Patent Publication Number: US-6222499-B1

Title: Solderless, compliant multifunction RF feed for CLAS antenna systems

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a feed system for an antenna, and, more particularly, to a feed system used in connection with a low observable, multifunction, conformal, load bearing structure antenna on an aircraft that provides an RF feed for VHF/FM, VHF/AM, UHF and L-band frequencies. 
     2. Discussion of the Related Art 
     Modern manned and unmanned tactical military aircraft require radio communications over several frequency bandwidths and communication modes to support the communications, navigation and identification (CNI) functions necessary for operation of the aircraft. These radio frequency (RF) bandwidths generally include the VHF frequency modulation (FM) band (30-88 MHz), the VHF amplitude modulation (AM) band (118-174 MHz), UHF band (225-400 MHz), and L BAND (962-1213 MHz). The required L-band functions are JTIDS, MIDS, and TACAN. 
     To transmit and receive signals in these frequency bands, a suitable antenna system is required that is configured on the aircraft. Generally, multiple blade antennas are required for the CNI functions, including one for the VHF/FM frequency band, one for the VHF/AM frequency band and another one for the UHF frequency band. the available antenna installation sites on an aircraft may not support the number of antennas needed by the required CNI functions if each function requires its own antenna. Because modern tactical aircraft are usually low-observable aircraft, it is necessary that the antenna elements conform to the aircraft structure to minimize the radar cross-section (RCS) of the aircraft and that require a feed that connects separate conductive structures. 
     To overcome the requirements of multiple antennas to support the CNI functions, and to eliminate the need for blade antennas, U.S. Pat. No. 5,825,332 issued Oct. 20, 1998, titled “Multifunction Structurally Integrated VHF-UHF Aircraft Antenna System,” assigned to the assignee of this application and herein incorporated by reference, discloses an aircraft antenna that is totally integrated within the aircraft, and operates over a wide range of frequencies, including the VHF/FM, VHF/AM and UHF frequency bands. To operate in this manner, the antenna system uses an electrically conductive element that is part of the aircraft structure and an antenna element positioned and shaped to form a notch therebetween. The notch is generally uniform in width over part of its length and flares to a larger width over the remainder of its length. Broadband impedance matching electronics are provided to couple the antenna system to a transceiver to provide efficient transfer of energy to and from the antenna. 
     Other antenna systems have also been proposed that conforn with the existing aircraft structure. For example, U.S. patent application Ser. No. 09/178,356, filed Oct. 23, 1998, titled “A Conformal Load-Bearing Antenna System,” assigned to the assignee of this application, and herein incorporated by reference, discloses a conformal load bearing antenna structure (CLAS) that is configured within the skin of the aircraft so that the antenna element does not increase the RCS of the aircraft. The CLAS manufacturing processes allow the antenna elements to be integrated within a composite RF window that carries a load that would be carried by the replaced skin panel. The CLAS antenna elements also use part of the aircraft skin as the radiating element. 
     Antenna elements manufactured with the CLAS manufacturing process require a suitable RF feed that connects the antenna element to the matching electronics. U.S. patent application Ser. No. 09/233,361, titled “A Dual-Feed System for a Multifunction, Conformal, Load-Bearing Structure Excitation Antenna,” filed Jan. 19, 1999, assigned to the assignee of this application, and herein incorporated by reference, discloses a dual-feed system for providing a feed structure within the notch between the antenna element and the conductive aircraft structure of the type discussed in the &#39;332 patent. The feed structure includes two feed posts strategically positioned along the notch so that the feed posts provide impedance matching at the desirable frequencies. In one particular design, an aft feed post provides impedance matching at a feed point location for the UHF and VHF/FM bands, and a forward feed post provides impedance matching at a feed point location for VHF/AM frequencies. The feed posts can be made of any suitable conductive material, such as brass, gold, nickel, etc., and can have any suitable shape, such as cylindrical and conical shapes. The feed posts are connected to the antenna element and the aircraft structure by a solder connection. 
     The aircraft structure is subject to high vibration and temperatures during operation. Therefore, the feed connection must be able to withstand this environment. The CLAS antenna elements are actual load bearing parts of the aircraft, such as a vertical tail end cap or fin cap. Feed structures with a soldered or rigid mechanical connection would quickly fail. Additionally, it is necessary to provide a feed connection that provides impedance matching for all of the VHF/AM, VHF/FM, UHF and L-band frequencies. 
     What is needed is a compliant RF feed connection that satisfies the above requirements. It is therefore an object of the present invention to provide such an RF feed. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a compliant RF feed system is disclosed that provides impedance matching for VHF/AM, VHF/FM, UHF and L-band frequencies. The feed system includes a feed post electrically connected to an antenna element at one end and a matching circuit package at an opposite end, where the feed post extends across a non-conductive gap between the antenna element and a conductive aircraft component. The matching circuit package includes a compressible bellows mounted in the package, where the feed post contacts the bellows in a compression engagement to provide an electrical connection between the feed post and a RF matching circuit in the matching package. An impedance transformer is provided in the matching package to match the impedance of the bellows to the matching circuit. The bellows allows the electrical connection between the antenna element and the aircraft component to accommodate the vibrations during aircraft operation. 
     In order to accommodate L-band frequencies, a feedthrough is provided from an L-band antenna element to an L-band matching network in the package. The feedthrough extends through the feed post and the bellows, and is electrically isolated therefrom. 
     Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a three function compliant feed for a CLAS antenna system, according to an embodiment of the present invention; 
     FIG. 2 is an enlarged partial view of the antenna system shown in FIG. 1; 
     FIG. 3 is a perspective view of a bellows mounted within a matching circuit package for the compliant feed shown in FIG. 2; and 
     FIG. 4 is a cross-sectional view of a four function compliant feed for a CLAS antenna system, according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a compliant RF feed for a CLAS in an aircraft is merely exemplary in nature, and is no way intended to limit the invention or its applications or uses. 
     FIG. 1 is a cross-sectional view of a compliant antenna feed system  10  for use in connection with an antenna system of the type discussed in the &#39;332 patent referred to above, that is made by the CLAS manufacturing process. FIG. 2 is an enlarged partial view of the antenna system shown in FIG.  1 . The antenna feed system  10  includes a feed post  14  positioned across a notch  16  between an antenna excitation element  18  and a conductive aircraft structural component  20 . The feed post  14  is able to replace one of the feed posts in the feed structure discussed in the &#39;361 patent application referred to above. The notch  16  provides electrical insulation between the antenna element  18  and the structural component  20 , and can be any suitable non-conductive material, such as air or a phenolic honeycomb, as would be understood by those skilled in the art. The feed post  14  is positioned at a suitable location along the length of the notch  16  to provide impedance matching for the desirable frequency band, such as the UHF, VHF/AM or VHF/FM band. The complete feed system in this embodiment would include another feed post for the other UHF or VHF band. 
     The feed structure  10  directs the signals received by the antenna element  18  to matching electronics  24  within a matching circuit package  22 . Additionally, the feed post  14  directs signals to be transmitted by the antenna element  18  received from the matching electronics  24 . The matching electronics  24  converts the feed point impedance to the transceiver output impedance, usually 50 ohms, and the transceiver output impedance to the feed point impedance. The aircraft structural component  20  can be a tail, fin, wing, stabilizer, or other aircraft structure of the aircraft. Both the antenna element  18  and the aircraft component  20  are excited as a result of currents flowing in these structures from transmitted signals from the transceiver (not shown) or electromagnetic signals received from the air. These currents create electromagnetic fields across the notch  16 . The notch  16  radiates generally omnidirectionally, and both the antenna element  18  and the component  20  radiate as a result of the currents flowing in these structures. 
     The feed post  14  includes a threaded protrusion  28  extending from a head  30  of the post  14 . The feed post  14  also includes a cylindrical portion  36  extending across the notch  16  to an opposite end  34  of the feed post  14 . The shape of the end  34  allows the threaded protrusion  28  of the feed post  14  to be readily threaded into an embedded feed plate  32  within the antenna element  18  by a wrench. The feed plate  32  allows the antenna element  18  to first be manufactured by a CLAS process, so that the feed post  14  can be attached to the element  18  thereafter to provide ease of manufacture. The feed post  14  is made of any suitable conductive material, as would be well understood by those skilled in the art. 
     In accordance with the teachings of the present invention, a conductive bellows  38  is press fit into an opening  26  in a non-conductive bellows interface element  46  mounted in the matching package  22 . FIG. 3 is a perspective view of the bellows  38  in connection with the matching package  22  separated from the feed system  10 . The bellows  38  includes a plurality of ridges  42  and a central cylindrical opening  50 . The bellows  38  makes electrical contact with a conductive impedance transformer  40  in the matching package  22 . The impedance transformer  40  includes steps  52 , as shown, to provide impedance matching between the bellows  38  and a conductive trace  44  connected to the matching electronics  24 . The transformer  40  is positioned in a nonconductive transformer interface element  48  to electrically isolate the impedance transformer  40  from the conductive portion of the aircraft. The elements  46  and  48  can be made of any suitable non-conductive material, such as Teflon. Bolts  56  and  58  are threaded into holes  60  and  62 , respectively, to hold the interface elements  46  and  48  and the impedance transformer  40  within the package  22 . 
     When the feed structure  10  is assembled, the end  34  of the feed post  14  is pressed against the bellows  38 , causing it to compress, and make an electrical contact thereto. FIG. 3 shows the bellows  38  in an uncompressed format prior to the combination of the feed post  14  and the antenna element  18  being assembled thereto. The feed post  14  and the bellows  38  complete the electrical contact between the antenna element  18  and the matching network  24  in a non-soldered engagement. 
     While in flight, the aircraft will undergo vibrations and the like which will go through the aircraft component  20  and into the post  14 . The configuration of the bellows  38  and the connection of the feed post  14  to the bellows  38  allows the feed post  14  and bellows  38  to move relative to each other, so that they do not fail as a result of the vibrations. 
     The feed system  10  discussed so far provides a feed for the frequency bands in the UHF, VHF/AM or VHF/FM bandwidths. The feed system  10  can also be used in connection with a feed in the L-band frequency range. FIG. 4 is a cross-sectional view of a feed structure  70  that satisfies this purpose. The antenna feed system  70  is similar to the antenna feed system  10 , and therefore like components will be identified with the same reference numeral. The feed system  70  provides a feed for an L-band antenna element  72 . The antenna element  72  can be configured in any suitable location within the aircraft structure relative to the element  18 , as would be well understood to those skilled in the art. A coaxial feedthrough  74  is connected to the feed element  72  and extends through suitable aligned openings in the protrusion  28 , the post  14 , the bellows  38 , the impedance transformer  40 , and the spacer element  48 . The feedthrough  74  is then electrically connected to an L-band impedance matching circuit  76 . In the embodiments discussed herein, the bellows  38  includes the opening  50  for accommodating the coaxial cable associated with the L-band feedthrough  74 . In an alternate embodiment, the bellows  38  can be a solid member that provides the necessary resistance and compression for electrically connecting to the feed post  14  in a desirable manner. The coaxial feedthrough  74  is electrically isolated from these conductive components by an insulator layer  80 . A ground connection  82  is provided for the outer conductor of the feedthrough  74 . Therefore, the feed system  70  can be used for both UHF and L-band connections, or VHF and L-band feed connections. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims: