Patent Publication Number: US-8125400-B2

Title: Compact antenna feed assembly and support arm with integrated waveguide

Description:
FIELD OF THE INVENTION 
     The invention relates to antenna feed and feed support arm assemblies, including those employed by single-offset antenna assemblies of microwave terminals. 
     BACKGROUND OF THE INVENTION 
     Portable communications systems that transmit high bit rate data have high performance demands. Such high performance systems include Satellite News Gathering (SNG) systems, systems for logging and transmitting data from remote exploration sites, and portable military communication systems. In order to achieve high performance while preventing undue interference to or from other systems, such communication systems generally employ an antenna with a suitably sized parabolic reflector. The most practical and least expensive option for such systems is a single-offset antenna in which the feed support arm (with the feed assembly at the top end) can be removed from the parabolic reflector to enhance portability. 
       FIG. 1  shows a conventional prior art portable satellite communications terminal unit. The unit has a base  106  that includes means for stabilizing the unit on a surface. The base also houses electronic components for processing incoming and outgoing communications signals. The distal end of an elongate support arm  104 , commonly referred to in the art as a “boom,” holds a feed horn  110 . Throughout this disclosure and the claims, “distal” refers to structures along the feed arm and support arm at, near, or toward the feed horn; “proximal” refers to structures along the feed arm and support arm at, near, or toward the reflector or base. 
     The support arm is attached by its proximal end to a parabolic reflector  107 , commonly referred to as a “dish.” The support arm is shown in  FIG. 1  attached to the top of reflector  107 . Although this is typical, it is not uncommon to have the support arm attached to the bottom of the reflector or at other points about the periphery of the reflector. 
     A feed assembly  101  includes the feed horn  110  aimed at the reflector to collect incoming (down-link) received signals reflected from the reflector and to direct outgoing (up-link) transmitted signals to the reflector. The feed assembly also includes an exposed flexible guide  108  for conducting the transmit signal to the feed horn. A receive line  109  conducts receive signals from the feed assembly to the processing circuitry. As shown, a low noise block (LNB) downconverter  102  is often integrated into the receive signal pathway.  FIG. 1  also shows a transmit amplifier  105  as typically attached to the back of the reflector  107 . 
     A transmit filter  103 A, when used, is often attached to the support arm as shown and runs generally parallel to the arm. Such a transmit filter is particularly important in cases where a high power transmit amplifier is used to meet the up-link requirements because high power transmit amplifiers typically produce a high amount of noise in the receive frequency band that passes through the receive filter. This noise interferes with the performance of the receiver unless preventive measures are taken. The transmit filter, if properly designed, will pass the transmit frequency band with minimum signal loss while suppressing the noise in the receive frequency band. However, in some of the lower frequency bands such as X-band and C-band, filters having sufficiently high performance are relatively large. Placing such a filter near the feed horn results in bulky, awkward structures that cause problems due to weight loading of the support arm and possibly wind loading caused by a large cross-sectional area. Partial obstruction of the signal radiated from the reflector may also occur. Thus the size of the transmit filter typically requires placing it alongside the feed support arm  104  with appropriate attachments to the arm. However, this arrangement significantly complicates assembly and disassembly of the unit in field conditions. 
       FIG. 2  shows a typical conventional feed assembly in more detail. As shown, the feed assembly typically includes a feed horn  201 , a polarizer  202  (in cases of circular polarization), an ortho-mode transducer (OMT)  203 , and LNB  102  with an associated receive filter  204 . In some cases the receive filter employed is too large to be incorporated into the feed assembly and, together with the LNB  102 , is placed outside the feed assembly. 
     The OMT separates vertically and horizontally polarized signals in the case of linear polarization. The two signals are physically accessed at two waveguide flanges oriented in different directions, as discussed below. 
     For circular polarization, either a polarizer  202  is placed between the feed horn  201  and the OMT  203 , as shown, or a polarizer incorporating the OMT function is used. The latter category is well represented by septum polarizers. A number of patents can be found for various types of septum polarizers, such as U.S. Pat. No. 6,661,390 to Gau et al.; U.S. Pat. No. 6,507,323 to West et al.; U.S. Pat. No. 6,118,412 to Chen, and U.S. Pat. No. 6,724,277 to Holden et al. 
       FIG. 3  shows a typical septum polarizer  301  in cross-section. Feed horn  302  is connected to the distal end  301 A of the septum polarizer. At the proximal end  301 B of the septum polarizer are two ports. For instance, port  303  carries the linearly polarized transmit signal  304 , which is gradually converted into a left-hand circularly polarized signal as it progresses along the septum to the distal end  301 A. Similarly, a right-hand circularly polarized signal entering the distal end of the polarizer is gradually converted along the septum into a linearly polarized receive signal  306  emerging at port  305 . Thus, septum  307  converts circular to linear polarization (or vice versa) and separates the transmit and receive signals at the proximal end, hence the name septum polarizer. For the purpose of comprehending the present invention it is important to note that the septum of the prior art septum polarizer is limited just to the polarizer, because the two signals diverge at the proximal end of the septum polarizer into separate waveguides  308 ,  309 , the axes of which often subtend an angle of 180°, as shown in the figure. 
     As a general rule, the two ports of a septum polarizer are oriented in different directions, usually opposite each other as shown in  FIG. 3 . While this conventional design is convenient for physical separation of transmit and receive components, it also contributes to a bulky feed assembly in antennas, particularly those used for the lower microwave bands such as X-band and C-band. 
     As noted above, the prior art devices have a number of disadvantages and problems, particularly with respect to portable units used in the field. Many of these disadvantages and problems are related to the fact that waveguides are handled separately. As a result the feed assemblies have exposed waveguide adapters, waveguide filters, receive-lines, and bulky opposing polarizer ports. These exposed structures on the end of the support arm produce significant weight and wind loading on the arm. In addition, external transmit filters attached to the support arm increase the complexity and time of assembly and disassembly and increase the risk of damage should the unit be knocked over by wind or other forces. 
     Although all of these problems have not hitherto been resolved in a single device, there have been ad hoc attempts to resolve some of them. For instance, an attempt to improve the mechanics of the feed support arm is disclosed by Canadian patent 2,424,774 to Russell et al, which describes a portable satellite terminal for Ku-band operation in which the transmit filter is contained within a hollow support arm, rather than using the more conventional placement beside the arm. This arrangement is shown in  FIG. 2  in which the hollow arm  104  houses the alternative transmit filter  103 B. The support arm connects to either the reflector or the base by a flange or other suitable connector means. This allows the integrated support arm and filters to be attached or removed as a single unit. 
     Another example of attempts to integrate various functions is shown in U.S. Pat. No. 5,905,474 to Ngai et al. wherein a single, appropriately bent waveguide is used to provide both the signal connection to the feed assembly and mechanical support (i.e. feed support arm). However, Ngai does not disclose integrated waveguides and filters. In U.S. Pat. No. 5,708,447 to Kammer et al., two bent waveguides running in parallel are used in a similar way to achieve a similar result. This approach enables both a transmit and receive function with different polarizations or a dual receive only (or dual transmit only) function with different polarizations. But again, there is no disclosure of integration of the waveguides or the filters, nor of any means for integrating multiple waveguides into a single structure that also includes transmit and/or receive filters. 
     Finally, there have been attempts to place some of the RE front end electronics into the feed support arm; however, these attempts have so far been limited to small receive components such as mixer/amplifiers and either microstrip or coaxial filters. One example of this approach is U.S. Pat. No. 5,523,768 to Hemmie et al. uses a hollow arm containing a mixer/amplifier and a coaxial filter but no waveguide components. 
     In view of the functional and structural limitations of the present art, what is needed is a rugged, high performance, high speed portable communications system for transmission and reception of data and/or video communications in which the components of the feed assembly and its support arm are unobtrusively integrated into a single streamlined structure that is free of exposed waveguides and filters and that minimizes weight and wind loading to the support arm. 
     SUMMARY OF THE INVENTION 
     This invention is a novel, multiple-integrated feed assembly and support arm of the type used by, for instance, single-offset parabolic antennas. The feed assembly includes a waveguide integrator (WGI), which combines two or more waveform pathways into a single, integrated waveguide structure. The WGI may be, for instance, an OMT or a septum polarizer modified for parallel arrangement of transmit and receive ports. The WGI has a transition portion for effectuating the transition of two or more waveform pathways into parallel waveguides integrated into a single structure, such as a waveguide or waveguide adapter or support arm having an internal separating wall or partition. Preferably the ports and the waveguide structure have square cross-sections. A flange may be used for mating the WGI and the integrated waveguide structure. 
     Individual WGI ports may be functionally continuous with transmit and receive filters joined together in parallel to also form a square-profile structure that serves as a feed support arm. In such embodiments, the receive filter terminates in an LNB while the transmit filter connects to a transmitter through a connector incorporating a waveguide flange at the base or at the bottom of the reflector. Alternatively, a specially designed power amplifier is integrated into the support arm and communicates with the rest of the transmitter circuitry housed behind the reflector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The features and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a prior art portable satellite communications terminal using a conventional antenna design, discussed above. 
         FIG. 2  is a side elevation of a prior art feed assembly, discussed above. 
         FIG. 3  is a cut-away view of a prior art feed horn and septum polarizer, discussed above. 
         FIG. 4  is a cut-away view of a waveguide integrator (WGI) in the form of a modified septum polarizer. 
         FIG. 5  is a perspective drawing of a feed assembly comprising a WGI in the form of a modified septum polarizer. 
         FIG. 6  is a top elevation of an antenna incorporating elements of the invention. 
         FIG. 7  is a perspective view of a WGI in the form of an OMT with parallel ports. 
         FIG. 8  is a top elevation of an antenna showing the integration of a waveguide power amplifier into the support arm structure. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 4  shows a polarizer  401  acting as a waveguide integrator (WGI), which WGI serves the function of integrating at least two waveform pathways into a single structure, as described herein. A WGI is defined as any structure, modification, or device that performs this integration function. In this embodiment the WGI is a modified septum polarizer that integrates transmit and receive pathways to and from a feed horn  402 . At its distal end  401 A, the septum polarizer is attached to and in communication with the feed horn. At its proximal end  401 B, the septum polarizer has two ports  403 ,  404 , which are arranged in parallel, in contrast to the typical prior art arrangement of having the ports mutually opposed and linear, as shown in  FIG. 3 . The two parallel ports of  FIG. 4  form a square-profile interface equipped with a square flange  405  that allows each of the ports to communicate with one of the two waveguides (not shown) that are integrated into a single structure such as a waveguide adapter proximal to the septum polarizer, as shown in  FIG. 5  and disclosed below. 
     A separator or partition  406  called a “septum” separates the two parallel transmit  410  and receive  409  spaces in which conversion from linear to circular or vice versa occurs as the signals travel along the septum. Arrows indicate transmit  407  and receive  408  signals, which are kept separate by the partition. 
       FIG. 5  shows an embodiment of the invention employing a streamlined, in-line arrangement of all parts of the feed assembly and boom that is possible as a result of using a WGI to integrate the waveguides into a single structure. In this embodiment the WGI is a modified septum polarizer  501  of the type, for instance, disclosed above in relation to  FIG. 4 . The embodiment shown in  FIG. 5  is for circular polarization. 
     The modified septum polarizer  501  is oriented toward and communicates with the feed horn  502  by means of a connector that connects the distal end of the WGI to the feed horn. In the present embodiment, this connector includes mating flanges  511  and  512 . The septum polarizer has a septum or partition  514 , which separates the signals within the polarizer. At its proximal end, the septum polarizer has at least two parallel ports, which communicate with the distal end  505 B of waveguide adapter  505  by means of a connector. In the present embodiment, this connector includes square flanges  503  (on the polarizer side) and  504  (on the adapter side). The proximal end  505 A of waveguide adapter  505  is connected to the distal end of support arm  510  by means of a connector, such as circular mating-flanges  506  and  507 . 
     This waveguide adapter differs from prior art adapters in that its rigidity is enhanced by a square cross-section, and it encompasses two or more internal waveguides (typically, transmit and receive) separated by an internal partition  515  that runs from the distal end to the proximal end of the waveguide adapter. This is possible because the septum polarizer acts as a WGI to integrate the two waveguides into the unitary structure of the waveguide adapter. Although only two waveguides are shown in the drawing, after reading this disclosure the advantages and means of adapting the device to accommodate multiple waveguides will become obvious to those skilled in the art. 
     Preferably support arm  510 , like waveguide adapter  505 , has a square-profile. The support arm may house two or more internal waveguides separated by an internal partition  513 , which internal partition is functionally a continuation of partition  515  and partition  514 , thereby producing two waveguides that are continuous from the distal end of the WGI to the proximal end of the support arm. Alternatively, support arm  510  may house transmit filter  509  and receive filter  516 . The mating flanges  507  and  506  contain corresponding waveguide flanges internally (not shown) for maintaining functional continuity of the transmit and receive filters or the waveguides in support arm  510  with the waveguides in the adapter  505 . 
       FIG. 6  shows a top view of the antenna, including support arm  610 , which is connected to the bottom portion of reflector  602  by means of flange  601 , which provides functional continuity between the transmit filter housed within support arm  610  and components of transmitter  603  on the back of the reflector. 
     Thus, although the antenna components may be assembled as one piece without connectors depending on the application and the specifications, if connectors are used, they are of a type that maintain the continuity and separation of the waveguides. 
     Also shown in  FIG. 6  is LNB  605 , which is in communication with the receive filter by means of waveguide bend  606 . The receive signal is output from the LNB processing circuitry by coaxial cable  604 . 
     It will be noted from  FIG. 6  that the distal portion of the feed arm assembly is clean and un-cluttered relative to the prior art. For instance, the LNB  605  and waveguide bend  606  are moved proximally and away from the exposed distal end of the boom to the more massive base, thereby providing greater protection for these elements and reduced load on the boom. These are additional advantages of integrating the waveguides into a single structure. 
       FIG. 7  shows an embodiment of the invention applicable to linearly polarized signals in which feed horn  715  combined with an OMT  701  as used for linear polarization. The OMT is modified as disclosed herein to act as a WGI, integrating two waveguides into a single structure. 
     OMT body  702  internally contains a circular waveguide  709  that has a side slot  710  to accommodate a first port  711 . The OMT also has a circular-to-rectangular transition  716  terminating in a rectangular end slot  712  to accommodate a second port  713 . The second port is continuous with waveguide  703  while the first port is continuous with waveguide  714  formed by bend  704 , twist  705  and bends  706  and  707 . The proximal ends of waveguide  703  and waveguide  714  are combined to form a square cross-section and they are connected by means of a square-profile flange  708  to the distal end of a waveguide adapter (not shown in  FIG. 7 ), which has a square-cross section that is complimentary to that of the combined ports. Thus, the modified OMT operates as a WGI by integrating the two waveguides into the single waveguide adaptor. 
       FIG. 8  shows a preferred embodiment of the invention that can be employed with either circular or linear polarization. A “waveguide style” power amplifier  801  is inserted between the end of the transmit filter in support arm  802  and flange  803 . In this position the power amplifier is effectively a continuation or extension of the support arm. With the power amplifier integrated into the support arm, the transmitter components behind the reflector, namely the block-up converter (BUC)  804 , can be reduced in size, thereby allowing incorporation of other electronics into its enclosure. The waveguide-style power amplifier as shown features the ability to spatially combine signals into several semiconductor chips, all housed within the waveguide. Amplifiers capable of being adapted in this way are now commercially available, such as the solid state power amplifiers (“SSPAs”) manufactured by Wavestream Corporation. 
     SUMMARY 
     The benefits of integrating waveguides, filters and other components of support arms and feed assemblies as disclosed and illustrated above include a streamlined, linear package that reduces the weight of the assembly; reduced wind loading on the distal components; increased stability of the antenna including stabilizing antenna “aim”, reducing the moment of the support arm by placing the heavy filters close to the reflector, thereby easing the stresses on the elevation adjustment/locking assembly for the reflector. With respect to portable antennas, these improvements enhance portability due to easy assembly/disassembly of the feed and support arm from the reflector as a whole unit. 
     While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various novel modifications of the illustrative embodiments, as well as other embodiments of the invention, that are within the scope of the following claims will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that any such modifications or embodiments fall within the scope of the claims and their equivalents.