Patent Publication Number: US-11641057-B2

Title: Coaxial feed for multiband antenna

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 62/865,631 filed Jun. 24, 2019 and entitled COAXIAL FEED FOR MULTIBAND ANTENNA, the entire contents of which is incorporated herein for all purposes by this reference. 
    
    
     BACKGROUND OF INVENTION 
     Field of Invention 
     This application relates, in general, to coaxial feeds for multiband antennas, and more particularly to coaxially feeds for multiband antennas used for satellite communications. 
     Description of Related Art 
     Coaxial feeds are well known in the tracking antenna field. For example, U.S. Pat. No. 6,222,492 discloses a dual coaxial feed having concentric waveguides including an inner waveguide for a “sum” radiation pattern and an outer waveguide for a “difference” radiation pattern. The dual coaxial feed also includes a variety of chokes near the open ends of the waveguides for improving and modifying impedance matches between free space and its coaxial waveguides. 
     Coaxial feeds may also be used with multiple band antennas, which are desirable for satellite communications because such antennas provide the ability to operate on multiple frequency bands. Coaxial feeds are particularly well suited for use with dual-band tracking antennas configured to track communications satellites. For example, U.S. Pat. No. 6,982,679 discloses a coaxial horn antenna system having a choke extending around the aperture of the inner horn to reduce currents on the outer surface of the inner horn to improve pattern performance. 
     One will appreciate that it may be desirable to operate a multiband antenna within frequency bands having wavelengths that are very close to one another, for example, operating a dual-band antenna in Ka and Ku bands. In the case of close frequency bands, the outer “low-band” coaxial waveguide may have an inner diameter that is relatively large compared to its outer diameter, which may cause unwanted cross polarization (X-pol) radiation. Radiation patterns are determined by the electric field at the radiation aperture, and larger inner diameters cause greater electric field bending at the aperture, which in turn leads to greater X-pol radiation. 
     It would therefore be useful to provide a multiband antenna with a choke structure that overcomes the above and other disadvantages of known coaxial-feed chokes. 
     BRIEF SUMMARY 
     One aspect of the present invention is directed to a coaxial feed for a multiband antenna, the coaxial feed including: a tubular high-band (HB) waveguide, the HB waveguide including an outer conducting surface, an inner HB conducting surface, and a HB aperture defined by the inner HB conducting surface; a tubular low-band (LB) waveguide disposed coaxially around the HB waveguide, the LB waveguide including an outer feed surface, an inner LB conducting surface, and an annular LB aperture defined by the inner LB conducing surface and the outer conducting surface of the HB waveguide; and an annular high-band (HB) choke located in the outer conducting surface of the HB waveguide, the HB choke being axially offset from the HB aperture. 
     The HB waveguide may be a Ka-band waveguide. 
     The HB waveguide may be dielectrically loaded with a dielectric member. 
     The dielectric member may have a relative permittivity equal to or greater than 2. 
     The dielectric member may be formed of a material selected from plastic, quartz, REXOLITE (cross-linked polystyrene) or a combination thereof. 
     The HB waveguide aperture may have a diameter in the range of approximately 0.2″ to 0.33″. 
     The HB choke may be axially offset from the HB aperture equal or larger than ¼ wavelength of the LB frequency. 
     The offset of the HB choke may be configured to provide impedance matching to free space for LB frequencies of the LB waveguide. 
     The LB waveguide may be a Ku-band waveguide. 
     The LB aperture may have an LB aperture inner diameter in the range of approximately 0.22″ to 0.35″. 
     The LB aperture may have an LB aperture inner diameter, and the HB choke may have an HB choke inner diameter that is approximately equal to the LB aperture inner diameter. 
     The LB aperture may have an LB aperture inner diameter, and the HB choke may have an HB choke outer diameter that is greater than the LB aperture inner diameter. 
     The HB waveguide may be tuned for a HB frequency having an HB wavelength, and the HB choke outer diameter is approximately 0.1 to 0.25 times the HB wavelength larger than the LB aperture inner diameter. 
     The HB choke outer diameter (OD HB Choke ) may be determined:
 
 ID   LB Aperture +0.1λ HB   ≤OD   HB Choke   ≤ID   LB Aperture +0.25λ HB ,
 
wherein ID LB Aperture  is the LB aperture inner diameter, and λ HB  is the HB wavelength.
 
     The LB waveguide may include a radial groove in the inner conducting surface axially disposed between the LB aperture and the HB choke, the radial groove defining a corrugation configured and dimensioned to provide phase tuning for the HB waveguide. 
     The LB waveguide may include a secondary HB choke disposed around the annular LB aperture. 
     The LB waveguide may include a plurality of secondary HB chokes concentrically disposed around the annular LB aperture. 
     Another aspect of the present invention is directed to a multiband antenna system including: a primary reflector; a subreflector affixed relative to the primary reflector; and any one of the coaxial feeds described above, wherein the coaxial feed extends from the primary reflector toward the subreflector. 
     The antenna may further include a tracking pedestal supporting the primary reflector, the subreflector, and coaxial feed, the tracking pedestal configured for tracking communications satellites. 
     The system may further include: a HB diplexer positioned behind the primary reflector and operatively connected to a HB throat of the HB waveguide; a LB turnstile junction positioned around the HB diplexer and operatively connected to a LB throat of the LB waveguide; and a LB orthomode transducer and diplexer positioned behind and operatively connected to the LB turnstile junction. 
     The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of an exemplary coaxial feed for a multiband antenna in accordance with various aspects of the present invention. 
         FIG.  2    is a side view of the coaxial feed and multiband antenna of  FIG.  1   . 
         FIG.  3    is a cross-sectional view of the coaxial feed taken along line  3 - 3  in  FIG.  2   . 
         FIG.  4    is an enlarged detail of the coaxial feed shown in  FIG.  3   . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     Turning now to the drawings, wherein like components are designated by like reference numerals throughout the various figures, attention is directed to  FIG.  1   , which shows an exemplary coaxial feed  30  for a multiband antenna  32 . The coaxial feed extends away from a primary reflector  33  and supports a subreflector  35  in a position that is affixed relative to the primary reflector in an otherwise conventional manner. For example, an RF-transparent subreflector support  37  may be utilized to support the subreflector on the end of the coaxial feed. In various embodiments, the multiband antenna is a circularly-symmetric dual-reflector antenna, in which both the primary reflector and the subreflector are circularly symmetric. 
     With reference to  FIG.  2   , multiband antenna  32  may be operatively supported on a tracking pedestal  39  for tracking satellites and/or other moving communications devices in an otherwise conventional manner. The multiband antenna may also be provided with a high-band diplexer  40  operatively connected to an HB throat of an HB wave guide, a low-band turnstile junction  42  positioned around the HB diplexer and operatively connected to an LB throat of an LB waveguide, a low-band orthomode transducer and diplexer  44  positioned behind and operatively connected to the LB turnstile junction. One will appreciate that the multiband antenna may also be provided with other suitable equipment in an otherwise conventional manner. 
     Turning now to  FIG.  3   , the coaxial feed generally includes a tubular high-band (HB) waveguide  46 , a coaxial low-band (LB) waveguide  47  disposed around the HB waveguide and held in place by at least one RF-transparent coaxial support  49 . The HB waveguide generally includes an outer conducting surface  51 , an inner HB conducting surface  53 , and a HB aperture  54  defined by the inner HB conducting surface, while the LB generally includes an outer feed surface  56 , an inner LB conducting surface  58 , and an annular LB aperture  60  defined by the inner LB conducing surface and the outer conducting surface of the HB waveguide. 
     One will appreciate that the multiband antenna may be configured as a dual band antenna, and each of the HB and LB waveguides may be configured dimensions to optimize reception and propagation of radio frequency waves of different frequencies. In various embodiments, the HB waveguide is configured as a Ka-band waveguide and the LB waveguide is configured as a Ku-band waveguide. 
     In accordance with various aspects of the present invention, HB waveguide may be dielectrically loaded with a dielectric member  61 . Dielectrically loading the HB waveguide advantageously allows for a smaller HB aperture diameter, which in turn, allows for a smaller inner diameter of the LB aperture and improved cross polarization (X-pol) radiation performance. In particular, a smaller inner diameter of the LB aperture reduces electric field bending at the LB aperture and thus reduces unwanted X-pol radiation. 
     The dielectric member preferably has a relative permittivity equal to or greater than 2. Suitable materials for the dielectric member include plastic, quartz, REXOLITE (a cross-linked polystyrene manufactured by C-Lec Plastics, Inc. of Philadelphia, Pa.), a combination thereof, and/or other suitable materials. 
     In accordance with various aspects of the present invention, an annular high-band (HB) choke  63  is provided on the outer conducting surface  51  of HB waveguide  46  and axially offset away from HB aperture  54 , as shown in  FIG.  3   . The offset of the HB choke is configured to provide impedance matching to free space for LB frequencies of the LB waveguide. In various embodiments, the HB choke is axially offset from the HB aperture a distance that is equal or larger than ¼ wavelength of the LB frequency. 
     In operation, and with reference to  FIG.  4   , high-band radiation travels through HB waveguide  46  and radiates from the HB aperture  54 . A majority of the wave energy radiates to free space (indicated by arrow A). However, some of the wave energy leaks onto the outer conducting surface  51  of the HB waveguide (indicated by arrow B). The axial offset of HB choke  63  may be properly tuned to reflect leaking wave energy back and re-radiate at the coaxial aperture (indicated by arrow B′) thus minimizing wave energy leaking into LB waveguide  47  (indicated by arrow B″). 
     The axial offset distance (D) of the HB choke determines the phase of the reflected wave energy (arrow B′). Preferably the majority of radiated wave energy (arrow A) and the reflected wave energy (arrow B′) are in phase so that the majority and reflected wave energy are constructively combined to maximize radiation energy from the coaxial feed. 
     Accordingly, axially offset HB choke  63  allows for the optimization of high-band performance by reducing energy leakage into the coaxial LB waveguide  47  and phase tuning the reflected radiation energy (arrow B′). 
     Significantly, the axial-offset HB choke configuration allows for the inner diameter of LB aperture  60  to be less than the outer diameter of HB choke  63 . In various embodiments, the inner diameter of the LB aperture is approximately equal to that of the HB choke, as is shown in  FIG.  4   . Thus, the axial-offset configuration of the HB choke also allows for a smaller inner diameter of the LB aperture and improved cross polarization (X-pol) radiation performance. 
     Such configuration also allows for an outer diameter of the HB choke to be greater than the inner diameter than the LB aperture inner diameter. In various embodiments, HB waveguide  46  is tuned for a specific HB frequency and LB waveguide  47  is tuned for a specific LB frequency, for example Ka and Ku respectively. The axial-offset HB choke configuration allows the outer diameter of HB choke  63  to be larger than the inner diameter of LB aperture  60  by approximately 0.1 to 0.25 times the HB wavelength. For example, the HB choke outer diameter (OD HB Choke ) may be determined:
 
 ID   LB Aperture +0.1λ HB   ≤OD   HB Choke   ≤ID   LB Aperture +0.25λ HB   Eq. (1)
 
     where ID LB Aperture  is the LB aperture inner diameter, and λ HB  is the HB wavelength. 
     In such cases, the HB/Ka-band waveguide aperture preferably has a diameter in the range of approximately 0.2″ to 0.33″, and the LB/Ku-band waveguide preferably has an LB aperture with an LB aperture inner diameter in the range of approximately 0.22″ to 0.35″. Preferably, the difference between the LB aperture inner diameter and the HB aperture diameter is merely the wall thickness of the HB waveguide. For example, the LB/Ku-band waveguide preferably has an LB aperture inner diameter in the range of approximately 0.21″ to 0.35″ when the HB waveguide has a tubular wall thickness of 0.01″, and the LB/Ku-band waveguide preferably has an LB aperture inner diameter in the range of approximately 0.24″ to 0.37″ when the HB waveguide has a tubular wall thickness of 0.02″. 
     Returning to  FIG.  3   , coaxial feed  30  may be provided with other tuning features to improve both high and low band performance. For example, LB waveguide  47  may include a radial groove  65  in its inner conducting surface  58  axially disposed between the LB aperture  60  and the HB choke  63 . The radial groove may be configured and dimensioned to provide phase tuning of the reflected wave energy of HB waveguide  46  (arrow B′) and the HB waveguide  46 . The radial groove may also be configured to provide phase tuning of low-band radiation traveling through LB waveguide  47 . In particular, HB choke may create a discontinuity of LB radiation (arrow C), in which case the radial groove may be tuned to provide additional phase tuning of the discontinuity whereby matching of LB radiation can be improved. Accordingly, the radial groove may be used to simultaneously optimize HB radiation performance and LB matching. 
     coaxial feed  30  may also include one or more aperture chokes  67  disposed around the annular LB aperture  60  to minimize undesired side lobes on the antenna radiation pattern. One will appreciate that such aperture chokes may be tuned to primary HB radiation, reflected HB radiation, or LB radiation in an otherwise conventional manner. 
     For convenience in explanation and accurate definition in the appended claims, the terms “inner” and “outer” are used to describe features of the exemplary embodiments with reference to the positions of such features as displayed in the figures. 
     The foregoing descriptions of specific exemplary embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.