Patent Publication Number: US-8994474-B2

Title: Ortho-mode transducer with wide bandwidth branch 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 waveguide devices used to combine or separate two orthogonal modes, also known as ortho-mode transducers (OMTs). 
     2. Description of the Related Art 
     Satellite broadcasting and communications systems may use a first signal having a first polarization state for an uplink to a satellite and a second signal having a second polarization state, orthogonal to the first polarization state, for a downlink from the satellite. Note that two circularly polarized signals are orthogonal if the e-field vectors rotate in the opposite directions. The polarization directions for the uplink and downlink signals may be determined by the antenna and feed network on the satellite. 
     A common form of antenna for transmitting and receiving signals from satellites consists of a parabolic dish reflector and a feed network where orthogonally polarized modes travel in a common waveguide. The common waveguide may typically be cylindrical or square, but may be elliptical or rectangular. 
     An ortho-mode transducer (OMT) is a three-port waveguide device having a common waveguide coupled to two branching waveguides. An ortho-mode transducer may be used to launch or extract the orthogonal linearly polarized modes into or from the common waveguide of an antenna feed network. 
     In this patent, the term “cylindrical waveguide” means a waveguide segment shaped as a right circular cylinder, which is to say the cross-sectional shape of the waveguide segment is circular. Similarly, the terms “elliptical waveguide”, “rectangular waveguide”, and “square waveguide” mean a waveguide segment having an elliptical, rectangular, or square cross-sectional shape, respectively. In this patent, the term “generally rectangular waveguide” means a waveguide having an asymmetrical cross-section with two long sides and two short sides where at least a portion of each side is flat (not curved). A generally rectangular waveguide may have, for example, rounded internal corners, a septum extending between the two long sides or the two short sides, and/or ridges extending into the waveguide from one or more sides. In this patent, the term “ridged waveguide” means a generally rectangular waveguide with ridges, or conductive protrusions extending from two opposed sides of the waveguide. Within this patent, the term “port” refers generally to an interface between devices or between a device and free space. A port of a waveguide device may be formed by an aperture in an interfacial surface to allow microwave radiation to enter or exit a waveguide within the device. 
     The common waveguide of an OMT typically supports two orthogonal linearly polarized modes. Within this patent, 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 cylindrical 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. When the cylindrical waveguide is partially filled with a dielectric material, the two orthogonal linearly polarized modes may be hybrid HE 11  modes which have at least some electric field component along the propagation axis. Two precisely orthogonal TE 11  or HE 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, which is to say that a 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, which may be a TE 10  mode. The mode supported by the first branching waveguide is orthogonal to the mode supported by the second branching waveguide. Within this patent, the term “orthogonal” will be used to describe the polarization direction of modes, and “normal” will be used to describe geometrically perpendicular structures. 
     A traditional OMT, for example as shown in U.S. Pat. No. 6,087,908, has one branch waveguide axially aligned with the common waveguide, and one branch waveguide normal to the common waveguide. The branch waveguide that is axially aligned with the common waveguide terminates at what is commonly called the vertical port. The linearly polarized mode supported by the vertical port is commonly called the vertical mode. The branch waveguide which is normal to the common waveguide is terminated at what is commonly called the horizontal port. The branch waveguide that terminates at the horizontal port also supports only a single polarized mode commonly called the horizontal mode. 
     The terms “horizontal” and “vertical” will be used in this patent 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 physical horizontal and vertical directions. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an OMT having a symmetry port and a septum. 
         FIG. 2  is perspective view of the internal airspace within the OMT of  FIG. 1 . 
         FIG. 3  is side view of the internal airspace within the OMT of  FIG. 1 . 
         FIG. 4A  is a plan view of the OMT looking into a common port. 
         FIG. 4B  is a partial plan view of the OMT providing dimensions of a common waveguide. 
         FIG. 5A  is a plan view of the OMT looking into a vertical branch port. 
         FIG. 5B  is a partial plan view of the OMT providing dimensions of a vertical branch waveguide. 
         FIG. 6A  is a plan view of the OMT looking into a horizontal branch port. 
         FIG. 6B  is a partial plan view of the OMT providing dimensions of a horizontal branch waveguide. 
         FIG. 7A  is a plan view of the OMT looking into a symmetry port. 
         FIG. 7B  is a partial plan view of the OMT providing dimensions of a symmetry waveguide. 
         FIG. 8  is a cross-sectional view of the OMT at a section plane A-A defined in  FIG. 4A . 
         FIG. 9  is a cross-sectional view of the OMT at a section plane B-B defined in  FIG. 7A . 
         FIG. 10  is a graph showing the simulated performance of the OMT of  FIGS. 1-9 . 
     
    
    
     Elements in the drawings are assigned reference numbers which remain constant among the figures. An element not described in conjunction with a figure may be presumed to be the same as a previously-described element having the same reference number. 
     DETAILED DESCRIPTION 
     Description of Apparatus 
       FIG. 1  is a perspective view showing primarily the front and top of an exemplary ortho-mode transducer (OMT)  100 . Throughout this patent, relative directional terms such as “top”, “front”, “back”, “bottom”, “left”, “right”, “up”, and “down” refer to the OMT as shown in a particular figure and do not imply any absolute orientation of the OMT. The OMT  100  may be formed as a series of machined cavities within an OMT body  105 . The OMT body  105  may be a conductive metal material such as aluminum, or a nonconductive material such as plastic with a conductive coating deposited on at least the interior surfaces of the OMT body  105 . 
     The OMT  100  may include a common waveguide  210  that terminates at a common port  200  on the front surface of the OMT  100 . In this example, the common waveguide  210  is a cylindrical waveguide. The common waveguide of an OMT may be cylindrical, elliptical, square, rectangular, or some other shape. 
     The OMT  100  may include a generally rectangular vertical branch waveguide  310  that terminates at a vertical port  300  on a back surface of the OMT  100 . The vertical branch waveguide  310  and the vertical port  300  are partially visible through the common port  200 . The vertical branch waveguide  310  may be configured to support a first TE 10  mode and to couple the first TE 10  mode into or from a first TE 11  mode in the common waveguide  210 . 
     A septum  320  (partially visible through the common port  200 ) may extend between two short sides of the vertical branch waveguide  310  near the intersection of the vertical branch waveguide  310  and the common waveguide  210 . The septum  320  may have a central stepped portion  325  having a smaller cross-section than the adjacent side portions of the septum  320 . 
     The OMT  100  may include a generally rectangular horizontal branch waveguide  410  (partially visible through the common port  200 ) that terminates at a horizontal port  400  (not visible) on a bottom surface of the OMT  100 . At least a portion of the horizontal branch waveguide  410  may be a ridged waveguide. The horizontal branch waveguide  410  may be configured to support a second TE 10  mode and to couple the second TE 10  mode into or from a second TE 11  mode within the common waveguide  210 . A polarization direction of the second TE 10  mode may be orthogonal to a polarization direction of the first TE 10  mode. The terms “vertical” and “horizontal” do not imply any absolute orientation of the OMT  100 . 
     The OMT  100  may include a generally rectangular symmetry waveguide  510  that terminates at a symmetry port  500  on the top surface of the OMT  100 . At least a portion of the symmetry waveguide  510  may be a ridged waveguide. The symmetry waveguide  510  may be configured to support the first TE 10  mode and to couple the first TE 10  mode into or from the waveguide  210 . The symmetry port  500  may be opposed to the horizontal port  400 . The symmetry waveguide  510  may be in line with and coaxial with the horizontal branch waveguide  410 . The symmetry port  500  may be closed by a shorting plate, not shown in  FIG. 1 , to create a closed cavity, which will be referred to herein as a “symmetry cavity”. 
     The characteristics of an OMT such as the OMT  100  are determined by the geometry of the common waveguide, the vertical branch waveguide, the horizontal branch waveguide, and other structures internal to the OMT. It may be difficult to visualize the internal structure based on drawings of the exterior of an OMT. To aid in understanding the structure of the OMT  100 ,  FIG. 2  and  FIG. 3  show a perspective view and a side view, respectively, of the air space within the OMT  100 . Both figures show the common waveguide  210 , the vertical branch waveguide  310 , the horizontal branch waveguide  410  and the symmetry waveguide  510 , essentially with the OMT body surrounding theses waveguides removed. 
       FIG. 3  and subsequent figures provide dimensions of the waveguides within an embodiment of the OMT  100  where the return at the horizontal port is less than −20 dB over a frequency band of 10.7 GHz to 12.75 GHz and the return at the vertical port is less than −20 dB over a frequency band of 10.7 to 14.5 GHz. Thus the vertical port provides a wide bandwidth equal to 30% of the center frequency. 
     In  FIG. 2 , the common port  200  (not visible) faces generally away from the viewer, and the symmetry port  500  faces upward. The vertical port  300  (not identified in  FIG. 2 ) faces generally towards the viewer but is obscured by a section of rectangular waveguide  390  coupled to the vertical port  300 . Similarly, a section of rectangular waveguide  490  is coupled to the horizontal port (not visible). The rectangular waveguides  390  and  490  may be standard WR-750 waveguides (0.750″×0.375″ waveguide dimensions). 
     In  FIG. 3 , the common port  200 , the vertical port  300 , the horizontal port  400 , and the symmetry port  500  face right, left, down, and up, respectively. 
     The common waveguide  210  may have a circular cross-section over its entire length. The vertical branch waveguide  310  may include three segments. A first vertical branch waveguide segment  312 , nearest the vertical port  300 , may have a generally rectangular cross-section with rounded corners. A second vertical branch waveguide segment  314  may be split into two generally rectangular portions separated by the septum  320 . The septum  320  may be centered on the shorter sides of the generally rectangular cross-section of the second vertical branch waveguide section  314 . 
     A third vertical branch waveguide segment  316  may overlap a portion of the common waveguide  210 . As will be discussed subsequently with respect to  FIG. 9 , the overlap of the third vertical branch waveguide segment  316  and the common waveguide  210  results in a waveguide cross-sectional shape that is a composite of the circular cross-section of the common waveguide  210  and the generally rectangular cross-section of the third vertical branch waveguide  316 . The overlap of the third vertical branch waveguide segment  316  and the common waveguide  210  is instrumental in providing efficient coupling between the vertical port  300  and the common port  200  over a wide frequency range. 
     Continuing the discussion of  FIG. 3 , the horizontal branch waveguide  410  may include three segments including a first horizontal branch waveguide segment  412  nearest the horizontal port  400 , a second horizontal branch waveguide segment  414 , and a third horizontal branch waveguide segment  416  coupled to the common waveguide  210 . Each of the first, second and third horizontal branch waveguide segments  412 ,  414 ,  416  may be a generally rectangular waveguide with rounded corners. One or more or all of the segments  412 ,  414 ,  416 , may be ridged waveguides having wide ridges extending into the waveguide along the long sides of the generally rectangular shape, forming a narrowed waist region. This ridged waveguide cross-sectional shape is commonly referred to as a “dog bone” shape due to a resemblance to a well-known style of dog biscuit. The dog-bone cross-sectional shape of the first, second and third horizontal branch waveguide segments  412 ,  414 ,  416  is more clearly visible in  FIGS. 6A and 6B . 
     Referring again to  FIG. 3 , the symmetry waveguide  510  may include two segments including a first symmetry waveguide segment  512  nearest the symmetry port  500 , and a second symmetry waveguide segment  514  coupled to the common waveguide  210 . One or both of the first and second symmetry waveguide segments  512 ,  514  may be ridged waveguides having a dog-bone cross-sectional shape. The dog-bone cross-sectional shape of the first and second symmetry waveguide segments  512 ,  514  is more clearly visible in  FIGS. 7A and 7B . 
     An OMT, such as the OMT  100 , may be designed such that the respective segments of the vertical branch waveguide  310 , the horizontal branch waveguide  410 , and the symmetry waveguide  510  having the largest cross-sectional areas are adjacent to the corresponding vertical, horizontal, or symmetry port. Additionally, an OMT may be designed such that the cross-sectional area of each succeeding waveguide segment is smaller than, and contained within, the cross-sectional area of the preceding waveguide segment. “Contained within” means that the entire perimeter of each succeeding waveguide section is visible through the aperture formed by the preceding waveguide section. With such a design, each waveguide section may be formed by machining through the aperture of the preceding waveguide section. Thus each waveguide section may be formed by a numerically controlled machining operation with an end mill or other machine tool, and the number of machining operation steps may be equal to the total number of waveguide segments. 
     The OMT  100  and other OMT devices designed according to the same principles may be formed in a series of machining operations without assembly or joining operations such as soldering, brazing, bonding, or welding. An OMT designed according to these principles may be formed from a single piece of material. The single piece may be initially a solid block of material. The OMT may be formed from a solid block of a conductive metal material such as aluminum or copper. The OMT may be also formed from a solid block of dielectric material, such as a plastic, which would then be coated with a conductive material, such as a film of a metal such as aluminum or copper, after the machining operations were completed. If justified by the production quantity, a blank approximating the shape of the OMT could be formed prior to 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. 
       FIG. 4A  is a plan view of the OMT  100  looking normal to and into the common port  200 . In this view the horizontal port  400  faces down and the symmetry port  500  faces up. A shorting plate  520  is attached to the symmetry port  500  to close the end of the symmetry waveguide to form a closed symmetry cavity (not visible). The septum  320 , including the notched portion  325 , is visible through the common port  200 . A plurality of tapped or through holes  230  may be provided about the common port  200  to facilitate attachment of a waveguide or other device to the common port. 
       FIG. 4B  provides a diameter of the common waveguide  210 . 
       FIG. 5A  is a plan view of the OMT  100  looking normal to and into the vertical port  300 . In this view the horizontal port  400  faces down and the symmetry port  500  faces up. The shorting plate  520  is attached to the symmetry port  500  to form the close symmetry cavity (not visible). The septum  320  and portions of the common waveguide  210  are visible through the vertical port  300 . A plurality of tapped or through holes  330  may be provided about the vertical port  300  to facilitate attachment of a waveguide (such as the waveguide  390  shown in  FIG. 2  and  FIG. 3 ) or other device (not shown) to the vertical port  300 . 
       FIG. 5B  provides dimensions of the vertical branch waveguide  310  and the septum  320 . 
       FIG. 6A  is a plan view of the OMT  100  looking normal to and into the horizontal port  400 . In this view the vertical port  300  faces down and the common port  200  faces up. The dog-bone cross-sectional shapes of the first, second, and third horizontal waveguide segments  412 ,  414 ,  416  are visible. The septum  320 , including the notched portion  325 , is partially visible through the horizontal port  400 . A plurality of tapped or through holes  430  may be provided about the horizontal port  400  to facilitate attachment of a waveguide (such as the waveguide  490  shown in  FIG. 2  and  FIG. 3 ) or other device (not shown) to the horizontal port  400 . 
       FIG. 6B  provides dimensions of the first, second, and third horizontal waveguide segments  412 ,  414 ,  416 . 
       FIG. 7A  is a plan view of the OMT  100  looking normal to and into the symmetry port  500 . In this view the vertical port  300  faces down and the common port  200  faces up. The dog-bone cross-sectional shapes of the first and second symmetry waveguide segments  512 ,  514  are visible. The septum  320 , including the notched portion  325 , is partially visible through the symmetry port  500 . A plurality of tapped or through holes  530  may be provided about the horizontal port  500  to facilitate attachment of a shorting plate ( 520  in  FIG. 4A  and  FIG. 5A ) to close the end of the symmetry waveguide. 
       FIG. 7B  provides dimensions of the first and second symmetry waveguide segments  512 ,  514 . 
       FIG. 8  is a cross-sectional view of the OMT  100  along section plane A-A, which was defined in  FIG. 4A . In this view, the common port  200  faces up and the vertical port  300  faces down.  FIG. 8  shows details and dimensions of the septum  320  and the notched portion  325 . 
       FIG. 9  is a cross-sectional view of the OMT  100  along section plane B-B, which was defined in  FIG. 7A . In this view, the symmetry port  500  (not identified) faces up and the horizontal port  400  (not identified) faces down. The first, second, and third horizontal branch waveguide segments  412 ,  414 ,  416  and the first and second symmetry waveguide segments  512 ,  514  are shown in cross-section. Section plane B-B transects the region of overlap between the third vertical branch waveguide segment  316  and the common waveguide  210 . The cross-sectional shape of the overlapped region is a composite of the circular shape of the common waveguide  210  and the rectangular-with-rounded-corners shape of the third vertical branch waveguide segment  316 . 
     An OMT, such as the OMT  100 , may be designed using a commercial software package such as CST Microwave Studio. An initial model of the OMT may be generated with estimated dimensions for the common waveguide, horizontal branch waveguide, and vertical branch waveguide. The structure may then be analyzed, and the reflection coefficients and cross coupling may be determined for two orthogonal linearly polarized modes introduced respectively at the two branch ports. The dimensions of the model may then be iterated manually or automatically to minimize the reflection coefficients across an operating frequency band. As previously described,  FIGS. 3-9  provide dimensions for an embodiment of the OMT for use in the frequency range of 10.7 to 14.5 GHz. These dimensions may be scaled (inversely with frequency) for operation in other different frequency bands. 
       FIG. 10  is a graph  1000  illustrating the simulated performance of the exemplary OMT  100  as shown in  FIGS. 1-9 . The exemplary OMT  100  was designed for a specific application in a communications terminal wherein the horizontal port operates over a frequency band of 10.7 GHz to 12.75 GHz and the vertical port operates over a frequency band of 10.7 to 14.5 GHz. The performance of the exemplary OMT was simulated using finite integral time domain analysis. The time-domain simulation results were Fourier transformed into frequency-domain data as shown in  FIG. 10 . 
     The dashed line  1010  is a graph of the return S2(1),2(1) at the horizontal port of the OMT, and the solid line  1020  is a graph of the return S3(1),3(1) at the vertical port of the OMT. The return S2(1),2(1) is less than −20 dB, equivalent to a voltage standing wave ratio (VSWR) of 1.22, over the frequency range from 10.7 GHz to 12.75 GHz. The return S3(1),3(1) is less than −20 dB over the frequency range from 10.7 GHz to greater than 14.5 GHz. Thus the bandwidth of the vertical port is greater than 3.8 GHz or 30% of the center frequency of 12.6 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.