Patent Grant
12261366
Aperture antennas are a form of radio frequency (RF) antenna used for directed transmission and reception of various RF signals, often employed in microwave radio transmissions or in reflector antenna feed systems. One example aperture antenna, a horn antenna, comprises a source port which feeds into a flared volume surrounded by walls that define the general horn-like shape. A horn aperture or opening then transmits/receives signals to/from external nodes. Arrays of horn antennas can be formed and used to produce multibeam Electrically Steerable Arrays (ESAs). ESAs are often deployed on satellites placed into various orbital configurations for communication with earth-based stations over a range of aiming configurations. When employed for microwave and millimeter-wave RF applications, horn arrays and connected waveguide filters offer low loss and high efficiency as compared to other antenna types.
Along with the horn antenna elements themselves, other components are often employed in concert to form a complete antenna system. These other components can be referred to as a feed network and include polarizers, filters, waveguides, interfacing elements, and other RF components. For example, polarizers can be employed in RF feed networks which convert polarizations of signals between linear and circular polarizations, and vice-versa. Typically, these components are machined or cast from separate metallic workpieces which are then screwed or bolted together to form individual horn antennas, and many individual horn antennas are then bolted together to form large arrays. Unfortunately, such arrangements are high complexity and high mass, and require complex manufacturing processes to assemble and ensure proper alignment and RF interfacing between separate pieces. This can limit applications and performance of horn antennas on weight-sensitive satellite-mounted ESA systems.
Provided herein are various enhancements for radio frequency (RF) horn antenna systems and arrays that integrate horn antenna elements with associated feed networks. These horn antenna elements and integrated feed networks can be manufactured using injection molding techniques that incorporate draft angles into the associated geometry and preclude undercuts or overhangs. Thus, the horn antenna elements and integrated feed networks discussed herein can be formed from a single workpiece using an injection molding technique, and later plated with a conductive material on RF-carrying surfaces. This leads to a large reduction in manufacturing complexity, cost, and mass—while simultaneously increasing performance characteristics with relation to other types of horn antenna arrays.
One example implementation includes an apparatus that includes a horn antenna array formed in a single workpiece of material comprising horn antenna elements with integrated feed networks. Each of the integrated feed networks comprise a waveguide interface flange having a port, and a filter element having a serpentine cavity coupled to the port that is at least partially formed by a base section, where a separation interface is established between the base section and an attachable cap that forms a remainder of the serpentine cavity. Each of the integrated feed networks also comprise a polarizer element having a waveguide cavity that couples between the serpentine cavity of the filter element and an associated horn antenna element that forms a signal aperture.
Another example implementation includes a method of manufacturing. The method includes forming a horn antenna array in a single workpiece of material comprising horn antenna elements with integrated feed networks. Forming each of the integrated feed networks comprises forming a waveguide interface flange having a port, and forming a filter element having a serpentine cavity coupled to the port that is at least partially formed by a base section, where a separation interface is established between the base section and an attachable cap that forms a remainder of the serpentine cavity. Forming each of the integrated feed networks also comprises forming a polarizer element having a waveguide cavity that couples between the serpentine cavity of the filter element and an associated horn antenna element. Forming each of the horn antenna elements comprises forming a horn structure and forming a signal aperture.
Yet another example implementation includes an apparatus comprising a horn antenna assembly formed in a single workpiece of material having a horn antenna element and an integrated feed network. The integrated feed network comprises a waveguide interface flange comprising a port, and a filter element comprising a serpentine cavity coupled to the port and is at least partially formed by a base section, where a separation interface is established between the base section and an attachable cap that forms a remainder of the serpentine cavity. The integrated feed network also comprises a polarizer element comprising a waveguide cavity that couples between the serpentine cavity of the filter element and a horn antenna element comprising a signal 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.
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.
Aperture antennas are often employed in microwave RF transmissions, such as in directional antenna feed systems or direct-radiating antenna systems. Aperture antennas and associated arrays are a class of antennas which emit RF energy from a corresponding aperture or opening, and include horn antennas, short backfire antennas, and waveguide aperture antennas. Large arrays of such antennas, perhaps using hundreds of elements, can form electronically steerable arrays (ESAs) for satellite communications, terrestrial backbone communications, aircraft communications, radar systems, directed energy applications, and other various applications using signal phase shifting among each antenna of the array to achieve a desired directionality.
In one example, a horn antenna comprises a source port which feeds into a flared volume surrounded by walls that define the general shape of the horn antenna. Along with the horn antennas, other components are often employed in series to form a complete aperture antenna system. These other components can be referred to as a feed network and include polarizers, filters, waveguides, interfacing elements, and other RF components. Arrays of horn antennas can be formed by joining together many individually-manufactured horn antennas or by forming a plurality of horn elements with a unified workpiece. Various subtractive or additive manufacturing techniques can be employed. Subtractive manufacturing techniques include machining and similar techniques, while additive manufacturing techniques include casting, molding, and 3D printing techniques, among others. However, use of plastic injection molding to manufacture horn arrays has been limited due to challenges in forming internal cavities and other various feed network features while maintaining draft angles required by the injection molding process, especially for the small feature sizes for RF wavelengths of the X-band (approximately 8 to 12 GHZ), Ku-band (approximately 12 to 18 GHZ), Ka-band (approximately 26.5-40 GHz), or millimeter wavelength bands. It should be understood that other RF bands and wavelengths can be supported with accompanying scaling in size or geometry suitable to the corresponding wavelengths.
Discussed herein are several enhanced techniques and structures for producing arrays of horn antennas having integrated feed networks, while allowing for various manufacturing techniques such as injection molding. The traditional approach is to manufacture each horn antenna and corresponding feed network elements (e.g. polarizer and filter elements), as individual components employing mechanical joints or connections between the components as well as using multiple assemblies within the components, such as split blocks. This traditional approach results in a longer/taller (i.e. less compact) structure along with higher recurring costs, higher assembly costs and time, higher testing labor, and higher mass to achieve a complete horn array assembly. In addition, critical performance parameters such as axial ratio and overall insertion loss can be impacted. The examples discussed herein can form a horn antenna, along with a polarizer and filter, as a single integrated component. Arrays of such elements can also be formed as a single integrated component. The entire array or assembly can use a meander or serpentine approach for filter cavities and other waveguide components to fit within a shorter length envelope. Other example implementations might not incorporate serpentine filter cavities, and instead include non-serpentine filter cavities. While serpentine cavities can reduce an overall aperture length of each horn antenna assembly for smaller packaging, generally straight filter cavities can instead be employed.
Moreover, injection molding techniques can be employed to achieve cost savings in large quantities, such as forming large arrays or horn antenna structures in ESAs. When injection molding or casting techniques are employed to manufacture polarizers, filters, and associated horn structures, draft angles are included to slope cross-sectional areas along certain axes. These draft angles, approximately 1° or 2°, are typically a requirement of the manufacturing tooling or process to prevent material overhangs or parallel surfaces in order to release the workpiece from a mold or die. However, the specific requirements of injection molding tooling, namely use of draft angles, can pose challenges for traditional feed networks and horn antennas. When an injection molding or casting process is employed an assembly, each portion of the assembly comprises geometry incorporating draft angles corresponding to the selected molding or casting technique, and can be formed from a single workpiece or molded piece of material. In some manufacturing scenarios, a reflection or change in draft angles is employed corresponding to a different tooling pull or extraction directions to form the single part/piece. For example, tooling used to form a first portion is pulled (or extracted) in a first direction, and tooling used to form a second portion is pulled (or extracted) in a second direction. The direction of draft angles for interior cavity features are typically mirrored or opposite from draft angles for exterior features (e.g. exterior walls). This change in draft angle direction accommodates mold elements (e.g. die or mandrel) inserted into cavities during an injection molding process. Thus, external features will generally increase in size/diameter, while internal features will generally decrease in size/diameter over the pull direction. Injection molding results in a significant mass decrease, fewer mechanical operations, and order of magnitude materials cost decrease when manufacturing large quantities of filters, polarizers, or horn antennas when compared to manufacturing using electrical discharge machining (EDM) or direct machining. Additionally the polarizers and associated horn antennas can utilize a square aperture which provides higher spatial efficiency and performance when deployed in an array. Circular, triangular, hexagonal, or irregular horn antennas can instead be employed using similar techniques.
Materials employed for the elements of the feed networks and horn antenna elements (or any of the various components discussed herein) can include any injection-moldable material. Examples include plastics, polymers, carbon composites, polyamide, acrylic, polycarbonate, polyoxymethylene, polystyrene, acrylonitrile butadiene styrene (ABS), polypropylene, polyethylene, polyurethane, thermoplastic rubber, including combinations thereof. Additionally, various additives can be included in the injected material, such as stabilizers, glass or organic fibers, structural elements, lubricants, mold release agents, or other additives. The material can be injected via at least one port into a mold or die which forms the shapes and cavities of the associated elements. Once formed, conductive surface treatments are typically applied at least to surfaces in contact with RF signals. These conductive surface treatments include various platings or electroplated materials, including conductive materials, metallic substances, metals, metal alloys, and the like, such as aluminum, copper, silver, gold, nickel, or other similar metals or associated combinations. The surface treatment might have a layer thickness of 0.001 inch or less.
Turning now to a first example polarizer feed,
Detailed labeling for elements for horn antenna assembly 150 are shown in
Interface flange 115 comprises a waveguide interface flange having features to couple a corresponding horn antenna assembly to a waveguide which transfers signals over signal port 116. Interface flange 115 thus comprises signal port 116 and sealing feature 117, along with various bolt patterns or holes. Although one signal port 116 is shown for interface flange 115, other examples of interface flange 115 can include more than one port, such as ports for both left-hand polarization and right-hand polarization signals, among other configurations. Interface flange 115 can be employed to transition an internal cavity of filter element 110 to signal port 116 having a standardized waveguide type or size. These standardized waveguide sizes can include WR-34, WR-42, WRD-580, among others. Interface flange 115 can also provide a structural mount for horn antenna assemblies 150-155. Interface flange 115 can include transformer portions, such as for a ¼ wavelength (λ) transformer to a rectangular waveguide. Although not shown in
Filter element 110 comprises a serpentine or meandering waveguide cavity formed by bandpass section 111 and mode suppression section 112. The filtering characteristics and performance of bandpass section 111 are determined by the geometry and structure of filter waveguide cavity 213, which establishes the ranges of RF signal frequencies that are passed and rejected. In some examples, bandpass section 111 can form a “type 1” bandpass filter. The serpentine cavity of filter element 110 also comprises mode suppression cavities 211-212 between bandpass section 111 and polarizer element 120. Mode suppression cavities 211-212 can aid in suppression of unwanted transmission modes as well as provide attenuation or cut off of polarization portions of transiting signals. This serpentine waveguide cavity is shown in more detail in
Filter element 110 is formed from two longitudinal portions or halves. Filter element 110 is at least partially formed by base section 113. Attachable cap 114 forms a remainder of filter element 110 and associated serpentine cavity. A separation interface is established between base section 113 and attachable cap 114 which is typically sealed against RF leakage. This arrangement of base section 113 and attachable cap 114 forms two half-block elements which are mated together. The separation interface is such that fasteners are not required and is able to seal along the entire peripheral of the affected sections/components. This arrangement also reduces misalignment between base section 113 and attachable cap 114. In some examples, the separation interface can be established along the middle of the H-plane or broad wall where no currents are crossing to allow a more resilient interface between base section 113 and attachable cap 114. This location can also be referred to as the zero current plane. Various adhesives or bonding material can be deposited at the separation interface, which may include one or more grooves or ridges to provide features into which the bonding material can reside. Example bonding material includes conductive adhesive liquid, conductive tape epoxy, cut sheets of conductive epoxy, metallic impregnated adhesives, conductive gaskets, or other materials applied on mating surfaces. The conductive impregnation material can include conductive or metallic components like silver. Example epoxy materials include Loctite® Ablestik 8175. Grooves or ridges can help contain or guide liquid adhesives or provide bonding stops for adhesives. Alternatively, screws or straps can bond base section 113 to attachable cap 114.
Polarizer element 120 comprises a waveguide cavity formed by polarizer body 121 which couples to the waveguide cavity of filter element 110 at two ports, namely ports 122-123 (see in
Polarizers, such as polarizer element 120, can be deployed in microwave RF feed networks to convert polarizations of signals between linear and circular polarizations, and vice-versa. Linear (or single) polarization typically refers to an electromagnetic signal propagating in a single plane along the direction of propagation, while circular polarization includes two linear components that are perpendicular to each other and having a phase difference of 90° (π/2). Other polarizations are possible, such as elliptical. Often, feed networks with polarizers are coupled to horn antennas used for transmitting or receiving microwave communications. Conversion of polarizations of signals in such communication systems can enable more effective communications between endpoints having varied or unpredictable orientations. For example, it can be helpful to use circular polarization to communicate from a satellite to ground stations, aircraft, or vehicles. Some polarizers comprise septum orthomode transducer (OMT) polarizers (SPOLs) that include a stepped separator or septum positioned between polarization ports of the polarizer.
Horn element 130 forms RF aperture 133 for horn antenna assembly 150, from which RF signals can be transmitted and RF signals can be received with respect to remote communication nodes. Horn element 130 also includes horn body 131 and signal port 132. Signals are transferred between polarizer element 120 via signal port 132, and horn body 131 forms the horn-shaped shell of material which tapers outward in diameter from signal port 132 to aperture 133. As mentioned, although a square aperture geometry is employed for horn element 130, other geometries can instead be used.
Turning to detailed inset view 302, portions of end seal 118 on base section 113 is shown as providing a portion of the overlapping RF seal at interface flange 115. Signal port 116 (hidden from view) couples to inner port 323 which is covered by end seal 118 when attachable cap 114 is bonded to base section 113. Inner port 323 couples to filter waveguide cavity 213, which further couples to mode suppression cavities 211-212. Also included in view 302 are RF seal surfaces 321-322. These form an arc or ‘U’ shaped overlapping interface between base section 113 and attachable cap 114, which also provides for alignment between base section 113 and attachable cap 114. Conductive adhesive can be applied at RF seal surfaces 321-322 when attachable cap 114 is bonded to base section 113 to prevent leakage of RF signals from the associated joints between base section 113 and attachable cap 114. Other alignment or sealing features, such as grooves pins/holes, or ridges can be incorporated to catch/hold adhesive and align base section 113 to attachable cap 114.
Interface flange 115 also can include rigidity element 325. Rigidity element 325 comprises a structural insert into the molded portion of the workpiece that forms interface flange 115. Rigidity element 325 can comprise a metallic piece or other suitable material which reduces or eliminates viscoelastic creep at the joint between interface flange 115 and any coupled waveguide. Properties of rigidity element 325 are selected to reduce viscoelastic creep of an interface or joint between the interface flange 115 and a mating waveguide to below a threshold level. Rigidity element 325 can be a separately manufactured piece, such as formed by a stamped metal, casting, or machining process. During formation of the material of interface flange 115 and horn antenna assembly 150, rigidity element 325 can be inserted into a mold to have material formed around rigidity element 325 and thus become embedded into such material. Common or shared mounting holes can be included in rigidity element 325 as well as interface flange 115 to provide pass-through for bolts or other fasteners. Rigidity element 325 also can include protrusions or cavity features to hold or grab the material (e.g. plastic material) that forms interface flange 115 to ensure proper purchase and alignment within interface flange 115. Although one interface flange 115 is shown with one rigidity element 325, arrays or rows of horn antenna assemblies can include rigidity elements that are coupled by webbing or metal so as to couple across interface flanges of many horn antenna assemblies. This can reduce the part count to a single rigidity element per workpiece.
View 401 shows even further details on the mode suppression cavities in relation to polarizer signal ports 122-123 and junction 424. It should be noted that draft angles (noted by a) are included in all of the surfaces of all of the cavities noted in
It should be noted that draft angles (noted by a) are included in all of the surfaces of all of the cavities noted in
The configuration of base section 113 and attachable cap 114 shown in the included Figures describes two half-block elements that, when mated, complete the filter and waveguide cavities. The interface between base section 113 and attachable cap 114 is such that no fasteners are required, and that same interface is able to seal along the entire periphery of base section 113 and attachable cap 114. The use of alignment holes with mating pins reduces mis-alignment. Moreover, groove 550 is included on attachable cap 114. Groove 550 can accommodate conductive adhesives in liquid or tape form, and provides channeling to allow adhesives to be retained within the groove and at the mating surfaces between base section 113 and attachable cap 114. An RF seal is made when mounting base section 113 to attachable cap 114 when using conductive adhesive within groove 550. This RF seal can be tuned to prevent RF signals of a particular frequency range from radiating outward from the contained cavities, as well as prevent incursion of external RF signals over that range.
View 600 shows polarizer element 620 and horn element 630 aligned along a longitudinal axis. Horn element 630 forms RF aperture 633, from which RF signals can be transmitted and RF signals can be received with respect to remote communication nodes. Horn element 630 also includes horn body 631 and horn port 632. Signals are transferred between polarizer element 620 via horn port 632, and horn body 631 forms the horn-shaped shell of material which tapers outward in diameter from horn port 632 to aperture 633. The quantity, angle, and length of each of the tapered sections of horn body 631 can be selected according to target performance characteristics, such as efficiency, bandwidth, frequency range, and other parameters. Although a generally square aperture geometry is employed for horn element 630, other geometries can instead be used.
Polarizer element 620 comprises a waveguide cavity formed by polarizer body 621 which couples to the waveguide cavity of a filter element at two ports, namely ports 622-623. In this example, port 623 corresponds to a LHP signal and port 622 corresponds to a RHP signal. Other configurations of ports and polarizations are possible depending on the polarizer configuration. Polarizer element 620 then couples to horn element 630 at horn port 632. Polarizer element 620 comprises a septum orthomode transducer (OMT) polarizer (SPOL) in this example. Septum 625 splits the waveguide cavity of polarizer element 620 at ports 622-623 and tapers in a stepped fashion until merging with a corresponding sidewall of polarizer body 621. The waveguide cavity also forms aperture 626 which joins with horn port 632. Interface section 624 is included and transitions ports 622-623 to the waveguide cavity of polarizer body 621. Sidewall ridges on sidewalls of polarizer body 621 can be included in some examples, but are omitted for clarity in
Various manufacturing techniques can be employed to form the horn antennas, assemblies, and arrays in the preceding Figures. Some techniques and operations have been noted above.
In operation 701, a horn antenna array comprising a plurality of horn antenna assemblies is formed in a single workpiece of material comprising horn antenna elements (130) with integrated feed networks (110 and 120). As a part of this process, various horn antenna assemblies are formed concurrently or semi-concurrently, such as in an injection molding technique. In a first operational subset (710), each of the integrated feed networks are formed, as noted in operations 711-714. In a second operational subset (720), each of the horn antenna elements is formed, as noted in operations 721-722. Finally, a conductive surface treatment is applied in operation 730.
Turning first to the formation of each of the integrated feed networks in operational subset 710, operation 711 includes forming waveguide interface flange 115 having port 116. Interface flange 115 also includes optional features like sealing feature 117, bolt holes or other waveguide connection features, an embedded rigidity element, and cap mating features which form a portion of end seal 118. Forming interface flange 115 might also include forming a plurality of flanges each with associated ports, where the flanges are coupled together by webbing material or into a common portion among all antenna assemblies of the array. Rigidity elements in such configurations can be independent for each interface flange or a single piece which spans more than one interface flange. Operation 712 includes forming filter element 110 having a serpentine cavity coupled to port 116 that is at least partially formed by base section 113. Forming filter element 110 can also include forming various mating features to couple to attachable cap 114, such as the arch portions of end seal 118, adhesive grooves or ridges, and alignment holes or pins. Forming filter element 110 can also include forming several cavity regions including filter waveguide cavity 213 of bandpass section 111, mode suppression cavities 211-212 of mode suppression section 112, and interfacing features to couple associated cavities to polarizer ports 122-123. From here, operation 714 can form polarizer element 120 having polarizer body 121 with internal waveguide cavity and septum features. Polarizer element 120 is formed such that it couples between the serpentine cavity of filter element 110 and associated horn antenna element 130.
As a separate workpiece, attachable cap 114 is formed. Attachable cap 114 can be formed in a different mold than that of the remainder of the serpentine cavity (e.g. sub-assembly 102 of antenna array 101) and later assembled onto sub-assembly 102 using adhesive or other means. In some examples, attachable cap 114 can be formed with sub-assembly 102 from the same workpiece, but with webbing material or other temporary material which is removed or trimmed before final assembly onto sub-assembly 102.
Turning first to the formation of each of horn antenna elements 130 in operational subset 720, operations 721-722 include forming horn body 131 and signal port 132. Horn body 131 comprises a hollow shell of material which defines the horn shape and aperture 133, and signal port 132 comprises an aperture coupling to the waveguide cavity of polarizer element 120. In some examples, such as when forming an array, more than one horn body can be formed together at common/adjacent edges or via webbing material. Thus, the horn elements of an array might form junction points that hold together all of the horn antenna assemblies of the array.
As mentioned above, an injection molding or casting process can be employed to form the various elements of horn antennas, assemblies, caps, and arrays, and each element can comprise geometry incorporating draft angles corresponding to the selected molding or casting technique. This provides for formation from a single workpiece or molded piece of material. Different tooling pull or extraction directions can be employed in a single workpiece, while having draft angles corresponding to the particular tooling pull direction. These draft angles of approximately 1° or 2° are typically a requirement of the manufacturing process tooling to prevent material overhangs or parallel surfaces in order to release the workpiece from a mold or die.
Once the elements of the horn antennas, assemblies, caps, and arrays have been manufactured in the operations noted above, operation 730 indicates that surface coatings or platings can be applied to all surfaces in contact with RF signals. These surfaces include filter cavity surfaces, mode suppression cavity surfaces, sidewalls, septum surfaces, interior surfaces of ports, interior surfaces of apertures, and interior walls of waveguides. Although not required for all applications, certain feature size-sensitive cavities can be formed oversized in diameter and then plated selectively on RF-contacting surfaces. The selective plating process can achieve +/−0.001 inch tolerances for an injection molded part, and thus the oversizing of cavities can provide for correct sizing once plated. Materials employed for the elements of the horn antennas, assemblies, caps, and arrays discussed herein can include any injection-moldable material. Examples include plastics, polymers, carbon composites, polyamide, acrylic, polycarbonate, polyoxymethylene, polystyrene, acrylonitrile butadiene styrene (ABS), polypropylene, polyethylene, polyurethane, thermoplastic rubber, including combinations thereof. Additionally, various additives can be included in the injected material, such as stabilizers, glass or organic fibers, structural elements, lubricants, mold release agents, or other additives. The material can be injected via at least one port into a mold or die which forms the shapes and cavities of the associated elements. Conductive surface treatments include various platings, including conductive materials, metallic substances, metals, metal alloys, and the like, such as aluminum, copper, silver, gold, nickel, or other similar metals or associated combinations.
Thus, the operations in
It should be understood that various communication bands and frequencies can be employed for the components discussed herein, with corresponding geometry scaling to suit the frequency ranges. For example, the components can support a frequency range corresponding to the Institute of Electrical and Electronics Engineers (IEEE) bands of S band, L band, C band, X band, Ku band, Ka band, V band, W band, among others, including combinations thereof. Other example RF frequency ranges and service types include ultra-high frequency (UHF), super high frequency (SHF), extremely high frequency (EHF), or other parameters defined by different organizations.
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.
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