Abstract:
There is disclosed an ortho-mode transducer. An annular common waveguide may be defined by an outside surface of an inner conductor and an inside surface of an outer conductor, the outside surface and the inside surface concentric about a waveguide axis. A first port may couple a first TE 11  mode to the annular common waveguide. A second port may couple a second TE 11  mode to the annular common waveguide, the second TE 11  mode orthogonal to the first TE 11  mode. A first back-short may be disposed adjacent to the first port. A second back-short may be disposed on the outside surface of the inner conductor between the first port and the second port.

Description:
NOTICE OF COPYRIGHTS AND TRADE DRESS 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. This patent document may show and/or describe matter which is or may become trade dress of the owner. The copyright and trade dress owner has no objection to the facsimile reproduction by anyone of the patent disclosure as it appears in the Patent and Trademark Office patent files or records, but otherwise reserves all copyright and trade dress rights whatsoever. 
     BACKGROUND 
     1. Field 
     This disclosure relates to ortho-mode transducers for coupling orthogonally polarized TE 11  modes into or from coaxial waveguides. 
     2. Description of the Related Art 
     Satellite broadcasting and communications systems commonly use separate frequency bands for the uplink to and downlink from satellites. Additionally, one or both of the uplink and downlink typically transmit orthogonal right-hand and left-hand circularly polarized signals within the respective frequency band. 
     Typical antennas for transmitting and receiving signals from satellites consist of a parabolic dish reflector and a coaxial feed where the high frequency band signals travel through a central circular waveguide and the low frequency band signals travel through an annular waveguide coaxial with the high-band waveguide. Note that the terms “circular” and “annular” refer to the cross-sectional shape of each waveguide. An ortho-mode transducer may be used to launch or extract orthogonal TE 11  linear polarized modes into the high-band and low-band coaxial waveguides. A linear polarization to circular polarization converter is commonly disposed within each of the high-band and low-band coaxial waveguides to convert the orthogonal TE 11  modes into left-hand and right-hand circular polarized modes for communication with the satellite. 
     An ortho-mode transducer (OMT) is a three-port waveguide device having a common waveguide coupled to two branching waveguides. Within this description, the term “port” refers generally to an interface between devices or between a device and free space. A port may include an interfacial surface, an aperture in the interfacial surface to allow microwave radiation to enter or exit a device, and provisions to mount or attach an adjacent device. 
     The common waveguide of an OMT typically supports two orthogonal linearly polarized modes. Within this document, the terms “support” and “supporting” mean that a waveguide will allow propagation of a mode with little or no loss. In a feed system for a satellite antenna, the common waveguide may be a circular waveguide or an annular waveguide. The two orthogonal linearly polarized modes may be TE 11  modes which have an electric field component orthogonal to the axis of the common waveguide. Two precisely orthogonal TE 11 , modes do not interact or cross-couple, and can therefore be used to communicate different information. 
     The common waveguide terminates at a common port aperture. The common port aperture is defined by the intersection of the common waveguide and an exterior surface of the OMT. 
     Each of the two branching waveguides of an OMT typically supports only a single linearly polarized mode. The mode supported by the first branching waveguide is orthogonal to the mode supported by the second branching waveguide. Within this document, the term “orthogonal” will be used to describe the polarization direction of modes, and “normal” will be used to describe geometrically perpendicular structures. 
     The two branching ports and the associated waveguides are commonly termed the “vertical” and “horizontal” ports. The terms “horizontal” and “vertical” will be used in this document to denote the two orthogonal modes and the waveguides and ports supporting those modes. Note, however, that these terms do not connote any particular orientation of the modes or waveguides with respect to the actual physical horizontal and vertical directions. 
     In order to minimize coupling between orthogonal TE 11  modes, the OMT that launches the TE 11  modes must provide high isolation between the orthogonal TE 11  modes, and must avoid launching or coupling the TEM (transverse electromagnetic) mode and higher order modes. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an exemplary OMT for a coaxial waveguide. 
         FIG. 2  is an end view of the exemplary OMT. 
         FIG. 3A  is a side view of the exemplary OMT. 
         FIG. 3B  is a detail from  FIG. 3A  showing the dimensions of a waveguide. 
         FIG. 4A  is another side view of the exemplary OMT. 
         FIG. 4B  is a detail from  FIG. 4A  showing the dimensions of another waveguide. 
         FIG. 5  is a cross-sectional view through the axis of the exemplary OMT. 
         FIG. 6  is another cross-sectional view through the axis of the exemplary OMT. 
         FIG. 7  is a perspective view of the inner conductor of the exemplary OMT. 
         FIG. 8  is a graph showing the simulated performance of an OMT. 
         FIG. 9  is another graph showing the simulated performance of an OMT. 
     
    
    
     Throughout this description, elements appearing in views of the OMT are assigned three-digit reference designators, where the most significant digit is the figure number where the element was first introduced and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator. 
     DETAILED DESCRIPTION 
     Description of Apparatus 
     Referring now to  FIG. 1 , an exemplary OMT  100  may include an inner conductor  110  and an outer conductor  120 . The outer conductor  120  may also function as the body of the OMT  100 . A generally cylindrical opening in the inner conductor  110  may define a circular waveguide  115 . A space between the inner conductor  110  and the outer conductor  120  may define an annular waveguide  125 , which may be coaxial with the circular waveguide  115 . The annular waveguide  125  may be the common waveguide of the OMT  100 . 
     The circular waveguide  115  and the annular waveguide  125  may terminate at a common port  130 . The common port  130  may be defined by the intersection of the annular waveguide  125  and a common port flange  132 . The common port flange may be provided with tapped or thru mounting holes  136 . Both the cylindrical waveguide  115  and the annular waveguide  125  may be coupled to other waveguide components (not shown) that may be bolted via the mounting holes  136 , or otherwise coupled to the common port flange  132 . 
     A horizontal port  140  may be adapted to couple a horizontal TE 11  mode to the annular waveguide  125 . The horizontal port  140  may be defined by the intersection of a horizontal waveguide  144  and a horizontal port face  142 . The horizontal waveguide  144  may have a generally rectangular cross-sectional shape. As shown by the dashed arrow, the electric field vector of the horizontal TE 11  mode may be aligned with the shorter dimension of the horizontal waveguide  144 . Tapped holes  146  may be provided in the horizontal port face  142  to allow attachment of additional waveguide components (not shown). 
     A vertical port  150  may be adapted to couple a vertical TE 11  mode to the annular waveguide  125 . The vertical port  150  may be defined by the intersection of a vertical waveguide  154  and a vertical port face  152 . The vertical waveguide  154  may have a generally rectangular cross-sectional shape. As shown by the dashed arrow, the electric field vector of the vertical TE 11  mode may be aligned with the shorter dimension of the vertical waveguide  154 . Tapped holes  156  may be provided in the vertical port face  152  to allow attachment of additional waveguide components (not shown). 
     The horizontal port  140  and the vertical port  150  may be disposed on the OMT such that the horizontal TE 11  mode and the vertical TE 11  mode are orthogonal. To this end, the plane of the horizontal port face  142  may be normal to the plane of the vertical port face  152 . Further, the axis of the horizontal rectangular waveguide  144  and the axis of the vertical rectangular waveguide  154  may be normal. 
     The circular waveguide  115  may terminate at the common port  130  at one end, and at a circular port  190  (not visible in  FIG. 1 ) at the other end. 
       FIGS. 2 ,  3 B,  4 B,  5 , and  6  include dimensions defining a specific embodiment of the OMT  100 . The specific embodiment is intended for use in a frequency band from 19.4 GHz to 21.2 GHz, and was designed to satisfy a specific set of requirements. These dimensions are provided as representative example of an OMT. Other embodiments of the OMT  100  intended for use in other frequency bands and for other applications may have significantly different dimensions. 
       FIG. 2  is an end view of the exemplary OMT  100  normal to the plane of the common port  130 . For clarity, certain internal features of the OMT, visible through the annular waveguide  125 , are not shown. The OMT  100  may include an inner conductor  110  and an outer conductor/body  120 . The inner conductor  110  may have an inner surface  212  and an outer surface  214 . The inner surface  212  of the inner conductor  110  may define and bound the circular waveguide  115 . The outer conductor  120  may have an inner surface  222 . The surfaces  212 ,  214 , and  222  may be generally cylindrical and coaxial. The outer surface  214  of the inner conductor  110  and the inner surface  222  of the outer conductor  120  may define and bound the annular waveguide  125 . 
     The annular waveguide  125  may have an inner diameter Di, as defined by the surface  214 , and an outer diameter Do, as defined by the surface  222 . In the specific embodiment of the OMT  100 , Di may be 0.280 inches and Do may be 0.420. 
       FIG. 3A  is side view of the exemplary OMT  100  normal to the plane of the horizontal port face  142 . Looking into the horizontal waveguide  144 , three segments a, b, c having differing cross-sectional areas can be seen. Segment a, having the largest cross sectional area, opens to the horizontal port face  142 . Segment c, having the smallest cross-sectional area, opens to the annular waveguide  125  (not visible). The section line A-A defines a plane containing the axis of the annular waveguide  125  and the axis of the horizontal waveguide  144 . A cross-sectional view of this plane will be shown in  FIG. 5 . 
     The three segments a, b, c of the horizontal waveguide  144  may function as matching sections to couple the horizontally polarized TE 11  mode from the annular waveguide  125  (not visible), while simultaneously rejecting the vertically polarized TE 11  mode. The term “rejecting” as used in this document means that the vertically polarized mode is cut-off in the horizontal waveguide  144  such that power is not transferred from the annular waveguide to the horizontal port  140 . 
     The cross-sectional shapes and lengths of the three segments a, b, c of the horizontal waveguide may be designed to minimize the return loss for a horizontally polarized TE 10  mode introduced via a standard waveguide (not shown) attached to the horizontal port face  142 . The cross-sectional shape of segment a of the horizontal waveguide  144  may define a horizontal port aperture in the horizontal port face  142 . The cross-sectional shape of the horizontal port aperture may be different from, and not coaxial with, the cross-sectional shape of the standard waveguide (not shown) to be attached to the horizontal port face  142 . The transition from the cross-sectional shape of the horizontal port aperture and the cross-sectional shape of the attached standard waveguide may contribute to the matching function described in the prior paragraph. 
       FIG. 3B  is a detail from  FIG. 3A  showing the cross-sectional dimensions of the three segments a, b, c of the horizontal waveguide  144 . Since the cross-sectional areas of the three segments a, b, c of the horizontal waveguide  144  decrease in order without any hidden or undercut surfaces, the horizontal waveguide  144  may be inexpensively formed by machining with an end mill or other machining process. 
       FIG. 4  is another side view of the exemplary OMT  100  normal to the plane of the vertical port face  152 . Looking into the vertical waveguide  154 , two segments f, g having differing cross-sectional areas can be seen. Segment f, having the largest cross sectional area, opens to the vertical port face  152 . Segment g, having the smaller cross-sectional area, opens to the annular waveguide  125  (not visible). The section line B-B defines a plane containing the axis of the annular waveguide  125  and the axis of the vertical waveguide  154 . A cross-sectional view of this plane will be shown in  FIG. 6 . 
     The two segments f, g of the vertical waveguide  154  may function as matching sections to couple the vertically polarized TE 11  mode from the annular waveguide  125  (not visible), while simultaneously rejecting the horizontally polarized TE 11  mode. 
     The cross-sectional shapes and lengths of the two segments f, g of the vertical waveguide  154  may be designed to minimize the return loss for a vertically polarized mode introduced via a standard waveguide (not shown) attached to the vertical port face  152 . The cross-sectional shape of segment f of the vertical waveguide  154  may define a vertical port aperture in the vertical port face  152 . The cross-sectional shape of the vertical port aperture may be different from, and not coaxial with, the cross-sectional shape of the standard waveguide (not shown) to be attached to the vertical port face  152 . The transition from the cross-sectional shape of the vertical port aperture and the cross-sectional shape of the attached standard waveguide may contribute to the matching function described in the prior paragraph. 
       FIG. 4B  is a detail from  FIG. 4A  showing the cross-sectional dimensions of the two segments f, g of the vertical waveguide  154 . Since the cross-sectional areas of the two segments f, g of the vertical waveguide  154  decrease in order without any hidden or undercut surfaces, the vertical waveguide  154  may be inexpensively formed by machining with an end mill or other machining process. 
       FIG. 5  is a cross-sectional view of the OMT  100  at plane A-A, which was defined in  FIG. 3 . The lengths of the three segments a, b, c of the horizontal waveguide  144  (as defined by radial distances r a , r b , r c ) may be selected to transform the impedance of the annular waveguide  125  to the impedance of a waveguide component (not shown) that may be attached to the horizontal port face  142 . 
     A horizontal symmetry cavity  560  may be diametrically opposed to the horizontal port  140 . The horizontal symmetry cavity may include a horizontal symmetry waveguide  564 . The horizontal symmetry waveguide  564  may include two segments d, e. The horizontal symmetry waveguide  564  may be, for the extent of its length (defined by radial distance r d ), a mirror-image of the horizontal waveguide  144 . The horizontal symmetry waveguide  564  may have two segments d, e, which may have the same cross-sectional shape as the corresponding segments b, c of the horizontal waveguide  144 . The length of the two segments d, e of the horizontal symmetry waveguide  564  may be separately selected and may or may not be the same as the lengths of the corresponding segments b, c of the horizontal waveguide  144 . The horizontal symmetry waveguide may end at a horizontal symmetry cavity face  562 . A first shorting plate  566  may be affixed to the horizontal symmetry cavity face  562  to close the end of the horizontal symmetry waveguide  564 . The first shorting plate may be affixed by screws  568  or other fasteners, or by welding, soldering, conductive adhesive, or other attachment method or device. 
     The horizontal symmetry cavity  560  may be useful for the matching of both the horizontal and vertical ports and improving the isolation of the ports. For the horizontal port, the symmetry cavity  560  may act as a shorted stub whose length can be adjusted to help the coupling of the horizontal TE 11  mode in the annular waveguide to the TE 10  mode of a waveguide component (not shown) that may be attached to the horizontal port face  142 . To the vertical TE 11 , mode in the annular waveguide, the horizontal symmetry waveguide  564  and the horizontal waveguide  144  may look like identical cut-off waveguide stubs symmetrically placed on the common waveguide. To the vertical TE 11  mode, the junction of waveguides  564  and  144  may seem to have two planes of symmetry. This symmetry may prevent half of the higher order modes from being generated when the mode is scattered by the junction. 
     A vertical back short  580  may be disposed on the inner conductor  110  between the horizontal waveguide  144  and the vertical waveguide  154 . Referring to  FIG. 7 , which shows a perspective view of the inner conductor  110 , the vertical back short can be seen to be a pair of diametrically opposed fins extending from the outer surface  214  of the inner conductor  110 . The two fins of the vertical back short  580  may be divided into segments by one or more slots  782 . The number and location of the slots  782  may be selected to suppress resonances within an operating frequency band of the OMT  100 . 
     Referring again to  FIG. 5 , the vertical back short  580  may be disposed on the inner conductor  110  such that a distance L 1  exists from an edge  581  of the vertical back short  580  to the axis  555  of the vertical waveguide  154 . The distance L 1  and a length L 2  of the vertical back short  580  may be selected to minimize return loss for the vertical and horizontal ports and to maximize isolation between the vertical and horizontal ports. The two fins of the vertical back short  580  may extend close to but may not contact the inner surface  222  of the outer conductor  120 . Not requiring electrical contact between the two fins of the vertical back short  580  and the outer conductor  120  may reduce the cost of the OMT  100  by avoiding a soldering process or other assembly process (which may have been necessary to ensure electrical contact between the fins and the outer conductor). 
     A first horizontal back short  584  may be disposed on the inner conductor  110  adjacent to the horizontal waveguide  144 . Referring to  FIG. 7 , the first horizontal back short  584  can be seen to extend from a circular port flange  792  at the end of the inner conductor  110 . 
     Referring again to  FIG. 5 , the first horizontal back short  584  may be disposed on the inner conductor  110 . A distance L 3 , from the first horizontal back short  584  to the axis  545  of the horizontal waveguide  144 , may be selected to minimize return loss for the vertical and horizontal ports and to maximize isolation between the vertical and horizontal ports. 
     Still referring to  FIG. 5 , the inner conductor  110  may support a dielectric spacer ring  588  which may maintain the concentricity of the annular waveguide  125 . The presence of the dielectric spacer ring  588  may result in an impedance change. The inner conductor  110  may have a region  586  of increased diameter to both sides of the dielectric ring  588  to provide impedance matching. 
       FIG. 6  is a cross-sectional view of the OMT  100  at plane B-B, which is defined in  FIG. 4 . Plane B-B contains the axis of the annular waveguide  125  and the axis of the vertical waveguide  154 . 
     The lengths of the two segments f, g of the vertical waveguide  154  (as defined by radial distances r f  and r g ) may be designed to transform the impedance of the annular waveguide  125  to the impedance of the waveguide component (not shown) that may be attached to the vertical port face  152 . 
     A vertical symmetry cavity  670  may be diametrically opposed to the vertical port  150 . The vertical symmetry cavity  670  may include a vertical symmetry waveguide  674 . The vertical symmetry waveguide  674  may be a mirror-image of the vertical waveguide  154 . The vertical symmetry waveguide  674  may have two segments h, i, which may have the same cross-sectional shape as the corresponding segments f, g of the vertical waveguide  154 . The length of the segments h, i of the vertical symmetry waveguide (as defined by radial distance r h ) may be separately selected and may or may not be the same as the lengths of the corresponding segments f, g of the vertical waveguide  154 . The vertical symmetry waveguide  674  may end at a vertical symmetry cavity face  672 . A second shorting plate  676  may be affixed to the vertical symmetry cavity face  672  to close the end of the vertical symmetry waveguide  674 . The second shorting plate  676  may be affixed by screws  678  or other fasteners, or by welding, soldering, conductive adhesive, or other attachment method or device. 
     The vertical symmetry cavity  670  may be useful for the matching of both the horizontal and vertical ports and improving the isolation of the ports. For the vertical port, the symmetry cavity  670  may act as a shorted stub whose length can be adjusted to help the coupling of the vertical TE 11  mode in the annular waveguide to the TE 10  mode of a waveguide component (not shown) that may be attached to the vertical port face  152 . To the horizontal TE 11  mode in the annular waveguide, the vertical symmetry waveguide  674  and the vertical waveguide  154  may look like identical cut-off waveguide stubs symmetrically placed on the common waveguide. To the horizontal TE 11  mode, the junction of waveguides  674  and  154  may seem to have two planes of symmetry. This symmetry may prevent half of the higher order modes from being generated when the mode is scattered by the junction. 
     A second horizontal back short  686  may be disposed on the inner conductor  110  adjacent to the horizontal waveguide  144 . Referring to  FIG. 7 , the second horizontal back short can be seen to extend from a circular port flange  792  at the end of the inner conductor  110 . 
     Referring again to  FIG. 6 , the second horizontal back short  686  may be disposed on the inner conductor  110 . A distance L 4 , from the second horizontal back short  686  to the axis  545  of the horizontal waveguide  144 , may be selected to minimize return loss for the vertical and horizontal ports and to maximize isolation between the vertical and horizontal ports. 
     Each of the inner conductor  110  and the outer conductor  120  may be formed from a solid block of an electrically conductive metal material such as aluminum, aluminum alloy, or copper. Each of the inner conductor  110  and the outer conductor  120  may be formed from a solid block of dielectric material, such as a plastic, which may then be coated with a conductive material, such as a metal film, after the machining operations were completed. If justified by the production quantity, a blank approximating the shape of the inner conductor  110  and/or the outer conductor  120  could be formed prior the machining operations. The blank could be either metal or dielectric material and could be formed by a process such as casting or injection molding. Each of the inner conductor  110  and the outer conductor  120  may also be formed by assembling a plurality of components using screws or other fasteners, welding, soldering, adhesive bonding, or some other assembly technique. 
     The dielectric spacer ring  588  may be fabricated from a low-loss polystyrene plastic material such as Rexolite (available from C-LEC Plastics) or another dielectric material suitable for use at the frequency of operation of the OMT  100 . 
     An OMT, such as the OMT  100 , may be designed by using a commercial software package such as CST Microwave Studio. An initial model of the OMT may be generated with initial waveguide dimensions and relative positions that allow two orthogonal TE 11  modes to be supported in the annular common waveguide  125 , and that allow the horizontal and vertical branching waveguides to each support a single TE 10  mode, all over the desired operating frequency band. The structure may then be analyzed, and the reflection coefficients and isolation of the three ports may be determined. The dimensions of the model may be iterated and optimized manually or automatically to minimize the reflection coefficients and maximize the isolation of the dominant modes at each of the three ports. 
     Dimensions that may be manually or automatically optimized to minimize reflection coefficients and maximize isolation include the annular waveguide inner and outer diameters (Di, Do), the dimensions of the horizontal waveguide (w a , h a , r a , w b , h b , r b , w c , h c , r c ), the length (r d ) and other dimensions of the horizontal symmetry waveguide, the dimensions of the vertical waveguide (w f , h f , r f , w g , h g , r g ), the length (r h ) of the vertical symmetry waveguide, the dimensions (L 1 , L 2 , L 3 , L 4 ) of the horizontal and vertical back shorts, and other dimensions. The dimensions of the specific embodiment given in  FIGS. 2 ,  3 B,  4 B,  5 , and  6  may be suitable, if scaled, as the initial dimensions for the design of OMTs for other frequency bands or applications. 
       FIG. 8  is a graph  800  illustrating the simulated performance of an OMT similar to the specific embodiment of the OMT  100 . The dashed line  810  plots the isolation between the vertical and horizontal ports of the OMT. The isolation between the two ports may be 48 dB or greater over a frequency band from 19.4 GHz to 21.2 GHz. 
       FIG. 9  is a graph  900  illustrating the simulated performance of an OMT similar to the specific embodiment of the OMT  100 . The solid line  910  and the dashed line  920  plot the return loss of the vertical and horizontal ports of the OMT. The return loss may be less than −24 dB over a frequency band from 19.4 GHz to 21.2 GHz. 
     Closing Comments 
     Throughout this description, the embodiments and examples shown should be considered as exemplars, rather than limitations on the apparatus and procedures disclosed or claimed. Although many of the examples presented herein involve specific combinations of apparatus elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. Elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments. 
     For means-plus-function limitations recited in the claims, the means are not intended to be limited to the means disclosed herein for performing the recited function, but are intended to cover in scope any means, known now or later developed, for performing the recited function. 
     As used herein, “plurality” means two or more. 
     As used herein, a “set” of items may include one or more of such items. 
     As used herein, whether in the written description or the claims, the terms “comprising”, “including”, “carrying”, “having”, “containing”, “involving”, and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of”, respectively, are closed or semi-closed transitional phrases with respect to claims. 
     Use of ordinal terms such as “first”, “second”, “third”, etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. 
     As used herein, “and/or” means that the listed items are alternatives, but the alternatives also include any combination of the listed items.