Abstract:
An antenna for a compact satellite terminal. Antenna is a rigid parabolic structure of metal matrix composite capable of disassembly into segments affording a high degree of portability such as for man-packable satellite terminals and the like. A shallow feed horn assembly is joined to an orthomode transducer by a common hub, the hub also serving as the attachment point for a plurality of antenna segments, where a quick release means joins the segments to the hub. The feed horn, hub, orthomode transducer and antenna segments are designed for extremely compact stowability in a variety of applications.

Description:
PRIORITY CLAIM UNDER 35 U.S.C. §119(e) 
     This patent application claims the priority benefit of the filing date of provisional application Ser. No. 61/123,565, having been filed in the United States Patent and Trademark Office on Mar. 25, 2008 and now incorporated by reference herein. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalty thereon. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     This invention relates generally to the field of ground-based satellite communications equipment. More specifically the present invention relates to lightweight, portable, ground satellite communications terminals and stowable antenna structures to be used therewith. 
     2. Background 
     Communication by satellite is essential in remote locations of the world where terrestrial communications networks do not exist. Moreover, when moving about remote locations, satellite communications equipment must be mobile. Smaller, lighter satellite communications equipment affords greater mobility. Satellite communications in the higher frequency bands such as X, K and Ku require a minimum transmit and receive directed gain that is much higher than the non-directional gain of handheld satellite transceivers in the L- band. Therefore, to achieve the necessary directional gain, mobile satellite transceivers in the X, K and Ku bands require directional antenna systems generally comprising parabolically shaped reflecting surfaces. 
     Generally speaking, while electronics have become smaller and more efficient over the years, minimum antenna size remains bounded by the physics of electromagnetic radiation and the need for larger physical antenna size (i.e., aperture) to achieve a higher directed gain. It is not uncommon for antenna systems to comprise the least transportable component of modern portable satellite transceivers. 
     Efforts have been made to achieve a higher degree of transportability of satellite communications antenna systems. Early efforts employed umbrella-like unfolding antennas comprising Mylar material stretched over lightweight metallic frameworks. Other efforts incorporated parabolic-shaped recesses into the satellite terminal enclosures themselves. Many others efforts involved assembling sections of flat or semi-flat panels into mosaics to achieve a larger reflecting surface. While some of these designs may indeed increase directed gain at low satellite frequencies such as in the L-band, they provide inherently unacceptable directive gain at X, K and Ku bands. The design constraint which prior attempts face at higher frequencies is their inability to provide true parabolic reflecting surfaces necessary for narrow, focused (i.e., directed) beamwidths required not only for gain, but also for discriminating among adjacent geostationary satellites position in equatorial orbits. 
     3. The Prior Art 
     U.S. Patent Application Publication 2005/0212715 A1 to Saunders (hereinafter, Saunders) attempts to overcome the effects of rain fade by increasing the physical reflecting surface of a fixed antenna reflector by adding extensions around its periphery. The invention in Saunders, however, provides no means for compacting the fixed portion of the antenna reflector. Therefore, the invention in Saunders would not solve portability issues in transportable satellite communications terminals. 
     U.S. Pat. No. 5,019,833 to Nonaka et al discloses a parabolic antenna for television signal reception that affords a degree of transportability by virtue of having its means for positioning incorporated into the rear of the parabolic antenna where both comprise a common assembly joined by hinges. The problem not solved by Nonaka is reducing the transportable size of the parabolic antenna reflector. 
     U.S. Pat. No. 4,862,190 to Palmer et al discloses a deployable parabolic dish antenna where alternating sections of triangular and rectangular reflector surfaces are connected about the periphery of a stationary main reflector surface by hinges. Upon deployment, the triangular and rectangular sections rotate outward to form a larger resultant parabolic reflecting surface centered about the main reflector. The problem with this approach is that the triangular and rectangular sections, when not deployed, are positioned perpendicularly to the main reflector, resulting in the overall displaced volume of the antenna structure to be as great when stowed as when deployed. 
     U.S. Pat. No. 3,618,101 to Emde et al discloses a collapsible parabolic antenna for use on-board satellites. The antenna in Emde employs at least one fixed semicircular segment and at least one movable semicircular segment which, when rotated into position, provide a 360 degree reflecting surface. Because this antenna is designed for automatic deployment, the movable segments remain connected to the primary axis of the antenna structure at all times. The result, therefore, is that the stowed volume of the antenna is less, but not significantly less, than the deployed volume of the antenna. 
     U.S. Pat. No. 5,554,999 to Gupta et al discloses a collapsible flat antenna that provides phasing so as to simulate the antenna radiation characteristics of a parabolic dish reflector antenna. Phasing is accomplished by a plurality of reactive elements responsive to different frequencies within the antenna&#39;s bandwidth. The antenna disclosed in Gupta is intended to be a flexible structure allowing stowage by collapsible folding. One limitation of this approach is that phased antennas yield optimum radiation patterns at the specific frequency their reactive elements are designed for, whereas parabolic reflecting antennas exhibit optimum radiation patterns across frequency bands. Another limitation of a flexible structure is the difficulty in physically supporting it and maintaining its orientation. 
     U.S. Patent Application Publication 2004/0196207 A1 to Schefter et al discloses a collapsible antenna for portable satellite terminals which employs a reflector assembly comprising a plurality of panels which may be connected to each other for deployment and disconnected for stowage. Connection of each panel to the other is by means of quarter turn quick release cam nuts. A separate boom arm is used to mount the feed assembly at a focused distance from the reflector assembly. Schefter discloses that the reflector is comprised of four (4) panels. However, the antenna design suffers from large overall size because, it is not a true parabolic structure and because it is a truncated structure, the feed focus is necessarily deep, as opposed to shallow feed focuses with non-truncated parabolic structures. 
     U.S. Pat. No. 5,061,945 to Hull et al discloses a lightweight, collapsible satellite communications dish antenna having a plurality of identical pre-shaped sectors joined at their apex which can be stowed by rotating all of the sectors about their apex so as to result in their lying substantially atop each other. The invention in Hull inherently requires that the sectors be made of a highly flexible material so as to be capable of being drawn into a curvature shape upon deployment while also capable of returning to a flat shape for stowage. Hull does not disclose any cognizable means for mounting a signal feed means at the dish focus. 
     What the prior art fails to provide and what is needed, therefore, is an antenna which (1.) is extremely compactable when stowed and (2.) still retains true parabolic reflector properties when deployed. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for terrestrial-based satellite communications which provides improved transportability over the prior art. 
     It is therefore an object of the present invention to provide an antenna for a compact satellite terminal which exhibits characteristic parabolic radiation patterns at all angles. 
     It is a further object of the present invention to provide an antenna for a compact satellite terminal which utilizes a shallow, compact feed to reduce both its deployed and stowed volume. 
     It is still a further object of the present invention to provide an antenna for a compact satellite terminal which requires no tools for assembly, disassembly, and to change polarization. 
     It is yet still a further object of the present invention to provide an antenna meeting all of the above objectives yet being adaptable to a variety of satellite terminals operating at a variety of different frequencies. 
     An additional object of the present invention is to overcome a lack in the prior art of portable satellite antenna designs, some of which are compact but offer neither true parabolic characteristics nor ruggedness. 
     Briefly stated, the present invention achieves these and other objects by providing an antenna for a compact satellite terminal. The antenna is a rigid parabolic structure of metal matrix composite capable of disassembly into segments affording a high degree of portability such as for man-packable satellite terminals and the like. A shallow feed horn assembly is joined to an orthomode transducer by a common hub, the hub also serving as the attachment point for a plurality of antenna segments, where a quick release means joins the segments to the hub. The feed horn, hub, orthomode transducer and antenna segments are designed for extremely compact stowability in a variety of applications. 
     In a fundamental embodiment of the present invention, an antenna for a compact satellite terminal has a hub with a input side, an output side, and a plurality of slots equidistantly located on the periphery of the hub where a plurality of antenna segments being equal in number to said plurality of the slots are removably attached into and where a feed horn and an orthomode transducer are removably attached to the input and the output sides of the hub. 
     Still according to a fundamental embodiment of the present invention, an antenna for a compact satellite terminal, where each of the plurality of antenna segments are removably attached into each of the like plurality of said slots by means of a tab, one end of which forms an anchor being fastened to each antenna segment and the other end of which forms a tenon-like projection and also by detents located on at least one surface of each tenon-like projection into which spring actuated balls located inside at least one surface of said slot of said hub captively mate to secure each tab into its respective slot. 
     Still yet, according to a fundamental embodiment of the present invention, an antenna for a compact satellite terminal, where each of the plurality of antenna segments are conductive composite structures fabricated from a nickel nanostrand metal matrix composite material. 
     The above and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts the present invention as part of a very compact transportable satellite terminal. 
         FIG. 2  depicts an exploded view of the present invention. 
         FIG. 3  depicts a view of how the present invention is assembled. 
         FIG. 4  depicts a view of how the individual arts of the present invention assemble. 
         FIG. 5  depicts a rear view of the assembled present invention. 
         FIG. 6  depicts a view of the hub, feed and OMT of assembled present invention. 
         FIG. 7  depicts a cutaway view of how the hub mechanically captivates the feed and the OMT of present invention. 
         FIG. 8  depicts a view of the present invention in a stowed configuration. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention describes an antenna design that collapses into an optimally-dense package for stowage and carrying, and which can be easily set up and taken down. Such an antenna would find application in very compact and highly portable satellite communications terminals. The invention incorporates a number of unique features which collectively result in a very lightweight and compact system which can be configured to support full-duplex communications over satellites in earth orbit. These satellites are in most cases envisioned to be in geosynchronous orbit, with the satellite terminal antenna in a fixed orientation during the communications session. However, transportable terminal designs should be readily modifiable to provide for active tracking of the antenna for use on a moving platform, or with non geosynchronous satellites. Those skilled in the art would appreciate a practical implementation of the invention as readily applicable to a backpack transportable system weighing at 20 lbs or less and having rough stowed package dimensions of about one cubic foot or less (i.e., 12 inches by 12 inches by 12 inches). Such a terminal is depicted in  FIG. 1 . Typical operating frequency bands would be around 7 Ghz to 40 Ghz (e.g. X-band, Ku band, Ka band). The present invention is by no means limited to these frequency bands, however. It should be noted that the present invention is entirely scalable so as to provide whatever aperture gain is required at any frequency. 
     Still referring to  FIG. 1 , the present invention provides compact, transportable terminal aperture gain by a novel parabolic antenna having a single reflecting surface  10  (no secondary reflector) to a shallow feed  20  positioned at the reflector focus. For supporting significant full-duplex data rates of around 512 Kbs to 2 Mbs, and to maintain reasonably narrow beamwidths to minimize potential intersatellite interference, aperture sizes of 18 to 30 inches in diameter would be required. However, apertures of any size whatsoever are within the scope of this invention. 
     Referring to  FIG. 2 , because a single piece parabolic reflector would be too large to conveniently transport, the present invention provides a means to assemble a parabolic reflector from a plurality of trapezoidal shaped segments which plug into a hub with the antenna feed at its center. One skilled in the art would note that while a smaller or larger number of segments could be employed without limiting the utility if the invention, six segments have been chosen for a particular embodiment of the invention intended to fit within a terminal package for stowage and transport having an approximate 12 inch by 12 inch footprint. An exploded view shows the segmentation of the antenna reflector  10  with feed  20  in center, antenna hub block  30 , composite antenna segments  10  with integral end tabs  40  for attachment to the hub  30 . 
     Referring to  FIG. 3 , depicting a partially assembled view of the antenna assembly with feed  30  in center. The reflector segments  10  are preferably made from a single shell (about 0.050″ thick) of a high electrical conductivity graphite composite material which has an attached (or molded-in) end connector tab  40  by which it is pressed into the hub. Suitable composite materials for the reflector segments  10  include metal matrix composites such as but not limited to nickel nanostrand. The tab receiver slot (not shown, see  50 ,  FIG. 4 ) in the hub  30  preferably incorporates a small spring plunger (not shown, see  60 ,  FIG. 5 ) that provides for a snap fit to hold the tab in place, while allowing it to be easily pulled free with a light tug. Alternatively, the spring plunger may be located within the hub slot and a corresponding detent located on the tab. Additionally, the tab may be tapered and or lengthened in more than one dimension to mate with a similar taper and length in the hub slot (not shown, see  50 ,  FIG. 4 ) for greater retention. Other means for retention of the tab into the slot are clearly within the scope of this invention. 
     Referring to  FIG. 4 , a close-up view of antenna hub and pedal shows how tab  40  at end of petal interlocks with slot  50  in hub  30 . Note detent (not shown) in tab  40  for spring-loaded pin  60  inside hub  30  slot  50 . Also note that hub  30  slots  50  and tabs  40  are slightly tapered on three sides for ease of assembly. Segments  10  may also incorporate a spline  140  along their edges that would interlock with corresponding grooves or slots (not shown) on the edges of adjacent segments for adding rigidity to the antenna when assembled. Other modes of interlocking the segments to accomplish the same certainly exist. In an embodiment of the present invention, the metal matrix composite composition of the segments  10  provides sufficient rigidity without the need for splines  140 . 
     Referring to  FIG. 5  shows a rear view of the segmented antenna  10  of the present invention as fully assembled. The orthomode transducer (hereinafter OMT)  70  is visible from this rear view. 
     Referring to  FIG. 6  shows the antenna hub  30  relationship to the feed horn  20  OMT  70  as an assembly. Signals feed into and out of the OMT  70  into and out of a terminal electronics box (not shown, see  80 ,  FIGS. 1 and 8 ). The feed horn  20  and the OMT  70  rotate radially with respect to each other and are held together by the antenna hub  30 . 
       FIG. 7  depicts the mechanical relationship between the feed horn  20 , antenna hub  30 , and OMT  70 . In the cutaway view the feed assembly shows how hub  30  with attached cap  100  captivates flanges on the ends of both the feed horn  20  and OMT  70 . In this fashion, the feed horn  20 , hub  30  and OMT  70  are joined as an assembly. The OMT  70  is attached to elevation support arms (not shown, see  90 ,  FIG. 1 ). 
     Still referring to  FIG. 7 , the antenna feed comprises two sections, an OMT  70  portion behind the reflector  10 , and the feed horn  20  section at the front surface of the reflector  10 . The feed horn  20  can be hand rotated relative to the OMT  70 , which is fixed to the hub  30 , for the purpose of changing polarization. Detents (not shown) on the OMT  70  mating surface combined with a small spring plunger (not shown) on the feed horn  20  mating surface serve to index the position of each polarization (eg. LHCP, RHCP). The feed horn side can thus be rotated along the radial axis (about + or −90 degrees) to change polarization. Circular polarizations are also with the scope of the present invention. Additionally, in one embodiment of the invention, an internal pin (not shown) located in a 90 degree arced groove or slot (not shown) is employed to index the polarization by limiting the rotation of the feed horn  20  relative to the OMT  70  to 90 degrees. A slight amount of friction between the feed horn  20  and the cap  100  eliminates slop and backlash and is provided by an elastomeric ring functioning as a bearing surface  130 . 
     Referring to  FIG. 8 , the antenna of the present invention is depicted with an exemplary compact satellite terminal in the stowed position. The feed horn, hub, and OMT assembly (hidden from view), with the antenna reflector segments  10  detached, rotates backward into a position in which the feed horn is aligned parallel with the feed support arms. From here the elevation arms  90  can be lowered into a position of zero degrees with the horizon with the feed flat with the top of the box. The antenna reflector segments  10  are then stacked  120  on top of the feed with the curved edge over the protruding edge of the hub for maximum storage compactness. A cover typically would snap over the top of the box  80  for transport. 
     To deploy an exemplary compact satellite terminal incorporating the antenna of the present invention, the elevation arms  90  are raised to an angle where the feed and hub assembly can be rotated (around feed pivot point) into a position perpendicular to the elevation arms. A spring-loaded pin is employed to hold the feed and hub assembly in this position. The antenna reflector segments  10  are then snapped into place and the deployed configuration take the exemplary form of that depicted in  FIG. 1 . The pointing and signal acquisition process can typically now begin by first setting the elevation angle. 
     Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims.