Abstract:
A sub-reflector for a dish reflector antenna with a waveguide supported sub-reflector. The sub-reflector formed from a dielectric block, concentric about a longitudinal axis. The dielectric block having a first diameter waveguide junction portion adapted for coupling to an end of the waveguide and a sub-reflector surface coated with an RF reflective material having a periphery with a second diameter larger than the first diameter. A leading cone surface extends from the waveguide junction portion to the second diameter at an angle. The sub-reflector surface and the leading cone surface having a plurality of non-periodic perturbations concentric about the longitudinal axis.

Description:
BACKGROUND OF INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates to microwave dual reflector antennas typically used in terrestrial point to point, and point to multipoint applications. More particularly, the invention provides a low cost self supported feed solution for use in frequency bands between 5 GHz and 60 GHz wherein stringent regulatory standard compliance and or specific system electrical characteristics are required. The invention is particularly suited to “deep dish” designs overcoming performance limitations of prior art devices and obviating the need for a conventional shroud assembly. It is also applicable to more conventional dish profiles.  
         [0003]     2. Description of Related Art  
         [0004]     Dual reflector antennas employing self-supported feed direct a signal incident on the main reflector onto a sub-reflector mounted adjacent to the focal region of the main reflector, which in turn directs the signal into a waveguide transmission line typically via a feed horn or aperture to the first stage of a receiver. When the dual reflector antenna is used to transmit a signal, the signals travel from the last stage of the transmitter system, via the waveguide, to the feed aperture, sub-reflector, and main reflector to free space.  
         [0005]     Dual reflector antennas utilizing a sub-reflector supported and fed by a waveguide are relatively cost efficient. This configuration also facilitates the mounting of an “Outdoor Unit” comprising the initial stages of a transceiver system, directly onto the back of the main reflector and also eliminates the need for a separate feed support structure that would conventionally span the face of the main reflector, thereby introducing some loss in operating efficiency. The waveguide can have either a rectangular cross-section, whereby the antenna is single polarized, or can have a square or circular cross-section facilitating dual-polarization operation.  
         [0006]     The electrical performance of an antenna used in terrestrial communications is characterized by its gain, radiation pattern, cross-polarization and return loss performance efficient gain, radiation pattern and cross-polarization characteristics are essential for efficient microwave link planning and coordination, whilst a good return loss is necessary for efficient radio operation.  
         [0007]     These principal characteristics are determined by a feed system designed in conjunction with the main reflector profile. Conventional antenna designs used extensively in terrestrial point to point communications utilize a parabolic main reflector together with either a “J-hook” type waveguide feed system, or a self supported sub-reflector type feed system. In order to achieve “high performance” radiation pattern characteristics, these designs typically use an RF energy absorber lined cylindrical shroud around the outer edge of the main reflector antenna in order to improve the radiation pattern particularly in directions from approximately 50 to 180 degrees from the forward on axis direction. Shrouds however increase the overall weight, wind load, structural support and manufacturing costs of the antenna.  
         [0008]     An alternative method to improve the radiation pattern in these angular regions is to use a “deep” dish reflector, i.e. the ratio of the reflector focal length (F) to reflector diameter (D) is made less than or equal to 0.25 (as opposed to an F/D of 0.35 typically found in more conventional dish designs). Such designs can achieve “high performance” radiation pattern characteristics without the need for a separate shroud assembly when used with a carefully designed feed system which provides controlled dish illumination, particularly toward the edge of the dish. One such design which uses corrugations proximate to the outer radius of the sub-reflector to inhibit surface propagation and or field diffraction around the outer edge of the sub-reflector is described in U.S. Pat. No. 5,959,590 issued Sep. 28, 1999 to Sandford et al.  
         [0009]     In dual-reflector feeds employing dielectric cone supported sub-reflectors, adequate feed radiation pattern characteristics may be designed for conventional (F/D&gt;0.25) reflectors using simple unperturbed conic surfaces. Such a design presents a requirement for the feed to efficiently illuminate the main reflector over a total subtended angle of typically 130 degrees.  FIG. 1   a  illustrates one such design.  FIGS. 1   b  and  1   c  show models of the typical resulting amplitude and phase feed radiation patterns of this configuration.  
         [0010]     In order to provide the larger angular illumination for a “deep dish” reflector (subtended angle &gt;180 degrees), such a simple design is limited by internal and multi-path reflections prevalent within the cone structure between the rear reflecting surface and the leading edge boundary resulting in poorly controlled amplitude and phase radiation patterns with deep nulls at some frequencies within a typical operating band.  FIG. 2   a  illustrates one such design.  FIGS. 2   b  and  2   c  show typical models of the resulting amplitude and phase feed radiation patterns for this configuration.  
         [0011]     Multiple internal reflections can be reduced by the use of a regular array of corrugations positioned on the leading edge (cone surface closest to the main reflector).  FIG. 3   a  illustrates one such design.  FIGS. 3   b  and  3   c  show typical models of the resulting amplitude and phase feed radiation patterns of this configuration, as described in European Patent Application 0 439 800 A1 by Kuhne filed December 1990. Such a configuration improves the impedance match between the cone medium and that of free space, thus presenting a less severe impedance boundary to the RF signal path. However such a configuration only partially resolves the internal reflections and can have a detrimental effect on both amplitude and phase radiation match between E and H planes.  
         [0012]     Therefore it is the object of the invention to provide an apparatus that overcomes limitations in the prior art, and in so doing present a solution that allows such a feed design to provide reflector antenna characteristics which meet the most stringent electrical specifications over the entire operating band used for a typical terrestrial communication microwave link. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
         [0014]      FIG. 1   a , is a partial schematic side cross-section view of a prior art embodiment of a dielectric cone supported sub-reflector used, for example, in conventional dual reflector antennas using shallow dish reflectors.  
         [0015]      FIG. 1   b  is a model of a typical amplitude feed radiation pattern for an antenna with the sub-reflector configuration of  FIG. 1   a.    
         [0016]      FIG. 1   c  is a model of a typical phase feed radiation pattern for an antenna with the sub-reflector configuration of  FIG. 1   a.    
         [0017]      FIG. 2   a  is a partial schematic side cross-section view of a prior art embodiment of a dielectric cone supported sub-reflector cone body used in conventional dual reflector antennas using deep dish main reflectors.  
         [0018]      FIG. 2   b  is a model of a typical amplitude feed radiation pattern for an antenna with the sub-reflector configuration of  FIG. 2   a.    
         [0019]      FIG. 2   c  is a model of a typical phase feed radiation pattern for an antenna with the sub-reflector configuration of  FIG. 2   a.    
         [0020]      FIG. 3   a  is a partial schematic side cross-section view of a prior art embodiment of a dielectric cone supported sub-reflector as disclosed for example by the Kuhne reference, above.  
         [0021]      FIG. 3   b  is a model of a typical amplitude feed radiation pattern for an antenna with the sub-reflector configuration of  FIG. 3   a.    
         [0022]      FIG. 3   c  is a model of a typical phase feed radiation pattern for an antenna with the sub-reflector configuration of  FIG. 3   a.    
         [0023]      FIG. 4   a  is a cut-away side view of a deep dish dual reflector antenna with a self supported feed assembly with a tuned perturbation cone feed sub-reflector according to one embodiment of the invention.  
         [0024]      FIG. 4   b  is an angled front isometric view of the antenna shown in  FIG. 4   a.    
         [0025]      FIG. 5   a  is an angled external lower side isometric view of a dielectric cone supported sub-reflector according to a first embodiment of the invention.  
         [0026]      FIG. 5   b  is an angled external upper side isometric view of the dielectric cone supported sub-reflector shown in  FIG. 5   a.    
         [0027]      FIG. 5   c  is an external side view of the dielectric cone supported subreflector shown in  FIG. 5   a.    
         [0028]      FIG. 5   d  is a top view of the dielectric cone supported sub-reflector shown in  FIG. 5   a.    
         [0029]      FIG. 5   e  is a cut-away side view along the section line A-A of  FIG. 5   d.    
         [0030]      FIG. 6   a  is a chart of measured 22 GHz E-plane co-polar radiation patterns achieved using the sub-reflector of  FIG. 5   a - e  within a 1″ diameter shaped deep dish main-reflector, compared to ETSI E-plane and FCC regulatory radiation pattern specifications.  
         [0031]      FIG. 6   b  is a chart of measured 22 GHz H-plane co-polar radiation patterns achieved using the sub-reflector of  FIG. 5   a - e  within a 1″ diameter shaped deep dish main-reflector, compared to ETSI E-plane and FCC regulation pattern specifications.  
         [0032]      FIG. 7  is a chart of measured and modeled return loss for the embodiment shown in  FIGS. 5   a - e.    
         [0033]      FIG. 8   a  is an angled external lower side isometric view of a dielectric cone supported sub-reflector according to a second embodiment of the invention.  
         [0034]      FIG. 8   b  is an angled external upper side isometric view of the dielectric cone supported sub-reflector shown in  FIG. 8   a.    
         [0035]      FIG. 8   c  is an external side view of the dielectric cone supported subreflector shown in  FIG. 8   a.    
         [0036]      FIG. 8   d  is a top view of the dielectric cone supported sub-reflector shown in  FIG. 8   a.    
         [0037]      FIG. 8   e  is a cut-away side view along the section line A-A of  FIG. 8   d.    
         [0038]      FIG. 9   a  is a chart of measured 22 GHz E-plane co-polar radiation patterns achieved using the sub-reflector of  FIG. 5   a - e  within a 1″ diameter shaped deep dish main-reflector, compared to ETSI E-plane and FCC regulation pattern specifications.  
         [0039]      FIG. 9   b  is a chart of measured 22 GHz H-plane co-polar radiation patterns achieved using the sub-reflector of  FIG. 5   a - e  within a 1″ diameter shaped deep dish main-reflector, compared to ETSI E-plane and FCC regulation pattern specifications.  
         [0040]      FIG. 10   a  is a partial schematic side cross-section view of a third embodiment of a dielectric cone supported sub-reflector cone body according to the invention.  
         [0041]      FIG. 10   b  is a model of a typical amplitude feed radiation pattern for the antenna with the sub-reflector configuration of  FIG. 10   a.    
         [0042]      FIG. 10   c  is a model of a typical phase feed radiation pattern for the antenna with the sub-reflector configuration of  FIG. 10   a   FIG. 11   a  is a partial schematic side cross-section view of a fourth embodiment of a dielectric cone supported sub-reflector cone body according to the invention.  
         [0043]      FIG. 11   a  is a partial schematic side cross-section view of a fourth embodiment of a dielectric cone supported sub-reflector cone body according to the invention.  
         [0044]      FIG. 11   b  is a model of a typical amplitude feed radiation pattern for the antenna with the sub-reflector configuration of  FIG. 11   a.    
         [0045]      FIG. 11   c  is a model of a typical representative phase feed radiation pattern for the antenna with the sub-reflector configuration of  FIG. 11   a   FIG. 12   a  is a partial schematic side cross-section view of a fifth embodiment of a dielectric cone supported sub-reflector, having radial chokes (corrugations), according to the invention.  
         [0046]      FIG. 12   a  is a partial schematic side cross-section view of a fifth embodiment of a dielectric cone supported sub-reflector cone body, having radial chokes (corrugations), according to the invention.  
         [0047]      FIG. 12   b  is a model of a typical amplitude feed radiation pattern for an antenna with the sub-reflector configuration of  FIG. 12   a.    
         [0048]      FIG. 12   c  is a model of a typical phase feed radiation pattern for the antenna with the sub-reflector configuration of  FIG. 12   a    FIG. 13   a  is a partial schematic cross section view of a sixth embodiment of a dielectric cone supported sub-reflector configured to provide un-equal E and H-plane primary patterns.  
         [0049]      FIG. 13   a  is a partial schematic side cross-section view of a sixth embodiment of a dielectric cone supported sub-reflector configured to provide un-equal E and H-plane primary patterns, according to the invention.  
         [0050]      FIG. 13   b  is a model of a typical amplitude feed radiation pattern for the antenna of  FIG. 13   a.    
         [0051]      FIG. 13   c  is a model of a typical phase feed radiation pattern for the antenna of  FIG. 13   a   FIG. 13   d  is a chart of measured 38 GHz E-plane co-polar radiation patterns achieved using the sub-reflector of  FIG. 13   a  within a 1″ diameter shaped main-reflector, compared to ETSI and FCC radiation pattern specifications.  
         [0052]      FIG. 13   e  is a chart of measured 38 GHz H-plane co-polar radiation patterns achieved using the sub-reflector of  FIG. 13   a  within a 1″ diameter shaped main-reflector, compared to ETSI and FCC radiation pattern specifications. 
     
    
     DETAILED DESCRIPTION  
       [0053]     The self-supported feed system described herein integrates the waveguide transmission line, aperture and sub-reflector into a single assembly comprising a length of waveguide, the aperture of which is terminated with a corrugated dielectric cone sub reflector assembly, the front and back surfaces of which are geometrically shaped and corrugated to provide a desired amplitude and phase radiation pattern suitable for efficient illumination of the main reflector profile.  
         [0054]     A typical dual reflector antenna according to the invention is shown in  FIGS. 4   a  and  4   b . The sub-reflector assembly  1  is mounted on and supported by a waveguide  2  to position the sub-reflector assembly  1  proximate a focal point of the dish reflector  3 , here shown as a dish reflector  3  having a “deep dish” configuration.  
         [0055]     Details of the sub-reflector  1  assembly according to the invention will now be described in detail. A first embodiment of a sub-reflector  1  according to the invention is shown in  FIGS. 5   a - e . Representative and measured performance of the first embodiment is shown in  FIGS. 6   a - 7 . Further embodiments and their respective representative and or measured performance is shown in  FIG. 8   a - 13   e . The sub-reflector assembly  1  may be formed, for example, by injection molding and or machining a block of dielectric plastic. A sub-reflector surface  5  of the sub-reflector assembly  1  may be formed by applying a metallic deposition, film, sheet or other RF reflective coating  10  to the top surface of the dielectric block. A waveguide junction portion  15  of the sub-reflector assembly  1  is adapted to match a desired circular waveguide  2  internal diameter so that the sub-reflector assembly  1  may be fitted into and retained by the waveguide  2  that supports the sub-reflector assembly  1  within the dish reflector  3  of the reflector antenna proximate a focal point of the dish reflector  3 .  
         [0056]     One or more step(s)  20  at the end of the waveguide junction portion  10  and or one or more groove(s)  25  may be used for impedance matching purposes between the waveguide  2  and the dielectric material of the sub-reflector assembly  1 .  
         [0057]     The sub-reflector surface  5  and a leading cone surface  30  (facing the dish reflector  3 ) of the sub-reflector assembly  1  may have a plurality of concentric non-periodic perturbation(s)  35  in the form of corrugations, ridges and protrusions of varied heights, depths and or widths. Internal, external and combinations of internal and external perturbations may be applied. Also, a leading angle selected for pattern and VSWR matching between the waveguide junction portion  15  and a first perturbation, along the leading cone surface  30 , may then change as the leading cone surface  5  continues to a periphery of the sub-reflector assembly  1 , for example as shown on  FIG. 13   a . Where the prior art may have utilized a single perturbation for VSWR matching purposes, the present invention utilizes multiple perturbations to control internal reflections and thereby form a desired radiation pattern. Calculated using a full wave solution with the assistance of commercially available full wave RF radiation pattern calculation software rather than ray tracing, the location and specific dimensions of the perturbations and angle changes may be calculated and then further iteratively adjusted to minimize multi-path reflections within the dielectric material, control amplitude and phase distribution from the feed and improve the impedance match (VSWR) between the feed and free space.  
         [0058]     Further, as shown for example by  FIGS. 13   a - e , contrary to common practice requiring manipulation of the waveguide entry dimensions, where electrical requirements are non-equivalent between the vertical and horizontal (E and H-plane, or Etheta and Ephi) polarizations, for example for the 38 GHz band (ETSI EN 300833 Class 5  FIG. 3C ), the ridges height and width separately affect the different polarizations, at different frequency bands, even though the perturbation(s)  35  are concentric.  
         [0059]     Because the perturbation(s)  35  are concentric, the sub-reflector assembly  1  need not be keyed to a specific orientation with the waveguide or reflector antenna. Also, machining of perturbation(s)  35  that would be difficult to form by injection molding, alone, is simplified if a concentric design is selected.  
         [0060]     Adapting the perturbation(s)  35  to a desired configuration provides efficiencies that previously were obtained in part by correcting the profile of the dish reflector  3 . When these adaptations are made via the perturbation(s)  35 , the invention provides the advantage of higher performance over a wide frequency range, for example 10-60 GHz, with the same reflector dish profile.  
         [0061]     The combination of a “deep” phase corrected reflector with a sub-reflector assembly  1  according to the invention results in a reflector antenna operable over a wide frequency range with electrical characteristics previously available only with shallow profile reflector dishes with RF absorbing shrouds.  
         [0062]     From the foregoing, it will be apparent that the present invention brings to the art a sub-reflector assembly  1  for a reflector antenna with improved electrical performance and significant manufacturing cost efficiencies. The sub-reflector assembly  1  according to the invention is strong, lightweight and may be repeatedly cost efficiently manufactured with a very high level of precision.  
                                         Table of Parts                                1   sub-reflector assembly       2   waveguide       3   dish reflector       5   sub-reflector surface       10   RF reflective coating       15   waveguide junction portion       20   step       25   groove       30   leading cone surface       35   perturbation                  
 
         [0063]     Where in the foregoing description reference has been made to ratios, integers, components or modules having known equivalents then such equivalents are herein incorporated as if individually set forth.  
         [0064]     Each of the patents and published patent applications identified in this specification are herein incorporated by reference in their entirety to the same extent as if each individual patent was fully set forth herein for all each discloses or if specifically and individually indicated to be incorporated by reference. While the present invention has been illustrated by the description of the embodiments thereof, and while the embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, representative apparatus, methods, and illustrative examples shown and described. Accordingly, departures may be made from such details without departure from the spirit or scope of applicant&#39;s general inventive concept. Further, it is to be appreciated that improvements and/or modifications may be made thereto without departing from the scope or spirit of the present invention as defined by the following claims.