Patent Publication Number: US-11646476-B1

Title: Compact orthomode transducer assembly

Description:
TECHNICAL BACKGROUND 
     Orthomode transducers (OMTs) are employed in wireless transmission systems to separate or combine vertical and horizontal signal polarizations from a signal or concurrent signals which are transported by a waveguide. Often these components are employed in satellite communication system feed structures which utilize microwave radio frequency (RF) signals, and can be installed in communication uplinks and downlinks to separate transmit (Tx) or receive (Rx) signals or separate concurrent components of circularly polarized signals. However, waveguides and associated OMTs can be difficult to design and manufacture due in part to the high sensitivity of waveguides to manufacturing precision, symmetry, and geometric configurations which can lead to distortions like passive intermodulation (PIM). PIM results in undesirable distortion, non-linearity, or mixing among signals within waveguides from physical properties of the waveguide, such as mechanical interfaces, surface defects, materials, or improperly designed geometric features. Various techniques and structures have been developed to address PIM and other signal distortion concerns for waveguides and OMTs. 
     For example, recombination networks and various turnstile structures can be employed in OMTs to increase bandwidth capabilities due to improved junction symmetry. Recombination networks include several additional waveguide arms that recombine a portion of the signals and leverage waveform interference effects, but these arms increase the size and part count of OMTs, as well as increase manufacturing complexity since these recombination networks must typically be electroformed or shim tuned (per-article) to achieve necessary tolerances. Other techniques include asymmetric split block OMTs which ease manufacturing by placing manufacturing joints that split along horizontal or vertical port/arm portions of the OMT. However, certain microwave communication bands, such as Ku, contain strong odd-order PIM products which prevent split block manufacturing of OMTs. These odd-order PIM products can significantly disrupt receive channels and require further mitigation by avoidance of mating dissimilar and similar metals. Thus, asymmetric split block OMTs have limited bandwidth than other types of OMTs, or limit operation to non-concurrent transmit and receive of signals. 
     Overview 
     Presented herein are various enhanced techniques and assemblies for orthomode transducers (OMTs) and associated waveguide structures. OMTs are employed in antenna feed structures to route microwave signals to/from reflectors, horns, or other directional antenna elements while separating or combining signals according to corresponding signal polarization. The performance of OMTs can be highly sensitive to manufacturing tolerances and design parameters. The example implementations discussed below have a higher level of manufacturability than many other OMTs, using end-milling techniques and minimal usage of specialized techniques such as electrical discharge machining (EDM). High-cost EDM processes such as sinker EDM are avoided entirely. While a two-piece assembly is discussed herein, other manufacturing techniques, such as additive or 3D manufacturing, can produce one-piece assemblies. However, additive techniques might still require a surface smoothing process to ensure surface finish roughness is below a target level. Moreover, the enhanced structures and designs of the OMTs discussed herein provide for greater performance in more compact envelopes than many existing OMT types. 
     In one example implementation, an orthomode transducer includes a junction element comprising a vertically polarized signal aperture along a longitudinal axis and a horizontally polarized signal aperture along an axis perpendicular to the longitudinal axis. The orthomode transducer also includes a waveguide element having a cruciform cavity coupled to the vertically polarized signal aperture and the horizontally polarized signal aperture and extending along the longitudinal axis to a split plane. The orthomode transducer also includes a choke element formed about at least a portion of the waveguide element and extending to the split plane, and a snout element extending the cruciform cavity from the split plane along the longitudinal axis and comprising a snout cavity that transitions from the cruciform cavity to a cruciform-quatrefoil cavity and from the cruciform-quatrefoil cavity to a circular feed aperture. 
     Also presented herein is an assembly having two workpieces. A first workpiece includes a junction element comprising a vertically polarized signal aperture along a longitudinal axis and a horizontally polarized signal aperture along an axis perpendicular to the longitudinal axis. The first workpiece also includes a waveguide element having a cruciform cavity coupled to the vertically polarized signal aperture and the horizontally polarized signal aperture and extending along the longitudinal axis to a split plane. The first workpiece also includes a choke element formed about at least a portion of the waveguide element and extending to the split plane. A second workpiece includes a snout element extending the cruciform cavity from the split plane along the longitudinal axis and comprising a snout cavity that transitions from the cruciform cavity to a cruciform-quatrefoil cavity and from the cruciform-quatrefoil cavity to a circular cavity. 
     In another example implementation, a method includes forming a junction element comprising a vertically polarized signal aperture along a longitudinal axis and a horizontally polarized signal aperture along an axis perpendicular to the longitudinal axis. The method also includes forming a waveguide element having a cruciform cavity coupled to the vertically polarized signal aperture and the horizontally polarized signal aperture and extending along the longitudinal axis to a split plane. The method also includes forming a choke element formed about at least a portion of the waveguide element and extending to the split plane. The method also includes forming a snout element extending the cruciform cavity from the split plane along the longitudinal axis and comprising a snout cavity that transitions from the cruciform cavity to a cruciform-quatrefoil cavity and from the cruciform-quatrefoil cavity to a circular aperture. 
     This Overview is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. It may be understood that this Overview is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the disclosure can be better understood with reference to the following drawings. While several implementations are described in connection with these drawings, the disclosure is not limited to the implementations disclosed herein. On the contrary, the intent is to cover all alternatives, modifications, and equivalents. 
         FIG.  1    illustrates an orthomode transducer in an implementation. 
         FIG.  2    illustrates components of an orthomode transducer in an implementation. 
         FIG.  3    illustrates an air cavity view of an orthomode transducer in an implementation. 
         FIG.  4    illustrates an air cavity view of an orthomode transducer in an implementation. 
         FIG.  5    illustrates an air cavity view of an orthomode transducer in an implementation. 
         FIG.  6    illustrates example manufacturing techniques for producing an orthomode transducer in an implementation. 
         FIG.  7    illustrates performance characteristics of an orthomode transducer in an implementation. 
     
    
    
     DETAILED DESCRIPTION 
     Orthomode transducers (OMTs) and associated waveguide structures are discussed herein, sometimes referred to as polarization duplexers. OMTs are employed in antenna feed structures to route microwave radio frequency (RF) signals to/from directional antenna elements while separating or combining signals according to signal polarization. OMTs can be employed to concurrently receive (Rx) and transmit (Tx) signals through the same feed aperture and directional antenna element, where the Rx and Tx signals are polarized in orthogonal orientations, such as vertical and horizontal polarizations. The Tx/Rx signals can often have similar carrier frequencies and still be routed to different source apertures or ports based on the polarization properties. OMTs can also be employed to transmit (or receive) different concurrent signals via the same directional antenna element by combining (or separating) signals having vertical and horizontal polarizations to/from a shared feed aperture. This feed aperture can be coupled to a horn element, and the entire assembly can be positioned nearby a directional antenna element (e.g. parabolic dish). 
     Associated waveguide portions of OMT assemblies discussed herein comprise cavities or hollow conduit portions which allow for propagation of electromagnetic waves therein. Input/output ports of the OMT assemblies, which couple to transmitter and receiver circuitry, are shown as rectangular in cross-section and correspond to vertically polarized (V) and horizontally polarized (H) signal ports. The portion of the OMT assemblies configured to combine/separate the vertically polarized signals and horizontally polarized signals is referred to herein as a waveguide junction or junction. This junction, as used herein, is different than a mechanical interface or joint between two pieces or parts that comprise the OMT assemblies. Electromagnetic waves travel within cavities formed by the OMT assemblies, and the examples herein have an enhanced set of transitions in cross-section between the V/H ports and a shared feed aperture/port. Tracing a pathway from the V/H ports for illustrative purposes, the OMT assemblies discussed herein transition from the rectangular cross-sections of the input/output ports to a cruciform (cross) cross-section and combine the two orthogonally polarized signals in a shared waveguide cavity. Then, another transition in cross-section occurs to transition from the cruciform cross-section to a cruciform quatrefoil cross-section. Finally, a transition in cross-section occurs from the cruciform quatrefoil cross-section to reach the shared feed aperture of the OMT assemblies, typically a circular cross-section. 
     The OMT assemblies discussed herein can be comprised of two parts or workpieces which when joined form an interface or joint having a low PIM double choke flange and critical impedance step. This configuration of two parts can be referred to as a split block OMT or OMT assembly. The critical impedance step enables matching of the V/H junction in excess of 25 dB return loss on both V and H polarizations over a broad bandwidth (e.g. Ku band). Various manufacturing techniques are discussed herein to achieve these enhanced designs and structures. For example, the first part can be a fully direct machined part that comprises choke grooves which form the low PIM mating interface at a split plane, a cruciform junction which combines or separates the V and H polarizations, transformers to fit a standard waveguide size for the V polarization arm, transformers to fit a standard waveguide size for the H polarization arm, and an impedance matching pedestal. Example standard waveguide sizes include WR75 having dimensions of 0.75 inches (19.05 mm)×0.375 inches (9.525 mm) and provide for frequency ranges of 10 to 15 GHz. Other standard waveguide sizes can be employed. 
     A nearly entirely direct machined second part (with one use of wire EDM techniques) comprises the mating surface (split plane) for the choke grooves from the first part, along with cruciform waveguide section which when mated with the first part forms an impedance matching step down to a size which further suppresses higher order modes launched by the junction. A quatrefoil-cruciform waveguide section also included in the second part and can be formed by uniting four cylinders with a cruciform waveguide. This new waveguide type enables direct machining and serves as the first transformer step. The split plane is selected such that it resides in the second part (OMT snout section) rather than in an OMT sidearm. 
     Various materials can be selected for the elements of the OMT assemblies discussed herein. Typically, suitable materials include conductive materials, such as metals, metallic alloys, or conductive composite materials. Example metals include aluminum, magnesium, copper, iron/ferrous metals, and alloy thereof. Moreover, surface coatings can be applied to enhance conductivity or reduce surface roughness. Typically, dissimilar metals are avoided for joints and interfaces, as dissimilar metals can affect signal quality by producing PIM products, especially the odd 3rd order, which significantly disrupt the receive channel Thus, to mitigate such risk, material selection typically avoids usage of mating of dissimilar metals. 
     Turning now to  FIG.  1   , an example OMT assembly is discussed.  FIG.  1    includes external isometric view  100  that illustrates OMT assembly  101 . Assembly  101  includes first portion  110  and second portion  120  which mate at split plane  114 . First portion  110  comprises a junction element which includes two source ports which can couple to transmitter/receiver circuitry. First port  111  comprises a port for vertically polarized signals and is oriented along a longitudinal axis and is oriented along an axis perpendicular to the longitudinal axis, while second port  112  comprises a port for horizontally polarized signals. Ports  111 - 112  comprise rectangular waveguides which couple into a common or shared internal cavity at a junction. This style of junction comprises an asymmetric junction. The cavity associated with the junction then forms a cruciform waveguide which extends to split plane  114 . First portion  110  also includes choke element  113  which includes internal choke cavities (not shown in  FIG.  1   ). Second portion  120  comprises a snout element beginning at split plane  114  with joint element  124  that couples to choke element  113  of first portion  110 . Second portion  120  also includes body  125  that houses waveguide  121  formed along longitudinal axis  130  which leads to feed port  122  and couples via feed interface  123  to a feed horn or can directly emit/receive signals from a directional antenna element. 
     In receive operations, signals can be received at feed aperture  122  for propagation within waveguide  121  to split plane  114 . Choke element  113  and joint element  124  produce an impedance step related to a diameter/width change in the waveguide and choke element  113  attenuates various unwanted signals generated by passive intermodulation (PIM) and from other sources of distortion. From here, the received signals can be routed to a source port that corresponds to the polarization of the received signals. In transmission operations, signals received at either of source ports  111 - 112  can have a particular polarization and are introduced into a shared waveguide or cavity within first portion  110 . From here, the choke element  113  attenuates various unwanted signals generated by PIM of the impedance step and from other sources of distortion. Then, waveguide  121  propagates the transmitted signals toward feed aperture  122  for emission towards a directional antenna element. Choke element  113  provides reduced PIM for OMT assembly  101  by at least attenuating (choking) mixed signals generated by PIM, and permits concurrent transmit and receive signals by reducing PIM between transmit and receive frequencies. 
     First portion  110  might serve as an OMT by itself as it contains the junction between V/H ports to a shared waveguide, however, connection of first portion  110  to a feed horn or other output element might produce undesired performance characteristics. Higher order propagation modes above the dominant mode (i.e. the 3rd, 4th, 5th and 6th modes) would propagate into the feed horn having been launched strongly by the asymmetric junction if first portion  110  was used alone as an OMT. The addition of second portion  120 , which steps down to a smaller-size cruciform waveguide, attenuates the higher order modes generated by the asymmetric junction. This configuration enables a broad bandwidth of at least 10.70 to 14.80 gigahertz (GHz) and also allows for inclusion of choke element  113  at split plane  114 . Example dimensions for OMT assembly  101  include approximately 4.5 inches in length, suitable for frequencies approximately in the Ku microwave band (which covers frequencies from 12-18 GHz). Other frequency bands or ranges are possible, and the sizing/geometry of OMT assembly  101  can scale to suit these frequency bands. 
     Turning now to further illustrations of OMT assembly  101 ,  FIG.  2    is presented.  FIG.  2    shows separate views  200  and  201  illustrating first portion  110 , and second portion  120 , respectively. Example axes  290  and  291  are shown to compare relative orientations among views  200  and  201 . View  200  includes similar elements shown for  FIG.  1   , and also provides further detail on waveguide  115  and choke element  113  with internal choke cavities  116 . View  201  includes similar elements shown in  FIG.  1   , and also provides further detail on waveguide  121  and joint features  126 - 127  of joint element  124  at split plane  114 . 
     In view  200 , first portion  110 , referred to as a junction element, comprises a housing/body and includes waveguide  115  comprising a cavity which extends along longitudinal axis  130  from port  111  to split plane  114 . Waveguide  115  forms a cruciform waveguide that extends along longitudinal axis  130  of OMT assembly  101  and can be visualized as superimposed or overlapping rectangular lobes, each with width ‘x’ as shown. Further internal to first portion  110  is a waveguide junction site (not shown) where V/H signals are combined/separated with respect to the associated ports, and the cruciform cross-section transitions to individual rectangular cross-sections—one for each V/H port. Thus, the cavity that forms waveguide  115  is coupled to cavities that form ports  111 - 112 . Choke element  113  comprises a circular structure formed about at least a portion of waveguide  115  and surrounds walls of waveguide  115 . Choke element  113  is formed from one or more concentric ring structures  117  that establish concentric cavities  116  which extend to split plane  114 . Choke element  113  also forms an interface with joint element  124  of second portion  120  at split plane  114 . Choke element  113  forms a low-PIM double-choke flange and critical impedance step related to a diameter/width change in the waveguide. The critical impedance step enables matching of the V/H junction in excess of 25 dB return loss on both V and H polarizations over a broad bandwidth (e.g. Ku band). 
     In view  201 , second portion  120 , referred to as a snout element, comprises body  125  and includes waveguide  121  comprising a snout cavity that extends along longitudinal axis  130  from split plane  114  to feed aperture  122 . Waveguide  121  forms a cruciform waveguide, which can be visualized as superimposed or overlapping rectangular lobes, each with width ‘x-y’ as shown. Width ‘x’ is similar to that in view  200 , thus the size of waveguide  121  is smaller by an amount ‘y’. Waveguide  121  continues or extends a cruciform waveguide cross-sectional shape of waveguide  114  along longitudinal axis  130 , albeit at a smaller size and on an opposite side of the joint formed at split plane  114  by joint element  124 . From here, waveguide  121  transitions to a series of different cavity cross-sections, which are shown in further Figures below. Also shown in view  201  are joint features  126 - 127  of joint element  124  at split plane  114 . Joint feature  127  forms a cavity or gap relative to joint feature  126 . Joint feature  126  has a height greater than that of joint feature  127 , this height can be approximately 0.010 inches or any suitable gap to prevent electrical contact at split plane  114  through joint feature  127 . When joined to first portion  110 , joint feature  127  forms a cavity or gap region about waveguide  121  establishing a selected gap between first portion  110  and second portion  120  over at least an inner portion of a radius of split plane  114 , while joint feature  126  can couple in a conductive seal with outer ring of choke  113 . In this manner, an outer ring of choke  113  and joint feature  126  form flanges which couple to each other to join first portion  110  and second portion  120 . The flanges can be referred to as a broadband integrated double choke flange. When first portion  110  and second portion  120  are mated, concentric cavities  116  of choke element  113  are open to waveguide  121  via the selected gapping on a side proximate to second portion  120  and closed by an outer portion of the radius of split plane  114  by joint feature  126  of joint element  124  on the side proximate to second portion  120 . Various types of fasteners or clamps can be employed to couple first portion  110  and second portion  120  in accordance with a target interface strength. 
     The location within OMT assembly  101  for split plane  114  is selected to occur at a low current or zero-current region along the total waveguide length to mitigate PIM effects, as surface currents can act as parasitic frequency mixer. Other OMTs might put a split plane that divides a horizontal polarization port (referred to as a side arm in some cases), which precludes use of a choke as is done in the examples herein. The use of a choke allows for lower PIM, as well as concurrent/simultaneous propagation of Tx/Rx signals through the corresponding waveguide. This is in contrast to OMTs which lack a choke or have PIM above a threshold level, and require non-concurrent Tx/Rx signal propagation (e.g. timewise separated) to render PIM concerns for parasitic mixing of concurrent Tx/Rx signals irrelevant. Concurrent Tx/Rx signal propagation typically requires use of specialty structures or arms, such as recombination networks or turnstile structures, which are costly to manufacture and difficult to achieve precision high enough to provide for wide bandwidth and low PIM. In order to achieve broad bandwidth, recombination arms must be either manually tuned, electroformed or both. Also, the asymmetry of having a ‘side arm’ portion of OMT assembly  101 , such as having port  112  perpendicular to the longitudinal axis of the waveguide, can introduce undesired propagation modes. In the past, recombination networks or other structures were employed to suppress/reduce these undesired modes. Advantageously, the enhanced geometry and manufacturability of the waveguide cavities and structural configuration of OMT assembly  101  achieves mode suppression of undesired modes in a less complex mechanical arrangement than with recombination networks. Specifically, the geometry and structure of OMT assembly  101  includes cross-sectional transitions that reduce cross-sectional area of the corresponding cavity/waveguide and suppress higher order modes. The dimensions of the waveguide cross-section are selected to be small as possible to suppress undesired modes but not inhibit a dominant mode by having too small of a dimension. 
     To further illustrate the internal structures and geometries of the cavities that form waveguides, junctions, and ports/apertures of OMT assembly  101 ,  FIGS.  3 - 6    are included.  FIGS.  3 - 6    illustrate air cavity views or internal spaces, and not the material/walls seen in  FIGS.  1  and  2   . Walls or enveloping materials can be manufactured to achieve the air cavities seen in  FIGS.  3 - 6   . Also, although features of  FIGS.  3 - 6    might be employed for elements of  FIGS.  1  and  2   , it should be understood that the corresponding air cavities can be achievable using different external material/structural configurations than those shown in  FIGS.  1  and  2   .  FIG.  3    shows isometric view  300  of air cavity  301 .  FIG.  4    shows isometric cross-sectional view of air cavity  301 .  FIG.  5    shows a different orientation than view  300  and view  400  in isometric view  500  of air cavity  301 . 
     Air cavity  301  includes two main portions, first portion  310  and second portion  320  which are joined at split plane  302 . First portion  310  comprises junction waveguide  332  which combines/separates signals associated with horizontal arm  333  and vertical arm  331 . Horizontal arm  333  includes horizontal polarization aperture  314 . Vertical arm  331  includes vertical polarization aperture  311 . Junction waveguide  332  forms a waveguide cavity along longitudinal axis  330  and terminates at split plane  302 . Choke element  340  is formed around a portion of junction waveguide  332 . Choke element  340  includes two concentric cavities  341 - 342  which surround junction waveguide  332  and also terminates at split plane  302 . Thus, first portion  310  comprises a cruciform waveguide junction with an integrated double choke. Formation of first portion  310  can be achieved using direct machining, such as milling. In one example, first portion  310  is manufactured by forming junction element comprising a vertically polarized signal arm/aperture along a longitudinal axis and a horizontally polarized signal arm/aperture along an axis perpendicular to the longitudinal axis. A waveguide element of first portion  310  can be formed having a cruciform cavity, coupled to the vertically polarized signal arm/aperture and the horizontally polarized signal arm/aperture, and extending along the longitudinal axis to split plane  302 . Also, the manufacturing operations include forming choke element  340  about at least a portion of the waveguide element and extending to the split plane  302 . 
     A joint or interface between the two air cavities associated with first portion  310  and second portion  320  is established at split plane  302 . When joined, first portion  310  and second portion  320  form a combined longitudinal waveguide cavity. Contact between the two air cavities can be selected to occur in a zero-current region of the combined longitudinal waveguide cavity to mitigate PIM effects. A discontinuity or step  345  in impedance is also established at split plane  302  but a change in diameter of the combined longitudinal waveguide cavity. While the diameter of the waveguide associated with first portion  310  is a first diameter, the diameter of the waveguide associated with second portion is a second, smaller, diameter. Thus,  FIG.  3    shows a waveguide configuration that avoids mating between metals, both similar and dissimilar, in peak current regions of the waveguide. Also, by utilizing choke element  340  at the interface, the configuration ensures that any metal-to-metal contact happens in the near-zero current region of the waveguide. 
     Second portion  320  is comprised entirely of a waveguide which is divided into several segments  321 - 325  leading to feed aperture  315 . Second portion  320  is formed within a snout element extending (at step  345 ) from split plane  302  to feed aperture  315 . First segment  321  comprises a cruciform cross-sectional cavity that extends along axis  330  to second segment  322 . Second segment  332  comprises a cross-section that transitions from the cruciform cross-section of first segment  331  to a cruciform quatrefoil cross-sectional cavity of third segment  333 . Third segment  333  transitions from a first cruciform quatrefoil formed with circles/cylinders of a first diameter to a second quatrefoil formed with circles/cylinders of a second, larger, diameter. Fourth segment  334  transitions from the second quatrefoil cross-section to a circular cross-section associated with fifth segment  325 . Fifth segment  325  matches a cross-section of feed aperture  315 . Thus, the combined waveguide cavity of section portion  320 , or snout cavity, transitions from a cruciform cavity to a cruciform-quatrefoil cavity and from the cruciform-quatrefoil cavity to a circular cavity comprising the feed aperture. Other cross-sections might be employed for feed aperture  315 , and fifth segment  325  can be employed to transition from the cruciform-quatrefoil cavity to the cross-section of feed aperture  315 . 
     Although the diameters and lengths can vary among the waveguides of first portion  310  and second portion  320 , these parameters will typically be selected in accordance with the desired frequency range. For example, the longitudinal waveguide cavity comprising segments  321 - 325  of second portion  320  each comprise diameters selected to pass dominant modes of signals for a selected frequency range and suppress higher odd-order modes of the signals (i.e. the 3rd, 4th, 5th and 6th modes, among others). 
       FIG.  6    illustrates various manufacturing methods or processes which can be employed to form the waveguides and air cavities of the snout portion of an OMT assembly.  FIG.  6    illustrates air cavity  600  for a snout portion having a configuration with potentially a higher level of manufacturability than other configurations. Air cavity  600  might be formed in a workpiece using direct machining, end milling, wire electrical discharge machining (EDM) operations, or other processes. However, the configuration shown in  FIG.  6    avoids the need for electroforming and plunge EDM processes which can be more expensive and time consuming. When end-milling of a workpiece is employed, a single wire EDM operation might still be needed which pulls all the way from aperture  611  through port  612  without intersection. 
     Air cavity  600  comprises transitions among cross-sections from cruciform to quatrefoil to circular over the longitudinal length of air cavity  600 . Although four transitional segments  621 - 624  are shown in  FIG.  6   , it should be understood that a different quantity of segments might instead be employed to achieve similar shapes and cross-sectional transformations as shown. Segments  621 - 624  each have different geometric properties to make the transition from cruciform to cruciform quatrefoil to quatrefoil to circular. Aperture  611  comprises a cruciform (cross) cross-section which is reflected in first segment  621 . First segment  621  comprises a similar shape and cross-section throughout the length of segment  621 , and a cross-sectional transition occurs at the boundary with segment  622 . Segment  622  includes a first quatrefoil cross-section throughout the segment, and segment  623  includes a second quatrefoil cross-section through the segment. Finally, a circular cross-section for aperture  612  is established for segment  624 . 
     To manufacture air cavity  600 , operational views  601 - 602  are shown. First, a workpiece, such as a suitable length of metal stock, is machined to form a cruciform cross-section. The depth of the machining might occur only to the depth shown for segment  621 , or may instead penetrate the entire longitudinal length of the workpiece. When the machining operation penetrates the entire longitudinal length, a wire EDM operation might be employed. This step is reflected in shape  630  in view  601 . After this cruciform cross-section cavity has been established in the workpiece, then four (4) merged, overlapping, or superimposed cylindrical cuts (end mill operations  631 ) are performed to form the “leaves” of the quatrefoil shape. These circular end mill operations can occur sequentially. End mill operations  631  might occur to a particular depth corresponding to that of segments  622 ,  623 , and  624 , or may instead penetrate the entire longitudinal length of the workpiece (even through shape  630 ). From here, view  602  shows several end milling operations which produce an increasingly larger diameter and transition from cruciform to quatrefoil among segments  622  and  623 . Finally, a circular end milling operation forms segment  624 . 
     Advantageously, the operations shown in  FIG.  6    establish air cavity  600  without requiring any undercuts. The “leaves” are representative of a large endmill diameter which can reach deep into the snout. This enables direct machining, avoids any electroforming or sinker EDM operations and reduces length of the snout. When the leaves are united with the cruciform, a waveguide is formed which not only suppresses higher order modes but also serves as an impedance transform up to the remainder of the direct machined steps. 
       FIG.  7    is included to show example performance characteristics for an OMT assembly using the quatrefoil-cruciform or clover-cruciform shape discussed herein. Graph  700  includes two plots  711 - 712 . Plot  711  shows horizontal polarization (HPOL) return loss, while plot  712  shows vertical polarization (VPOL) return loss. A vertical graph axis corresponds to less in decibels (dB), while a horizontal graph axis corresponds to frequency in GHz. As seen in graph  700 , a greater than 25 dB return loss is exhibited for the full Ku band using the enhanced OMT assemblies discussed herein. Specifically, the enhanced OMT assemblies are comprised of two parts, a first part and a second part, which when joined form the device as well as a low PIM double choke flange and critical impedance step. This critical impedance steps enables matching of the junction in excess of 25 dB return loss on both V and H Polarizations over a broad bandwidth (full or partial Ku Band). 
     The functional block diagrams, operational scenarios and sequences, and flow diagrams provided in the Figures are representative of exemplary systems, environments, and methodologies for performing novel aspects of the disclosure. While, for purposes of simplicity of explanation, methods included herein may be in the form of a functional diagram, operational scenario or sequence, or flow diagram, and may be described as a series of acts, it is to be understood and appreciated that the methods are not limited by the order of acts, as some acts may, in accordance therewith, occur in a different order and/or concurrently with other acts from that shown and described herein. For example, those skilled in the art will understand and appreciate that a method could alternatively be represented as a series of interrelated states or events, such as in a state diagram. Moreover, not all acts illustrated in a methodology may be required for a novel implementation. 
     The various materials and manufacturing processes discussed herein are employed according to the descriptions above. However, it should be understood that the disclosures and enhancements herein are not limited to these materials and manufacturing processes, and can be applicable across a range of suitable materials and manufacturing processes. Thus, the descriptions and figures included herein depict specific implementations to teach those skilled in the art how to make and use the best options. For the purpose of teaching inventive principles, some conventional aspects have been simplified or omitted. Those skilled in the art will appreciate variations from these implementations that fall within the scope of this disclosure. Those skilled in the art will also appreciate that the features described above can be combined in various ways to form multiple implementations.