Abstract:
A multi-band reflector antenna having a main reflector, a sub reflector, a first feed and a second feed. The first feed, the sub reflector and the main reflector positioned in a Gregorian optic configuration wherein an output of the first feed is directed to the sub reflector, from the sub reflector to the main reflector and from the main reflector into a first beam. The sub reflector may be shaped to distribute the output of the first feed reflecting off the sub reflector onto the main reflector whereby a central area of the main reflector has a lower illumination than a surrounding outer area of the main reflector and or elongated vertically. The second feed projecting from a hole in the sub reflector is oriented whereby an output of the second feed is directed to the main reflector and from the main reflector into a second beam.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. Utility application Ser. No. 10/484,572 filed on Jan. 22, 2004 entitled “Co-Located Antenna Design”, now pending, which is the National Stage of International Application PCT/US02/28991 filed on Sep. 12, 2002, entitled “Co-Located Antenna Design” which was published in English as International Publication Number WO 03/026173 A1 on Mar. 27, 2003; This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/322,343 filed on Sep. 14, 2001, entitled “Multi Beam Co-Located Antenna”. Also, This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/601,396 filed on Aug. 13, 2004, entitled “Co-Located Multi-Band Antenna”. U.S. Provisional patent application Ser. Nos. 60/322,343, 60/601,396 and International Application PCT/US02/28991 are each incorporated herein by reference in their entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The field of the invention relates to communication systems and more particularly to multi-band antennas useful, for example, for satellite communication.  
       BACKGROUND OF THE INVENTION  
       [0003]     Satellite communication systems are known and generally well understood. Such systems are typically used in telephone, television and data communications over long distances.  
         [0004]     Satellite communication systems are typically used in conjunction with one or more ground stations. Ground stations are usually constructed as high value subsystems able to combine and disperse communication signals routed through the satellite.  
         [0005]     Because of the volume of signal traffic typically processed by ground stations, signal traffic may be divided among relatively large numbers of carrier signals. Relatively large dish antenna are often provided to transceive those signals with a desired satellite(s).  
         [0006]     Recently, special purpose systems have been developed for transceiving signals with satellites. One example of such a system is the Very Small Aperture Terminal (VSAT) used for the communication of data, voice and video signals, except terrestrial broadcast television.  
         [0007]     A VSAT may include a transceiver and antenna (placed outdoors in direct line of sight with the satellite) and an interface unit. The interface unit is typically placed indoors and functions to interface the transceiver with end-user equipment.  
         [0008]     One application of VSAT is an Internet/Satellite TV system that provides combined satellite TV and Internet services. The Internet/Satellite TV system interacts with two co-located or close-located satellites. A first satellite may provide two-way Internet access. Internet messages may be received in the 20 GHz band and transmitted on the 30 GHz band.  
         [0009]     The second co-located or close-located satellite may provide satellite TV. The second satellite may transmit satellite TV in the 12 GHz band. To provide an extended range of satellite TV channels, multiple satellites may be used. Depending upon position, multiple satellites may be targeted with a nominally coincident beam and or one or more scanned beams.  
         [0010]     While the Internet/satellite TV system works well, the three different carriers of 12, 20 and 30 GHz are typically transceived through a dual band antenna and a second, separate antenna for the third band or in a single tri-band antenna using frequency selective surface (FSS) techniques. The use of dual antennas or FSS techniques is expensive and or aesthetically unacceptable in a consumer environment. Further, FSS surfaces may be susceptible to environmental degradation and or fouling.  
         [0011]     Competition in the consumer VSAT market has focused attention on minimization of overall costs, improved reliability and ease of installation/use. Accordingly, a need exists for a multi-band antenna system that is cost efficient, compact and conveniently mountable to an exterior of an end-user&#39;s home.  
         [0012]     Similar efficiencies are also desirable for multi-band terrestrial microwave antenna systems with nominally coincident beams and or one or more scanned beams.  
         [0013]     Therefore, it is an object of the invention to provide a co-located multi-band antenna that overcomes deficiencies in the prior art. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     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.  
         [0015]      FIG. 1  is a side schematic ray diagram for a prior art Gregorian dual offset reflector antenna.  
         [0016]      FIG. 2  is an elevated isometric front view of an exemplary embodiment of an antenna according to the invention;  
         [0017]      FIG. 3  is a cut-away side view of the antenna, along line A-A of  FIG. 2 ;  
         [0018]      FIG. 4  is an angled isometric rear view of the antenna of  FIG. 2 ;  
         [0019]      FIG. 5  is a side view of a schematic ray diagram for the antenna of  FIG. 2 .  
         [0020]      FIG. 6   a  is a schematic side view of an antenna according to the invention with a scanned-beam feed.  
         [0021]      FIG. 6   b  is a front view of  FIG. 6   a.    
         [0022]      FIG. 6   c  is a top view of  FIG. 6   a.    
         [0023]      FIG. 7   a  is a schematic side view of an antenna according to the invention with a scanned-beam feed and oval sub reflector.  
         [0024]      FIG. 7   b  is a front view of  FIG. 7   a.    
         [0025]      FIG. 7   c  is a top view of  FIG. 7   a.    
         [0026]      FIG. 8   a  is a schematic side view of an antenna according to the invention with a scanned-beam feed and a distorted oval sub reflector.  
         [0027]      FIG. 8   b  is a front view of  FIG. 8   a.    
         [0028]      FIG. 8   c  is a top view of  FIG. 8   a.    
         [0029]      FIG. 9  is a chart identifying analytical and numerically tabulated functions for forming a sub reflector rim with a notched oval rim.  
         [0030]      FIG. 10  is a close-up front schematic view of a sub reflector with notched rim according to the numerically tabulated function of  FIG. 9  and associated feeds.  
         [0031]      FIG. 11  is an isometric schematic view of an antenna according to the invention with a third feed adjacent to the first feed, according to the invention.  
         [0032]      FIG. 12   a  is a schematic side view of an antenna according to the invention with a third feed adjacent to the second feed.  
         [0033]      FIG. 12   b  is a schematic top view of  FIG. 12   a.   
     
    
     DETAILED DESCRIPTION  
       [0034]     In a conventional, non-shaped Gregorian system, as shown for example in  FIG. 1 , the main reflector  50  is a section of a paraboloidal surface and the sub reflector  52  is a section of an ellipsoidal surface. Ray-optic analysis shows rays launched by the first feed  60  strike the sub reflector  52  to be reflected so as to pass through a single point, the focal point of the main reflector, before striking the main reflector  50  and reflecting again to form the main beam. The first feed  60  is ideally positioned at one focus of the ellipsoid-section sub reflector  52 , with the sub reflector  52  positioned so that the other focus of the ellipsoid is coincident with the focal point of the main reflector  50 . By stating that the main reflector  50 , sub reflector  52 , and first feed  60  collectively form an antenna  10  system with Gregorian optics, it is understood by those skilled in the art that these components are arranged so that the focal point or focal region of the main reflector  50  is located in front of the sub reflector  52 , i.e. on the same side of the sub reflector  52  as the first feed  60 .  
         [0035]     The familiar offset configuration shown is the preferred configuration of the present invention. Axisymmetric configurations of the present invention may also be applied. However, blockage of the main reflector  50  by the sub reflector  52  and first feed  60  is reduced by the offset configuration.  
         [0036]     Applicants research has demonstrated that one difficulty with the conventional system is that for a small-aperture main reflector  50 , the corresponding ellipsoid-section sub reflector  52  is typically too small to function as a proper sub reflector  52 . For example, a 74×59 cm main reflector  50  requires a non-shaped sub reflector  52  approximately 4 wavelengths in size at 12 GHz for the geometrical parameters selected. This requires a relatively large and expensive, high-gain first feed  60  to illuminate the sub reflector  52  if excessive feed spillover is to be avoided. Such a sub reflector  52  is also too small electrically for the theory of Geometrical Optics (GO) to hold, on which the ray-tracing analysis depends. In this case, even with a high-gain first feed, there is considerable loss in gain due to undesired scattering by the sub reflector  52  in directions other than the main reflector  50 .  
         [0037]     One skilled in the art will appreciate that configuration of an antenna involves trade-offs of component cost, electrical performance and overall antenna size. While reception bands may be degraded in the compromise chosen, broadcast bands are typically required to at least meet, for example, applicable regulatory agency earth station transmission specifications.  
         [0038]     The problems of the conventional configuration are reduced by the present invention, for example as demonstrated by  FIGS. 2-5 , through use of a GO shaping routine, whereby the sub reflector  52  size is increased to capture energy from the first feed  60  and redirect it to the main reflector  50  with minimum spill over. This allows use of a relatively inexpensive conventional first feed  60  with only moderate gain. For example, in embodiments where there is −12 dB illumination at the sub reflector  52  rim from a 14 dBi first feed, a sub reflector  52  roughly 9 wavelengths in size may be used. This is large enough electrically for ray-optic analysis to remain valid so that the main reflector  50  is illuminated as designed and excessive feed spillover is avoided. Alternatively, shaping methods other than GO can be used to similar effect in the present invention, for example surface optimization via Physical Optics (PO) analysis.  
         [0039]     However, while the GO sub reflector  52  shaping procedure fixes one problem another is created. As the sub reflector  52  is increased in size, and perturbed from the conventional section of an ellipsoidal surface, rays launched from the first feed  60  and reflected from the sub reflector  52  surface no longer pass exactly through the focal point of the main reflector  50 . Instead, they pass near the main reflector  50  focal point through a broader focal region B (see  FIG. 5 ) on their way to illuminating the main reflector  50 . This creates a roughly quadratic phase error across the main reflector  50  aperture. The more significantly perturbed the shaped sub reflector  52  surface is from the ideal ellipsoid-section surface the greater the phase error. That is, the larger the sub reflector  52  becomes, the more phase error is introduced as the rays pass farther away from the main reflector  50  focal point.  
         [0040]     The shaping-induced phase error related to the increased size of the sub reflector  52  can be either minimized or corrected for in at least three ways according to the present invention. First, the main reflector  50  can be shaped as well, perturbed from a section of a paraboloidal surface, to compensate for the phase error. This is a possible embodiment of the present invention, but not preferred because to achieve multi-band capability the main reflector  50  aperture is shared between the beam produced by the first feed  60  illuminating the sub reflector  52  and an additional alternative frequency band beam or beams produced by direct illumination of the main reflector  50  by a second feed  58  and, if desired, other additional feeds. Therefore, if shaping of the main reflector  50  is applied, it is preferred that only minimal shaping is used so as to not unacceptably degrade performance of the second feed(s)  58 , or the second feed(s)  58  themselves are also adapted to compensate for the main reflector shaping.  
         [0041]     A shaped main reflector  50  will not have a single focal point, but rather a distributed “focal region” roughly in the same position as the focal point of a best-fit paraboloidal surface, also positioned in front of the sub reflector  52 . In a parabolic main reflector  50 , the focal region will be a single focal point positioned in front of the sub reflector  52  as found in conventional Gregorian reflector systems. Similarly, the term “focal region” is also used herein with respect to the region near the focal point of a paraboloidal main reflector  50  through which rays from a shaped sub reflector  52  pass, as shown for example in  FIG. 5 .  
         [0042]     In a second method of phase error reduction the sub reflector  52  may be shaped. Using a sub reflector  52  roughly 9 wavelengths in size, the shaping-induced phase error may be minimized by leaving a “hole” or minima proximate the center of the main aperture illumination, as shown for example in the  FIG. 5  ray-tracing diagram. For clarity, the ray diagram demonstrates only the signal path with respect to the first feed  58 . By shaping the sub reflector  52  to direct most of the illuminating energy to the outer regions of the main reflector  50 , there is less variation in phase across the main reflector and the aperture efficiency is improved. This is apparent from an equal-path length analysis of rays striking the main reflector center, the main reflector  50  rim, and a point midway between the center and rim: there is less path-length difference between the last two rays than there is between the first two rays.  
         [0043]     Alternatively, the configuration tradeoff may be applied in a third form of phase error minimization by using a higher-gain first feed  60 , for example 17 dBi, and a smaller sub reflector  52  such as 6 to 7 wavelengths in size. This sub reflector  52  is just large enough to work electrically, yet is closer to the ideal ellipsoid-section shape so that phase error is minimized. In such embodiments, a more uniform amplitude illumination is preferred to get maximum aperture efficiency, so sub reflector  52  shaping is applied which does not form a minima in the main aperture central illumination. This has the additional advantage of reducing sidelobes in the beam produced by the first feed  60 .  
         [0044]     Returning to  FIGS. 2-5 , multi-band capabilities are added to the antenna  10  via a second feed  58 . A hole  62  formed in a generally central region of the sub reflector  52  locates the second feed  58  (e.g., a horn, waveguide, helical antenna, dielectric rod, etc.) oriented with a predominant axis of radiation aligned to illuminate the main reflector  50 . It should be understood that, as used herein, the term “feed” means a structure that is inherently capable of transmitting and/or receiving radio frequency energy. It should also be understood that while the second feed  58  is disposed within the sub reflector  52 , the phrase “disposed within” is also meant to include the situation where the end of the radiator extends towards the main reflector  50  beyond the reflecting surface of the sub reflector  52 , is flush with the sub reflector  52  or is recessed into the hole  62  of the sub reflector  52 . Preferably, the second feed  58  is a dielectric polyrod antenna projecting through the hole  62 , extending from and supported by a waveguide launch  59 , oriented along or close to the axis of a center reflected ray of the main reflector  50 .  
         [0045]     Where the second feed  58  is a dielectric polyrod antenna, the exact phase center of the dielectric polyrod antenna, as will be appreciated by those skilled in the art, is located some distance back from the tapered radiating end of the polyrod along the axis of the polyrod. This distance is generally a function of frequency. Typically in wideband or multi-band operation (in this embodiment, 20/30 GHz operation), some position along the polyrod is identified as a “best compromise” approximate phase center over the frequency band or bands of operation. This compromise position along the polyrod would normally be aligned with the focal point of the main reflector  50  or placed somewhere in the main reflector  50  focal region if a shaped main reflector  50  is used for optimal focusing of the beam or beams generated by the first feed  58 .  
         [0046]     Even if shaping is applied to the sub reflector  52  to leave a “hole” or minima proximate the center of the main aperture illumination, as indicated in  FIG. 5 , the second feed  58  interferes with a significant number of the incident and reflected rays of the sub reflector  52  corresponding to the first feed  60 . The interfering effect is greatest with respect to rays scattered from the forward tip of the second feed  58 . Therefore, minimizing the entry of the second feed  58  into the focal region B by positioning it closer to the sub reflector  52 , locating the phase center closer to the forward tip of the second feed  58  and or minimizing the overall width of the second feed  58  improves electrical performance of the first feed in a trade-off with performance of the second feed  58 .  
         [0047]     The sidelobes impacting Carrier/Interference (C/I) are also increased by the shaping of the sub reflector  52  for an amplitude distribution with a center minima. For a 74×59 cm main reflector, the GO shaping procedure stretches the sub reflector  52  to about 9 wavelengths in diameter. This makes the sub reflector  42  electrically large enough, but introduces a phase error that generally cannot be corrected in the main reflector  50  because of the shared aperture with the second feed  58 . If C/I is a priority, alternate embodiments which do not have the center minima may be used.  
         [0048]     In such embodiments, as described above, the required size of the sub reflector  52  may be reduced in size to about six wavelengths by using a higher gain first feed  60 , for example 17 dBi. As the first feed  60  gain is increased, the associated feed angle decreases. For example, a 17 dBi first feed  60  may have a feed angle of 26 degrees. With a smaller sub reflector  52 , the phase errors induced by using a GO-shaped, larger sub reflector  52  instead of a canonical ellipsoid sub reflector  52  are minimized. Therefore, it may not be necessary to apply the sub reflector  52  shaping to generate a center minima, further improving sidelobes and C/I.  
         [0049]     In further embodiments the size of the main reflector  50  and or first feed  60  may be increased to improve electrical performance. However, maintaining a minimal overall antenna  10  size is often a priority for consumer embodiments.  
         [0050]     Any beam or beams radiated by the second feed  58 , for example two beams in the 20 and 30 GHz bands, and a beam or beams radiated by the first feed  60 , for example a single beam in the 12 GHz band, are typically nominally coincident in the present invention. By “nominally coincident”, it is implied that all beams point in the same direction, or very nearly so. This includes the case where all beams communicate, for example, with transponders located on a single satellite, in which case the beams would point in as close to the same direction as possible. It also includes the case where one or more of the beams points in a slightly different direction, possibly to communicate, for example, with one or more satellites in very close proximity along the geostationary arc, for example less than 5°, away from the satellite or satellites in communication with the other nominally coincident beam or beams. By “very nearly” in the same direction, it is implied in satellite applications that nominally coincident beams point in directions closer than typical satellite spacings of, for example, 5° or 9° along the geostationary arc. In the present invention, “scanned beams” are those pointing away from the nominally coincident beams at angles of 5° or more away along the geostationary arc in satellite applications.  
         [0051]     For terrestrial microwave applications, the distinction between “nominally coincident” and “scanned” beams is similar, in that nominally coincident beams point in approximately the same direction to within a small angle defined for terrestrial applications such as a single tower or adjacent towers and scanned beams apply a larger angle such as between spaced apart towers.  
         [0052]     It may also be desirable to add fourth beam functionality to the antenna, where the fourth beam is scanned away from the three co-located main beams the antenna is ordinarily adapted for. This fourth beam may point to another satellite separated at some distance along the geostationary arc from the satellite(s) communicating with the VSAT via the three co-located beams. This separation could be, for example, 5° or 9° along the geostationary arc, with the desired fourth beam scanned roughly 5.5 or 9.9°, respectively, in the azimuth plane of the antenna. The fourth beam may be provided by the addition of an additional third feed  64  located next to the sub reflector  52 , as shown in  FIGS. 6-8  and  10 . The third feed  64  may be configured to illuminate the main reflector  50  in a standard single-offset reflector configuration. With the Gregorian optics used, rays clearing the third feed  64  are reflected away from it to illuminate the opposite side of the main reflector  50 .  
         [0053]     To avoid degrading performance of the co-located beam associated with the first radio frequency feed  60 , addition of a third feed  64  should not block the sub reflector  52 . A smaller third feed  64  could be used to minimize blockage, but at the expense of lower directivity for the feed and therefore much higher spillover loss for the scanned beam. The minimum scan angle available to a third feed  64  positioned adjacent the sub reflector  52  may be improved by shaping the sub reflector  52  to have an elevation dimension that is greater than an azimuth dimension, such as an oval shape as shown in  FIG. 7 . The oval rim sub reflector  52  has a larger dimension in the vertical plane than a circular-rim sub reflector  52  dimensioned for the same minimum scan angle. Similarly, a notched rim may be employed to provide clearance for the third feed  64 . Therefore, oval and notched sub reflector  52  rim designs, capture more energy from the feed to reduce sub reflector  52  spillover, improving efficiency relative to a circular rim sub reflector  52  with the same minimum scan angle.  
         [0054]     In another embodiment, the antenna  10  may be improved by further optimizing the oval shape of the sub reflector  52  into a notched and or distorted oval as shown for example in  FIG. 8 . Thereby, a higher gain feed may now be positioned for the, for example, 5.5° scan position without sub reflector  52  blockage—and without additional spillover loss.  
         [0055]     Further, as shown in  FIG. 9 , the notched and or distorted oval shape may be further optimized via the application of a numerically tabulated sub reflector rim function. This function may specify the feed angle at the sub reflector rim as a function of the feed angle phi in first feed  60  spherical co-ordinates to create a “notch” in the sub reflector, as shown in  FIG. 10 .  
         [0056]     Alternatively, the second feed  58  may be located such that any beam or beams radiated by the second feed  58  are also scanned away from the beam or nominally coincident beams radiated by the first feed  60 . Also, as shown in  FIGS. 12   a  and  12   b , a third feed  64  may be placed side by side proximate the second feed  58  position, oriented for scanned feed operation. To assist with scanned feeds of significantly large offsets, the main reflector may be formed with a toroidal or partially toroidal section.  
         [0057]     In further embodiments, as shown for example in  FIG. 11 , the fourth beam may be generated by a third feed  64  that may be located proximate the first feed  60 . In this configuration, the third feed  64  may be scanned away from the first feed  60  by between 2 and 8 degrees.  
         [0058]     Even if the antenna  10  is initially intended for only nominally coincident beams as described herein above, an oval and or notched shaped sub reflector  52  allows later cost effective addition of third feed  64  upgrades to an installed antenna  10 , for example if a satellite TV subscriber later decides they would like to upgrade their channel selection/services.  
         [0059]     Multiple third feed(s)  64  could be utilized, mounted at any of the described locations, to produce scanned beams at, for example, +5.5° azimuth scan and at −5.5° azimuth scan away from the three co-located beams. Similarly, multiple fourth feed(s)  64  may be mounted on the same side of the sub reflector  52  to acquire services from satellites stationed at +50 and +90 along the geostationary arc away from the satellite to which the three co-located beams are pointed.  
         [0060]     In the present embodiment(s), the invention is demonstrated as a multi-channel satellite communication system in the form of an Internet/Satellite TV system which receives satellite TV in a first frequency band such as the 12 GHz Band (Ku-Band) at the first feed. Outgoing Internet communications are transmitted in a second frequency band such as the 30 GHz Bans and received in a third frequency band such as the 20 GHz Band (KA Band) via the second feed. third feeds, if present, are envisioned as operating in the 12 Ghz Band for reception of additional satellite TV channels/services available on adjacent and or separate satellites. Although the embodiment demonstrated is described with respect to operation in 12, 20 and 30 GHz frequency bands, one skilled in the art will appreciate that the invention may be similarly applied to other frequency bands.  
         [0061]     The various embodiments of the invention create a multi-band antenna  10  with improved electrical performance and a compact form, without requiring additional measures such as FSS surfaces on the reflective surface(s) of the sub reflector  52  or the like. From the foregoing, it will be apparent that the present invention brings to the art a multiple band co-located antenna with improved performance that is compact, environmentally durable and has significant manufacturing and installation cost efficiencies.  
                                         Table of Parts                                10   antenna       50   main reflector       52   sub reflector       58   second radio frequency feed       59   waveguide launch       60   first radio frequency feed       62   aperture       64   third feed       B   focal region                  
 
         [0062]     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.  
         [0063]     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.