguide section. The waveguide switch is preferably used to receive and/or transmit dual polarized signals through an antenna feed input port between a linear output port using the bent waveguide section coupled to a linear polarization state sensitive probe and a circular output port coupled to a circular polarized probe using the straight waveguide section providing the capability to select either linearly or circularly polarized polarization state signal transmitted through the antenna feed port. This present invention provides a high level of polarization purity needed to separate two independent signals by polarization. The present invention is directed to selectable waveguides having selectable waveguide sections to perform the polarization state selection, and the loss incurred by these sections is much less than the losses in hybrid combining circuitry used in the conventional polarization state transformations. The waveguide sections can be sized, cascaded and coupled to frequency sensitive tapers and couplers for both polarization state selection and frequency selection of signals in applications where multiple frequency or multiple polarization state operation is required, for example, in simultaneous C band and Ku band operation.  
           [0015]    The preferred selectable waveguide has two positions for respectively selecting one of two waveguide sections within the selectable waveguide. The selectable waveguide is capable of propagating the two independent orthogonal channels. A waveguide is connected to an antenna feed capable of propagating two independent orthogonally polarized communication channels. A selector switch, knob, or other mechanical means on the waveguide is used to select one of the two waveguide sections to thereby select one of the two independent orthogonally polarized communication channels. Output ports of the selectable waveguide are used for separating the respective polarization states of the channels using respective polarization sensitive probes. The waveguide switch is thus used to route the transmitting or receive channel signals into either the circular polarized output port realized by an orthomode transducer capable of high polarization purity over wide bandwidths or to the linear polarized output port realized by an orthogonal linear polarized probe in the waveguide capable of high polarization purity over wide bandwidths.  
           [0016]    Preferably, the selector switch is used to transfer either linear or circular polarization signal components to respective ports. Like the conventional waveguide switch, the selection is preferably accomplished by mechanical rotation. Unlike conventional switches, however, the improved selectable waveguide has dissimilar waveguide sections that can respectively operate in two dominant modes. One switch setting consists of a straight waveguide section so that higher order modes and mode coupling does not occur. The second switch setting changes the direction of propagation by ninety degrees using a waveguide miter bend to avoid higher order mode generation. The axis of rotation is offset to permit the rotation of the switch and the port alignment. The improved selectable waveguide switch of the present invention is effectively a single-pole double-throw waveguide switch using three ports.  
           [0017]    These selectable waveguide switches can be frequency sized and cascaded for multiple frequency applications. Such cascading can be readily performed when the switch has the straight waveguide section. When the switch is placed in the position of bent section containing a miter bend, the conducting miter is replaced by a frequency selective surface to allow passage of the higher frequency signals to subsequent selector waveguide switches. Frequency sensitive couplers and tapers can be coupled to the switches to various operational configurations for selecting the signal of desired frequency and polarization. In addition to the ability to maintain polarization purity, the waveguide sections of the selector switch have little loss in comparison to hybrid network losses in the conventional approach. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a drawing of a selectable waveguide switch shown in the straight position.  
         [0019]    [0019]FIG. 2 is a drawing of the selectable waveguide switch shown in the bent position.  
         [0020]    [0020]FIG. 3 a  is a drawing of a modified selectable waveguide having a modified bent waveguide section.  
         [0021]    [0021]FIG. 3 b  is a drawing of the modified selectable waveguide in the straight position with an attached coupler for multiple frequency operation.  
         [0022]    [0022]FIG. 4 is a drawing illustrating a cascade arrangement of selectable waveguides for multiple frequency operation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    An embodiment of the invention is described with reference to the figures using reference designations as shown in the figures. Referring to both FIGS. 1 and 2, a selectable waveguide can be positioned into one of two positions, a straight waveguide position shown in FIG. 1 and a bent waveguide position shown in FIG. 2. An antenna feed port  10  communicates a feed signal  12  to and from an antenna feed  13 . In the straight position, the antenna feed port  10  communicates the feed signal  12  through a straight waveguide section  14  to a circular port  16  communicating a circular port signal  18 . The waveguide sections  14  and  20  are physically sized to transmit and receive signals within desired frequencies bands. The circular port signal  16  may be either a linearly polarized signal or a circularly polarized signal or may comprise a plurality of differing polarized signals. The circular port  16  is coupled a circular port probe  19  for communicating the circular port signals  18  to and from the antenna feed port  10 . The feed signal  12  is either a linear polarized signal or a circular polarized signal, or may be a composite signal having a plurality of differing polarized signals having respective polarized states. In the bent position, the selectable waveguide communicates the feed signal  12  through a bent waveguide section  20  to a linear port  22  communicating a linear port signal  24  that may be either a linearly polarized signal or a circular polarized signal and that may comprise a plurality of differing polarized signals. The linear port  22  is coupled to a linear port probe  25  for communicating the linear port signal  24  to and from the antenna feed port  10 .  
         [0024]    The bent waveguide section  20  has a reflecting surface  26  for reflecting the feed signal  12  and linear port signal  24  communicated through the bent waveguide section  20 . The direction of the signal path through the bent waveguide section  20  is reflected by ninety degrees using the reflecting surface  26  in the path of the bent waveguide section  20  to communicate linear polarized signals between the linear port  22  and the antenna feed port  10 . The reflecting surface  26  is for reflecting signals  24  and  12  communicated through the bent waveguide section  26 . This reflection is achieved by a miter bend to avoid mode conversion and coupling between the polarized component signals of the signals  24  and  12  that would reduce the separation between component signals.  
         [0025]    It should be apparent that the bent waveguide section  20  could be a curved waveguide or other suitable waveguide section so long at it connects the feed port  10  to the port  22  when in a first position, and is therefore dissimilar to in shape to the straight waveguide section  20 . Also, the straight waveguide section  20  could be a curved waveguide section or other suitable waveguide section so long at it connects the feed port  10  to the port  16  when in a second position, and is also therefore dissimilar in shape to the bent waveguide section  20 . That is, the two waveguide sections  14  and  20  must be dissimilar in shape for connecting the feed port  10  to respective output ports  16  and  22 . However, curved waveguide sections are limited to a single polarization state. Additionally, while the preferred form has only two sections  14  and  20 , additional sections could be added, so that there is at least a plurality of the dissimilar waveguide with respective sections and output ports.  
         [0026]    The port  10  is designated generally as an input port, and, the ports  22  and  16  are designated generally as output ports, but, ports  16  and  22  may transceive signals  24  and  18  to and from the port  10  as the feed signal  12 . The signal  12  is generally designated as an input signal having a plurality of component signals, such as signals  24  and  18 , having differing orthogonal polarization states, such as linear or circular polarization states, left hand circular or right hand circular polarization states, and linear horizontal or linear vertical polarization states. The signal separation and isolation by desired polarization states are realized by polarization sensitive probes  19  and  25  and waveguide switch selection at the respective straight and bent switch positions.  
         [0027]    To change positions from a bent waveguide position to and from a straight waveguide position, the selectable waveguide has a rotating selector knob  28  or other mechanical means for rotating a rotating element  30  supporting the bent waveguide section  20  and straight waveguide section  14  on a stationary housing  32 . The selectable waveguide preferably uses the rotating element  30  in the stationary housing  32  to change positions for respectively communicating signals  18  or  24 . As preferably designated, the selectable waveguide uses the bent waveguide  20  to communicate linearly polarized signals  24  and uses the straight waveguide  14  to preferably communicate circularly polarized signals  182 . The bent waveguide section  20  and the straight waveguide section  14  can have either a square or circular cross section and sized for the frequencies of interest. The manually actuated rotating knob  28  is rotated to connect either the bent waveguide  20  or the straight waveguide  14  between the antenna feed port  10  and either of the linear port  22  or the circular port  16 , respectively. Hence, the bent waveguide section  20  preferably communicates a linearly polarized signal  24  as feed signal  12  between the linearly polarized port  22  and the antenna feed port  10 , and, the straight waveguide section  14  preferably communicates circular polarized signals  12  as feed signal  12  between the circularly polarized port  16  and the antenna feed port  12 . Hence, the rotating knob  28  only has two positions, the first position connecting the linear port  22  to the antenna feed port  10  for linearly polarized signal communication, and the second position connecting the circular port  16  to the antenna feed port  10  for circularly polarized signal communication.  
         [0028]    The polarization sensitive probes  19  and  25  are preferably used to separate by polarization states the two orthogonal polarized signals  18  and  24 . The linear port  22  may communicate two independent signals separated by orthogonal polarization states, such as, linear horizontal and linear vertical polarization states. Likewise, the circular port  22  may communicate two independent signals separated by orthogonal polarization states, such as, left hand and right hand circular polarization signals. Each of the probes  19  or  25  are preferably responsive to a predetermined polarization state and as such are used to isolate and separate two independent orthogonally polarized component signals.  
         [0029]    By rotating the rotating element for waveguide section alignment, the probes  19  and  25  are thereby rotated into a position for receiving or transmitting one of the plurality of differing polarized signals, thereby perfecting a polarization state selection. The waveguide cross sections  14  and  20  remains unaltered from the antenna feed port  10  to either of the linear port  22  and the circular port  16 . The cross section areas of the waveguide sections  14  and  20  remain fixed within the selectable waveguide. Because the waveguide cross section remains unchanged, no mechanism exists for polarization modifications from antenna feed port  10  through the waveguide sections  14  and  20  to the ports  22  and  16 . Consequently, the waveguide does not degrade polarization isolation. The waveguide cross sections  14  and  20  may be square and in this case the signals are propagated on TE 01  and TE 10  waveguide modes. The waveguide cross section can also be circular and the signals  18  and  24  are propagated on orthogonal TE 11  waveguide modes. Hence, the waveguide cross section of the sections  14  and  20  is preferably preserved throughout the rotating member  30 .  
         [0030]    The waveguide section selection, and hence polarization state selection, by rotating the knob  28 , may be by conventional mechanical means to route the feed signals  12  to one of port  22  and  16  to thereby place a respective polarization sensitive probe  19  or  25  in the path of feed signal  12 . Like conventional waveguide baseball switches, the rotation can be manually performed or accomplished by using a motor drive that can be remotely controlled. However, the waveguide section selection knob  28  has the improved features of offering polarization state selection using dissimilar waveguide sections  14  and  20  and using respective dissimilar polarization state sensitive probes  19  and  25 . The rotating knob  28  is used to both select one waveguide section  14  or  20 , and to simultaneously select the one of the two respective probes  19  and  25  to perfect polarization state selection. The first switch selection position selects the straight waveguide section  14  and probe  19  to connect the antenna feed port  10  to the circular port  16 , and to select the polarization sensitive probe  19  communicating signal  24  of one polarization state. The second switch selection position is obtained by rotating the knob one hundred and eighty degrees to select the bent waveguide section  20  of the selectable waveguide to connect the antenna feed port  10  to the linear port  22 , and to select the probe  24  communicating signal  18  having a differing polarization state. Hence, the knob  28  is in effect a polarization state selection knob  28  to select one of a plurality of orthogonally polarized signals without coupling energy between the signals that would otherwise degrade the signal separation.  
         [0031]    Communication devices, such as probes  19  and  25 , connected at the circular port  16  and the linear port  22 , are designed to separate the component signals by their polarization states. A means for separating polarized signals  10  is to place probes in a waveguide section located ninety degrees apart in adjacent walls of the waveguides. Similarly, the ports  22  and  16  would separate polarized signals typically by an orthomode probe. These probes for separating signals by polarization are well known and capable of operation over wide bandwidths.  
         [0032]    The losses in dual polarized signal communication through the selectable waveguide result from the losses within the waveguide sections  14  and  22  which losses are very small. The losses in the waveguide sections  14  and  20  are less than the insertion losses associated with conventional hybrid networks. Thus, signal reception and transmission for the present invention are more efficient. The waveguide sections  14  and  20  are preferably used to select one of the two orthogonally polarized signals by virtue of the polarization sensitivity of the probes  19  and  25 , but can also be used to select signals  18  and  24  of differing frequencies.  
         [0033]    Referring to FIG. 3 a , a modified selectable waveguide  39  may be used for both polarization state and frequency selection of the feed signal  12  communicated to the antenna feed  13 . The modified selectable waveguide section  40  can be used in applications where multiple frequency operation is required. The waveguide  39  is initially sized to communicate signals within desired frequency bands. The modified selectable waveguide  39  includes a modified bent waveguide section  40  having an extended straight portion  42  and a frequency selective reflective surface  44 . The extended portion  42  is aligned to the port  16  when the bent waveguide section  40  is aligned to port  22  when the modified selectable waveguide  39  is switched to the bent position. The frequency selective reflective surface  44  is used to reflect signals  24  of one frequency, such as low frequency signals, to the port  22 , and to pass signals of another frequency, such as high frequency signals, to the port  16 . The probes  19  and  25  can then be used to select signals of differing polarization states, and by virtue of the frequency sensitive reflective surface  44 , concurrently select signals of differing frequencies.  
         [0034]    Referring to FIG. 3 b , the modified selectable waveguide  39  is attached to a coupler  46  having a left hand port  48  communicating left hand port signal  47  to a left hand probe  49 , and, having a right hand port  50  communicating right hand port signals  51  to a right hand probe  53 . As such, the probes  53  and  49  are used to isolate orthogonally polarized signals, such as right hand circular and left hand circular polarized signals. It should be apparent that the coupler  46  functions as a splitter providing two outputs, and that the coupler  46  and probes  49  and  53  could, as well, be attached to port  22  for respectively communicating horizontal linear and vertical linear orthogonally polarized signals  24 . The coupler  46  has a taper port  52  for attenuating low frequency component signals and passing high frequency component signals  54  to the probe  19 . Hence, the modified selectable waveguide  39  can be modified to include means that provide frequency selection while the probes  15  and  19  can be used to select desired polarization states to isolate signals of interest. It should now be equally apparent, that the selectable waveguide of FIGS. 1 and 2, and or the modified selectable waveguide  39  of FIGS. 3 a  and  3   b , can be used in combination with various probes, couplers and tapers to isolate signal of desired polarization states and frequencies. Further still, the selectable waveguide of FIGS. 1 and 2, and or the modified selectable waveguide  39  of FIGS. 3 a  and  3   b , can be cascaded and used in combination with various probes, couplers and tapers to isolate many different signals of respective desired polarization states and frequencies.  
         [0035]    Referring to FIG. 4, two modified selectable waveguides, a front end waveguide  39   a  and a back end waveguide  39   b , are cascaded for multiple frequency and multiple polarization state communication applications. Frequency selection and polarization state selection are enabled by the cascaded arrangement in combination with various probes, couplers and tapers. The two waveguides  39   a  and  39   b  are both shown in the straight position, but either or both may be rotated to the bent position, thereby providing a four position cascaded arrangement providing a straight-straight position, a straight-bent position, a bent-straight position and a bent-bent position.  
         [0036]    In the straight position, the waveguide dimension is chosen to permit propagation of all system frequencies. The straight position is preferably used for communicating circularly polarized signals at the lower frequencies. All of the signals propagate unmodified through the straight waveguide sections  14   ab . At the output circular port  16   a  of the modified selectable waveguide  39   a , the coupler  46   a  is used to separate the lowest frequency signals into ports  48   a  and  50   a . The port  48   a  can be used for left hand polarized signals, and the port  50   a  can be used for selecting right hand polarized signals in the lowest frequency band. The coupler  46   a  is transparent to the higher frequencies. The design of such couplers is well known and commonly used. The waveguide taper  52   a  follows the coupler  46   a  so that the waveguide size is reduced permitting propagation of signals of all frequencies except the lowest frequency signals. The second modified selectable waveguide  39   b  has smaller dimensions and follows the taper port  52   a . The selectable waveguide  39   a  is transparent to frequency bands above the lowest frequencies. The coupling of the lower frequency band to ports  22   ab  is enabled in the bent positions. The miter bends have frequency selection surfaces  44   ab  in place of a conducting surface  26  used by the single frequency selectable waveguide switch design. These frequency selective miter surfaces  44   ab  reflect the lowest frequency signals  24   ab  into the linearly polarized ports  22   ab  for connection to respective probes  25   ab . The frequency selective miter surfaces  44 ab are transparent to higher frequencies so that the higher frequency signals  54   a  can be communicated through the cascaded arrangement at the higher frequencies.  
         [0037]    Each of the modified selectable waveguides  39   ab , respectively includes ports  16   ab ,  22   ab ,  48   ab  and  50   ab , tapers  52   ab , straight waveguide sections  14   ab , and bent waveguide sections  40   ab . Waveguide  39   a  has the feed port  10   a  receiving the feed signal  12  and provides the output signal  54   a  that is fed into the feed port lob of waveguide  39   b  to provide the output signal  54   b  to probe  19 . Probes  25   ab  respectively communicating signals  24   ab , probes  49   ab  respectively communicating signals  47   ab , probes  53   ab  respectively communicating signals  51   ab , and probe  19  communicating signal  54   b , all of which can be used for selecting signals of differing frequencies and polarization states.  
         [0038]    The cascaded arrangement places the low frequency band modified selectable waveguide  39   a  closest to the antenna feed port boa and the antenna feed  13 , whereas the high frequency band modified selectable waveguide  39   b  may be used to communicate signals in a high frequency band. In the polarized selectable waveguide  39   a  closest to the antenna feed  13 , a modification can be made to miter bend. In single frequency designs, the miter bend  44   a  consists of a conducting surface  26 . In the multiple frequency design, the conducting miter surface  26  is replaced by a frequency selective surface  44   a  capable of reflecting the lowest frequency components and passing the higher frequency components. The coupler  46   a  passes only low frequency signals to ports  48   a  and  50   b . The coupler  46   b  passes only high frequency signals to the ports  48   b  and  50   b . Another frequency selective surface  44   b  can be used to prevent mode conversion and signal loss for the higher frequency components. The frequency selective surfaces  44   a  and  44   b  and taper ports  52   a  and  52  can be used for low, high, higher frequency band isolation.  
         [0039]    In the straight-straight position, the arrangement passes low frequency signals  47   a  and  51   a , passes high frequency signals  54   a ,  47   b  and  50   b , and passes higher frequency signals  54   b . In the bent-straight position, the arrangement passes low frequency signals  25   a , passes high frequency signals  54   a ,  47   b  and  50   b , and passes higher frequency signals  54   b . In the straight-bent position, the arrangement passes low frequency signals  47   a  and  51   a , passes high frequency signals  54   a ,  25   b ,  47   b  and  51   b , and passes higher frequency signals  54   b . In the bent-bent position, the arrangement passes low frequency signals  25   a , passes high frequency signals  54   a  and  25   b , and passes higher frequency signals  47   b ,  50   b , and  54   b . Preferably, the port  22   a  communicates low frequency linearly polarized signals  24   a  to probe  25   a , port  48   a  communicates low frequency left hand circularly polarized signals  47   a  to probe  49   a , port  50   a  communicates low frequency right hand circularly polarized signals  51   a  to probe  53   a , port  52   a  communicates high frequency signals to port lob, port  22   b  communicates high frequency linearly polarized signals  24   b  to probe  25   b , port  48   b  communicates high frequency left hand circularly polarized signals  47   b  to probe  49   b , port  50   b  communicates high frequency right hand circularly polarized signals  51   b  to probe  53   b , and port  52   b  communicates higher frequency signals to probe  19 .  
         [0040]    As may now be apparent, several selectable waveguides in combination with various frequency sensitive couplers and tapers can be coupled together and cascaded to provide a plurality of polarization states and frequency selections, all by means of simple rotation of the selectable waveguides. Hence, the selectable waveguide can be used for multiple frequency and multiple polarization selection and operation using both the straight and bent positions and using frequency selective tapers, coupler, and surfaces. These switches are cascaded so that the polarization selection can be made at desired frequencies. This cascade arrangement permits independent polarization selection at each of the used frequencies.  
         [0041]    The selectable waveguide switch can be readily applied a frequency selection application. However, other applications exist for the selectable waveguide. Waveguide switches are commonly used to connect other alternatives or redundant electronics into systems. For antennas designed to operate with several different satellites, the selectable switch can be used advantageously with different transmitters. As an example, a system may be required to provide both low and high data rate communications with different satellite systems. The transfer of high data rate information generally requires higher transmitted power than low data rate communications, and the frequency assignments within the band may differ somewhat between the satellite systems. The selectable waveguide switch can be used to connect two different transmitters having different power capabilities to the same antenna feed. Another common requirement is to be able to switch transmitters between the antenna and a dummy load. The dummy load permits operating the transmitters for diagnostic testing without radiating through the antenna causing needless interference. The selectable waveguide switch can be used advantageously in this application permitting use of a single dummy load for both orthogonal polarization states. Such a design can be more compact than one using two dummy loads for each polarization state. Those skilled in the art can make enhancements, improvements, and modifications to enhance the invention and extend the application of the selectable waveguide switch. However, those enhancements, improvements and modifications may nonetheless fall within the spirit and scope of the following claims.