Patent Publication Number: US-2021184397-A1

Title: Waveguide window/seal and portable antenna

Description:
RELATED APPLICATION INFORMATION 
     This patent is a continuation of application Ser. No. 16/673,633, filed Nov. 4, 2019, entitled WAVEGUIDE QUICK-CONNECT MECHANISM, WAVEGUIDE WINDOW/SEAL, AND PORTABLE ANTENNA, which claims priority from U.S. provisional patent application No. 62/756,431 entitled “WAVEGUIDE QUICK-CONNECT MECHANISM AND PORTABLE ANTENNA” filed Nov. 6, 2018, the entirety of which are incorporated by reference. 
    
    
     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 
     Field 
     This disclosure relates to antennas for satellite communications earth stations. 
     Description of the Related Art 
     Satellite communications systems use one or more orbiting satellites to relay communications between a pair of earth stations. Each earth station typically consists of a transmitter and a receiver coupled to a highly directional antenna. Given the large distance between each earth station and the satellite, each earth station must be configured to transmit a relatively powerful signal and to receive a very low power signal. A common form of antenna for transmitting to and receiving from a satellite consists of a parabolic dish primary reflector and a feed network. 
     Satellite communications systems commonly use separate frequency bands for the uplink to and downlink from satellites. Additionally, one or both of the uplink and downlink may transmit orthogonal right-hand and left-hand circularly polarized signals or orthogonal linearly polarized signals within the respective frequency band. 
     In many applications, such as disaster relief, it is desirable to set up an earth station in a remote and often inhospitable location. Such applications require an antenna that can be disassembled and compactly packaged, for example in a carrying case or backpack, for easy transport and then quickly and precisely reassembled. 
     In this patent, the term “circular waveguide” means a waveguide segment having a circular cross-sectional shape. Similarly, the term “annular waveguide” means a waveguide segment having a cross-sectional shape of an annulus between two concentric circles. The term “waveguide component” means a physical element containing at least one waveguide. The term “port” refers generally to an interface between waveguide components or between a waveguide component and free space. A port of a waveguide component may be formed by an aperture in an interfacial surface to allow microwave radiation to enter or exit a waveguide within the waveguide component. In this patent, the term “waveguide circuit” means an assembly of two or more waveguide components coupled such that microwave radiation can transit between waveguides within the waveguide components. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front view and simplified cross-sectional schematic view of a portable earth station antenna. 
         FIG. 2  is an exploded perspective view of a waveguide circuit including a quick-connect mechanism between two waveguide components. 
         FIG. 3  is an exploded perspective view of a portion of an earth station antenna including an antenna hub and parts of a feed network. 
         FIG. 4  is a side view of the hub and partial feed network of the antenna of  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the hub and partial feed network of the antenna of  FIG. 3  and  FIG. 4 . 
         FIG. 6  is a plan view and cross-sectional view of a waveguide window. 
         FIG. 7  is a plan view and cross-sectional view of another waveguide window. 
     
    
    
     Elements in the drawings are assigned two- or three-digit reference numbers. 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  includes a schematic front view and a greatly simplified schematic cross-sectional view of a portable earth station antenna  10  which includes a primary reflector  20 , a hub  30 , a back-side feed network  40 , and a front-side feed network  50 . In this patent, the “front” side of the antenna is the side that faces the satellite, which is to say the concave side of the primary reflector. The “back” side of the antenna is the convex side of the primary reflector. The hub  30  and/or the back-side feed network  40  may have provisions (not shown) to attach the antenna to a base or mount that allows the antenna to be aimed at a particular satellite. 
     To facilitate transporting the portable earth station antenna  10 , the primary reflector  20  may consist of a plurality of segments or petals, such as the petal  22 , that collectively form a parabolic reflector when attached to the hub  30 . When detached from the hub  30 , the petals may be nested and compactly packaged for transport. 
     The portable earth station antenna  10  is a center-fed antenna with the feed network located along the axis  25  of the primary reflector  20 . The back-side feed network  40  and the front-side feed network are coupled by a waveguide (not shown) that passes through the hub  30  along the axis  25 . This waveguide may be, for example a circular waveguide, an annular waveguide, or coaxial circular and annular waveguides. 
     The back-side feed network  40  includes components to couple the antenna to a transmitter and a receiver of the satellite earth station (not shown). The back-side feed network  40  may include, for example, a diplexer to separate signals in different uplink and downlink frequency bands and/or one or more orthomode transducers to separate orthogonally polarized signals. The internal configurations of the back-side and front-side feed networks  40 ,  50  will depend on the communications frequency band and protocols employed by the satellite with which the earth station will communicate. 
     The front-side feed network  50  includes a secondary reflector (not visible but housed in the “mushroom cap” at the end of the front side feed network) and a waveguide circuit along the axis  25 , which couples the secondary reflector to the back-side feed network  40 . Uplink signals from the earth station transmitter are coupled into the waveguide circuit by the back-side feed network  40 . The uplink signals travel through the waveguide circuit along the axis  25  to the secondary reflector, where the uplink signals are reflected towards the primary reflector  20 , which forms the uplink signals into a narrow beam aimed towards the satellite. Downlink signals from the satellite travel a reverse path to the earth station receiver. 
     When the uplink to and the downlink from the satellite use circularly polarized signals, the front-side feed network  50  may include a polarizing element that converts linearly polarized signals into circularly polarized signals. Specifically, the polarizing element will convert a signal with a first linear polarization direction into a right-hand circularly polarized signal and convert a second signal with a second linear polarization direction orthogonal to the first polarization direction into a left-hand circularly polarized signal. This conversion can be reversed, such that the first signal is converted to left-hand circular polarization and the second signal in converted to right-hand circular polarization, by rotating the front-side feed network  50  90 degrees about the antenna axis  25 . Since the polarization direction may have to be set or changed when a portable antenna is in the field, it is desirable for the front-side feed network  50  to be attachable to the hub  30  in two (or four) positions rotated by substantially 90 degrees about the antenna axis  25 . 
       FIG. 2  is an exploded perspective view of portions of a waveguide circuit  200  including a first waveguide component  210  and a second waveguide component  240 . The first waveguide component  210  include a first circular waveguide (not visible) concentric with an axis  215 . The first waveguide component  210  includes a first connecting member  220 . A body  222  of the first connecting member  220  may be shaped as a right circular cylinder or a frustum of a right circular cone. In either case, the body  222  is concentric with the first circular waveguide. The first circular waveguide terminates at a first port (not visible) in an end face  224  of the first connecting member  220 . Two or more pins  225  extend radially from the body  222  of the first connecting member  220 . The first waveguide component  210  may include a first waveguide window  230 . The first waveguide window  230  is inserted into the port of the circular waveguide to seal the port and prevent intrusion of moisture and foreign objects into the first waveguide when the waveguide circuit  200  is disassembled. 
     The second waveguide component  240  includes a second circular waveguide concentric with the axis  215  and a second connecting member  250  having a cavity  252  configured to fit over the first connecting member  220  of the first waveguide component  210 . An inside surface  254  of the cavity  252  may be a right circular cylinder or a frustum of a right circular cone. In either case, the cavity  252  is concentric with the second circular waveguide. The second circular waveguide terminates at a second port (not visible) within the cavity  252 . The second connecting member  250  includes slots  255  that accept the pins  225  when the second connecting member  250  is engaged with the first connecting member  220 . When the connecting member  250  is engaged with the first connecting member  220 , the first and second ports are brought into contact or close proximity such that the first and second waveguides are connected. The second waveguide component  240  may include a second waveguide window  245  inserted into the second port to seal the port and prevent intrusion of moisture and foreign objects into the second waveguide when the waveguide circuit  200  is disassembled. The first waveguide window  230  and the second waveguide window  245 , in combination, are substantially transparent to microwave radiation. 
     To hold the first and second waveguide components  210 ,  240  in the engaged position, a cup-shaped cap  270  may fit over the second connecting portion  250 . A wave spring  280  is compressed between an inside surface of the cap  270  and a shoulder  260  of the second connecting portion  250 . The cap  270  includes L-shaped slots  275  that allow the cap  270  to slide over the pins  225  and then rotate. The L-shaped slots  275  may include detents that allow the cap  270  to move slightly away from the first waveguide component  210  when the cap is in its fully rotated position. The cap  270  is considered to be “engaged” with the pins  225  when the pins  225  are disposed in the detents. Pressure from the spring  280  forces the second waveguide component  240  against the first waveguide component and retains the cap  270  in the fully rotated position. The spring  280  may be, for example, a hard steel spring material. When one or both of the second waveguide component  240  and the cap  270  are formed from a softer material, such as an aluminum alloy, flat washers  285  may be placed on one or both sides of the spring  280  to prevent the spring from marring the inside of the cap  270  or the shoulder  260 . 
     To connect the first and second waveguide components  210 ,  240 , the components must be rotated with respect to each other about the axis  215  such that the slots  255  align with the pins  225 . The number of possible orientations of the first and second waveguide components  210 ,  240  is determined by the number and arrangement of the pins and slots. To ensure uniform pressure on the shoulder  260  of the second waveguide component  240 , at least two pins  225  are required, and three or more pins may be preferred. In the example of  FIG. 2 , four pins  225  (one of which cannot be seen) are disposed at 90-degree intervals about a circumference of the first connecting member  220 , and four corresponding slots  255  are provided in the second connecting member  260 . The use of four pins and four slots allows the first and second waveguide components to be connected with four different relative positions, rotated by 90 degrees. Other configurations of pins and slots may be used. For example, three or more pins and a corresponding number of slots disposed at unequal angles about a perimeter of the first connecting member can be used to restrict the first and second waveguide component to be connected in exactly one orientation. 
       FIG. 3  is an exploded perspective view of a portion of a portable ground station antenna  300  including portions of a hub  310  and a front-side feed network  340  that are connectable by the quick-connect mechanism previously shown in greater detail in  FIG. 2 . A circular waveguide, centered on an antenna axis  315 , extends through the hub  310  into the front-side feed network  340 .  FIG. 4  is a side view of the same portions of the ground station antenna with the front-side feed network  340  mated with the hub  310  before the cap  370  is engaged with the pins  325 .  FIG. 5  is a cross-sectional view of the assembly of  FIG. 4  through the antenna axis. Unless otherwise noted, each of the elements described subsequently can be seen in two or three of these figures. 
     The hub  310  includes a first connecting member  320 , pins  325 , and a waveguide window  330 . The form and function of these elements is the same as the corresponding elements of  FIG. 2 . 
     The front-side feed network  340  is similar to the compact feed network described in U.S. Pat. No. 9,246,233. The front-side feed network  340  is made up of two waveguide components  390  and  395  that connect through a central aperture of the cap  370 . The waveguide component  390  includes a second connecting member  350  with slots  355  and a shoulder  360  configured to fit over the first connecting member  320  of the hub  310 . The front-side feed network  340  also includes a cap  370  with slots  375 , a spring  380 , and optional washers  385 . The form and function of these elements is the same as the corresponding elements of  FIG. 2 . 
     The use of four pins  325  and four slots  355 / 375  allows the front-side feed network  340  to be connected to the hub  310  in four different relative positions, rotated by 90 degrees about the antenna axis  315 . The front-side feed network  340  may contain a polarizing element  392  (shown only in cross-sectional view of  FIG. 5 ) to convert linear polarization into circular polarization. Rotating the front-side feed network  340  about the antenna axis  315  by 90 degrees reverses the circular polarization direction for a given linear polarization. For example, a linearly polarized wavefront from the transmitter of the ground station can be converted to right-hand circular polarization with the front-side feed network in a first position and converted to left-hand circular polarization with the front-side feed network in a second position rotated 90 degrees about the antenna axis  315 . 
     When a portable microwave system, such as the portable ground station antenna  10  of  FIG. 1 , is disassembled into components (such as the hub  310  and front-side feed network  340 ) for transport, it is preferable to have seals at each exposed waveguide port to prevent intrusion of foreign materials. Such seals must also be microwave windows, which is to say the seal must transmit microwave radiation with minimal insertion loss and no objectionable resonances. Resonances can occur due to reflections of microwave energy from the surfaces of the seals. 
     A known technique for sealing waveguide ports is to use a thin dielectric window. Since the reflections from two sides of a dielectric window differ in phase by 180-degrees, the reflections will cancel, or nearly cancel, if the window is sufficiently thin. However, a thin window may be subject to mechanical damage during handling and transport. 
     As an alternative to a fragile thin window, a window may have an electrical thickness of ½ wavelength at a selected frequency, which may be typically be a center frequency of a frequency band, or a frequency at the mid-point between uplink and downlink frequency bands. In this case reflections of microwave energy from the two sides of the window differ in phase by 540 degrees, and still substantially cancel over a useful frequency range about the selected frequency. 
       FIG. 6  shows a plan view and a cross-sectional view of a mechanically robust waveguide window/seal  610 . The waveguide window/seal  610  may be, for example, any of the waveguide windows  230  or  245  of  FIG. 2  or the waveguide windows  330 ,  345  of  FIG. 3 ,  FIG. 4 , and  FIG. 5 . 
     The waveguide window/seal  610  includes a central window  640 , an annular recess  650  surrounding the window  640 , and an annular rib  660  surrounding the annular recess  650 . Dimension t1 is a thickness of the central window  640 , t2 is a depth of the annular recess  650 , and t3 is a thickness of the annular rib  660 , where t2&lt;t1&lt;t3. An outside diameter d3 of the annular rib  660  may be configured to closely fit within an inside diameter of a circular waveguide to be sealed. The waveguide window seal  610  may optionally include a flange  670  to locate the waveguide window with respect to the waveguide. The central window  640 , the annular recess  650 , the annular rib  660 , and the flange  670  (when present) are concentric with each other and the circular waveguide. The waveguide window seal  610  may typically be affixed in the waveguide using an adhesive. 
     A waveguide window/seal may commonly face and be closely proximate to another, possibly identical, waveguide window seal (as indicated by the dashed outline  630 ). For example, in the portable earth station antenna  300  of  FIGS. 3, 4, and 5 , the waveguide windows  330 ,  345  face each other when the front-side feed network  340  is connected to the hub  310 . The waveguide windows  330 ,  345  may not touch, but may be separated by a gap ( 332  in  FIG. 5 ). This gap forms a radial waveguide which is terminated in a conductive O-ring and coupled to one or more choke grooves ( 334  in  FIG. 5 ). In such applications the total electrical thickness of the facing windows may be ½ wavelength at a selected frequency. Conveniently, the thickness t1 of the window  640  and the facing window may each be about ¼ wavelength. In some applications a waveguide window seal may face air (i.e. open space or an unsealed waveguide) rather than another window seal. In this case, the thickness t1 of the window  640  may be ½ wavelength. 
     The presence of a thick dielectric window in a waveguide presents a substantial change in impedance from the impedance of the empty waveguide. Additionally, the presence of a thick dielectric window may cause higher order mode (e.g. TM01) resonances at particular frequencies. The presence of a resonance within an operating frequency range of a waveguide device (e.g. either the uplink and or downlink frequency bands of a satellite antenna) causes high insertion loss and is generally unacceptable. The inner and outer diameters d1, d2 of the annular recess  650 , the depth t2 of the annular recess  650 , and the thickness t3 of the annular rib  660  provide degrees of freedom that can be used during the design of a waveguide circuit both to provide an impedance transition region between the empty waveguide and the window  640  and to tune the frequencies of any resonances to not fall within an operating frequency range of the waveguide circuit. 
     The waveguide window/seal  610  may be fabricated from a dimensionally stable, low-loss dielectric material suitable for use in an outdoor environment. The waveguide window seal  610  may be fabricated, for example, from a cross-linked polystyrene plastic material, such as REXOLITE® available from C-Lec Plastics. The waveguide window seal  610  may be fabricated from another low-loss dielectric material. 
       FIG. 7  shows a plan view and a cross-sectional view of another mechanically robust waveguide window/seal  710 . The window/seal  710  differs from the window/seal  610  in that an inside diameter d4 of the annular rib is greater than an outside diameter d2 of the annular recess. Additionally, the window/seal  710  includes a center conductor  780 , which may be a hollow tube of aluminum or other conductive metal. 
     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 method acts or system elements, it should be understood that those acts and those elements may be combined in other ways to accomplish the same objectives. With regard to flowcharts, additional and fewer steps may be taken, and the steps as shown may be combined or further refined to achieve the methods described herein. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments. 
     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.