Abstract:
The invention is a modular buoyant cable antenna that is towed on the surface of a body of water by a submerged underwater vehicle to allow communication coverage in an omnidirectional pattern that is also compatible with existing buoyant cable antenna deployment and retrieval systems. The antenna of the present invention comprises a floating cable having four identical antenna elements that are arranged in a cross configuration. The antenna uses the sea surface as a ground plane providing good antenna gain levels. The antenna is designed with an integrated impedance matching network to maximize radiation efficiency.

Description:
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 royalties thereon or therefore. 
    
    
     CROSS REFERENCE TO OTHER RELATED APPLICATIONS 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to antennas for use with an underwater vehicle, and more specifically to a buoyant cable antenna that is towed by a submarine to allow communication coverage in an omni-azimuthal pattern in the Very High Frequency (VHF) frequency bands (30 MHz to 300 MHz) and can also be scaled to cover other frequency bands. This invention is specifically designed to be compatible with existing buoyant cable antenna deployment and retrieval systems. 
     (2) Description of the Prior Art 
     Radio frequency communication for submerged underwater vehicles is currently limited to a buoyant cable antenna (BCA) system. Currently, this communication system only provides unidirectional signal coverage, which is of limited utility. The effectiveness of radio frequency communication for underwater vehicles would be greatly increased if omni-azimuthal signal coverage was possible throughout a desired frequency range to limit communication gaps and avoid the necessity of maneuvering an underwater vehicle to establish a good communication link. What is needed is a new and improved buoyant cable antenna that can provide omni-azimuthal signal coverage in a desired frequency range. 
     SUMMARY OF THE INVENTION 
     It is a general purpose and object of the present invention to provide omni-azimuthal signal coverage for submerged underwater vehicles through the use of a buoyant cable antenna. 
     It is a further object to use a matching technique to provide a very high frequency antenna that uses the sea surface as a ground plane with reasonable gain levels above the noise floor. 
     It is another object of the invention to provide a modular and tunable mechanism to match the impedance of a four element buoyant cable antenna joined to a transmission line of an existing buoyant cable antenna system. 
     It is another object of the invention to have one vertical component of the buoyant cable antenna perpendicular to the ocean surface at all times. 
     These objects are accomplished through the use of a modular buoyant cable antenna with a vertical antenna component that eliminates signal null areas. The antenna of the present invention comprises a floating cable having four identical antenna elements that are arranged in a cross configuration where two of the four elements are directly aligned at one hundred eighty degrees in a first spatial plane and the other two elements are directly aligned at one hundred eighty degrees in a second spatial plane parallel to the first spatial plane such that each element is spaced ninety degrees apart from an adjacent element in the parallel spatial plane. The antenna elements are attached to and protrude from the floating cable. While floating on the water surface, the antenna may rotate freely with minimal signal loss with one antenna element always extended above and perpendicular to the water&#39;s surface. Omni-azimuthal coverage is achieved by the vertical posture of the antenna elements. The modular buoyant cable antenna of the present invention is specifically designed for compatible use with existing systems onboard underwater vehicles. As such the dimensions of the antenna (particularly the cross-sectional diameter) and the connectors comply with size and connection standards of existing systems. The antenna employs a tunable impedance matching network in a modular chassis to decrease the reflections and increase the radiation properties between the transmission line and the antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon reference to the following description of the preferred embodiments and to the drawings, wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
         FIG. 1  illustrates the exterior structure of the modular buoyant cable antenna of the present invention; 
         FIG. 2  illustrates the connection of the antenna elements to the encapsulating cylindrical encasement; 
         FIG. 3 , illustrates the connection configuration of the ends of all four antenna elements electrically connected within the encasement at a single connection point; 
         FIG. 4 , illustrates the a tunable matching network; and 
         FIG. 5  illustrates the flexible shielded first section of the antenna elements. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the present invention teaches a modular buoyant cable antenna  10  that is attached to an underwater vehicle via a transmission line  12  and is aligned with an arrangement of existing buoyant cable antenna system components (not shown). The antenna  10  is electrically connected to transmission line  12  through connectors  13  that are compliant with the standards of the existing buoyant cable antenna system. The antenna  10  is towed by a submerged underwater vehicle as the antenna  10  floats on the surface of the water. The antenna  10  is composed of four sections; an encapsulating cylindrical encasement  16 , a modular chassis  18  containing a tunable impedance matching network  19 , a buoyant section  17  comprising a cable made of polyethylene foam that provides the buoyancy in seawater, and four identical antenna elements  14  that are attached to and protrude from encasement  16 . 
     In a preferred embodiment, encasement  16  is made from a potting compound such as a thermo-setting plastic or a silicone rubber gel that is water tight, flexible, tear resistant and meets the tensile requirements for towing a buoyant cable antenna at specified speeds as well as deployment and retrieval by existing cable antenna systems. In a preferred embodiment the potting compound that comprises encasement  16  is the commercially available PR-1592 manufactured by PPG Aerospace Inc. In a preferred embodiment encasement  16  encapsulates the electronic components (not shown) of the antenna  10 . In a preferred embodiment buoyant section  17  is a cable made of polyethylene foam that provides the buoyancy in seawater. Encasement  16  is joined to buoyant section  17  by means of a rubber coupler  15  that allows easier bonding between the potting compound of encasement  16  and the polyethylene of buoyant section  17 . In a preferred embodiment the diameter of encasement  16  and buoyant section  17  is 0.65 inch allowing them to conform to the required dimensions of existing cable antenna systems currently in use in underwater vehicles. 
     Referring to  FIG. 2 , the antenna elements  14  are held in place by the potting compound of encasement  16 . The four identical antenna elements  14  are arranged symmetrically around the encasement  16  in a cross configuration where two elements  14  are directly aligned at one hundred eighty degrees in a first spatial plane and the other two elements are directly aligned at one hundred eighty degrees in a second spatial plane parallel to the first spatial plane such that each element  14  is perpendicular to an adjacent element  14  in the parallel spatial plane. In operation, at least one element  14  is extended vertically above and perpendicular to the water surface when the antenna  10  is deployed regardless of rotations even as the antenna  10  moves along the surface of the water. 
     Each antenna element  14  is fabricated in sections from different materials. The first section  22  that is attached to the encasement  16  is fabricated using the alloy Nitinol, which has the physical property of high elasticity. The second section  24  is fabricated using carbon fiber, which has the physical properties of rigidity, corrosion resistance and sufficient electrical conductivity as needed for radio frequency communication. In a preferred embodiment the first section  22  protrude one inch from the encasement and the second section  24  is a quarter wave in length at a desired operational frequency plus five extra inches. The two sections  22  and  24  are joined through a mechanical and electrical crimp connection  26 . Whereas the carbon fiber increases stiffness for improved towing posture, the Nitinol allows the four antenna elements  14  to bend and fold against the sides of the encasement  16  when the antenna is not deployed. When bent and folded against the encasement  16 , the four antenna elements  14  are nested in machined grooves  30  in the encasement  16  such that the overall cross-sectional diameter of the modular section does not exceed the diameter of the encasement  16 . In a preferred embodiment, the diameter of the encasement  16  is 0.65 inch allowing it to conform to the required dimensions of existing buoyant cable antenna systems. 
     Referring to  FIG. 3 , the ends of all four antenna elements  14  are electrically connected within the encasement  16  at a single connection point  40  through transmission lines  31 . The connection configuration is illustrated in  FIG. 3 . Connecting the elements  14  facilitates impedance matching the antenna, a necessary process to avoid energy reflection, and to promote more efficient antenna radiation. 
     Referring to  FIG. 4 , there is illustrated an exemplary tunable matching network  42  for impedance matching the antenna  10 . The components of the matching network  42  as illustrated represents a single embodiment using a capacitor C and inductor L, however, other embodiments may also be suitable depending on the impedance of the transmission line  12 . Matching network  42  is electrically connected to the elements  14  after connection point  40 . The implementation of matching network  42  within antenna  10  is intended to match the impedance of the elements  14  with the impedance of the transmission line  12  in order to reduce the reflections between the transmission line  12  and the antenna  10 . 
     Matching network  42  is designed to allow modification/substitution of its internal electrical components in order to tune the antenna  10  to a specific frequency. In a preferred embodiment, the network  42  is designed on a removable circuit board  44  and placed in a modular chassis  18  that is then placed in a water proof housing  48  that is joined to encasement  16  and buoyant section  17 . 
     As illustrated in  FIG. 5 , for each antenna element  14 , the first section  22  is shielded. In a preferred embodiment the first section  22  is six inches long. The Nitinol  28  is covered by a layer  34  of insulating material. Disposed over the layer  34  of insulating material is a braid  36  of conductive material. The braid  36  is covered by an outer layer  38  of insulating material. The braid  36  is electrically connected to the ground in the antenna system (not shown). The arrangement of layer  34  covered by braid  36  covered by outer layer  38  combines to create a coaxial cable for the first section  22  of each antenna element  14 . The coaxial cable design provides shielding to protect the antenna  10  from signal dropout, which can occur when the plane of the sea water rises on the antenna elements of an unshielded antenna. 
     The advantages of the present invention are that the antenna  10  allows communication coverage in an omni-azimuthal pattern. The modular buoyant cable antenna  10  is compatible with existing buoyant cable antenna systems due to its dimensions and its use of standard connectors. This allows users to quickly add, remove, or interchange the antenna  10  with existing buoyant cable antenna systems. An advantage of the integrated impedance matching network is that even if the antenna elements rotate while the antenna is deployed causing the elements to change position the matching network continues to perform its function so long as one element remains above the water surface. The simplicity of the impedance matching network allows modification of the components in order to tune the antenna to a wide range of frequencies. 
     In light of the above, it is therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.