Abstract:
An antenna element with a single-conductor antenna is provided within a removable core. The removable core is mounted within a hollow liner by sliding the core into the hollow liner. A pull cable may be attached to an end of the removable core to pull the core through the outboard end of the hollow liner. Due to reduced strain on the core as compared to the liner, the single-conductor antenna may be formed in segments interconnected with capacitors for adjusting the antenna gain to a maximum in the vicinity of a desired operating frequency or frequencies.

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 PATENT APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a buoyant cable antenna (BCA) suitable for submarine communications and, more particularly, to a reconfigurable buoyant cable antenna element for the (BCA) system. 
     2. Description of the Prior Art 
     The buoyant cable antenna is one of a host of antennas currently in use by submarines. It is used for wireless electronic communications while the submarine is submerged. 
     The prior art antenna element for a buoyant cable antenna comprises a core that consists of a cylindrical insulated single-wire with a cylindrical layer of insulation. A buoyancy layer/jacket is either injection molded or extruded in place onto the core to make the final structure a solid object. This arrangement makes it impossible to remove the core if the core becomes damaged or if a new kind of core is needed to meet a specific mission. This arrangement also makes it impossible to reconfigure the antenna for other applications or specific applications. 
     Current designs for buoyant cable antennas suffer from limited gain and bandwidth performance due to the resonant behavior of the antenna element. For example, it is not possible to insert circuit elements into the single-wire antenna to change its behavior due to the stress that is applied to the core during towing. 
     The following U.S. patents describe various prior art systems that may be related to the above and/or other buoyant cable antenna systems: 
     U.S. Pat. No. 5,745,436, issued Apr. 28, 1998, to Bittleston, discloses a semi-dry marine seismic streamer cable that consists of a number of connected streamer cable sections, which each comprise a mechanical jacket surrounding a hollow core enclosing seismic sensor and signal transfer means. Elongated axial stress elements for transmitting axial loads and a radial reinforcement member for relieving radial loads are provided in the jacket. The core is filled with a fluid or a fluid-saturated foam and the sensor means are mounted in the core by vibration isolating elements. 
     U.S. Pat. No. 6,426,464, issued Jul. 30, 2002, to Spellman et al, discloses a cable sectional assembly that houses an electrically concatenated series of electronic circuit modules and has electric hook-up media interspersed between the modules. Each run of interspersed hook-up media is surrounded by a flexible tubular member, which is oversized to allow movement of the hook-up media without restriction under flexing of the assembly. The assembly is a construction and arrangement of two forms of moldingly bonded encapsulations. The first of these forms is made of a hard encapsulant and forms a unitary encapsulation local to each electronic module which (i) encapsulates the circuit module and associated electrical coupling connections between the adjacent end of each individual run of an adjacent interspersed run of media and an electrical terminal of the module, and (ii) is securely bonded to the adjacent marginal end portions of the tubular members which surround the adjacent interspersed runs of hook-up media. The second form is an overmolded annular encapsulation made of a softer flexible encapsulant which fills the annular spaces between the encapsulations of the first form, and which is securely moldingly bonded to the midportion of the tubular member and the adjacent lateral surfaces of the encapsulations of the first form. Both the first and second forms of encapsulations are made of cool curing types of encapsulants. 
     U.S. Pat. No. 6,473,365, issued Oct. 29, 2002, to Joh et al, discloses a support structure of hydrophones for a towed array sonar system, which includes elastic members arranged in each sensor package and adapted to support a hydrophone received in the sensor package while attenuating vibrations transmitted to the sensor package. External vibrations, such as vibrations generated due to the running of a towing vessel or vibrations induced due to motions of a towing cable, which are irrespective of acoustic waves reflected by a target moving underwater, are attenuated by the hydrophone support structure during the transmission thereof from the sensor package to the hydrophones, so that the hydrophones are suppressed from vibrating due to the external vibrations. 
     U.S. Pat. No. 6,982,383, issued Jan. 3, 2006, to Spellman et al, discloses a cable section assembly for marine applications having a core structure, such as a buoyant cable antenna interim manufacturing step subassembly, and a protective casing formed from heat shrinkable tubing, which together with the interim subassembly forms a complete assembly. For buoyant cable assembly applications the interim subassembly is at least partially formed of polyurethane material having glass microballoons distributed therein, with such material in any event occupying an outer layer portion of the subassembly. The cable section assembly of the present invention is formed by providing a length of heat shrinkable flexible tubing, inflating or expanding the heat shrinkable tubing to its approximate full diameter, inserting the core structure into the length of tubing, and shrinking the tubing around the core structure. 
     U.S. Pat. No. 7,429,957, issued Sep. 30, 2008, to Tonn, discloses a buoyant cable antenna element that employs a specific double-negative meta-material sheath with a negative permeability. The double-negative meta-material sheath is disposed over the insulated wire portion of the buoyant cable antenna element. The double-negative meta-material sheath enables a deliberate reduction in the antenna wire inductance to a zero value at a desired critical frequency. Reducing the antenna wire inductance to zero creates a traveling wave structure antenna having enhanced bandwidth. 
     The above cited prior art does not disclose reuse of the core of the antenna element of a buoyant cable antenna and/or provide a core that can be removed and/or replaced by a different core and/or a core which may be customized for a particular application. Moreover, the above-cited art does not disclose an antenna element core that floats within air in a liner and/or which is slidable within the liner to be isolated from strain. Moreover, the above-cited art does not disclose an antenna element core for a buoyant cable antenna that permits the use of miniaturized electrical components in the antenna element itself due to the strain exerted during towing. 
     The solutions to the above described and/or related problems have been long sought without success. Consequently, those skilled in the art will appreciate the present invention that addresses the above and other problems. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide an improved antenna element for a buoyant cable antenna. 
     Another possible object of the present invention is to provide a removable/replaceable antenna element core. 
     Another possible object of the present invention is to provide an antenna element for a buoyant cable antenna with improved gain in the frequency ranges of interest. 
     Another possible object of the present invention is to provide an antenna element for a buoyant cable antenna with improved gain in the high frequency (HF) radio band (3-30 MHz). 
     Accordingly, the present invention provides an antenna element in a buoyant cable antenna, which is towed by a submarine. The antenna element may comprise one or more elements such as, for example only, a removable core that may comprise a straight cylindrical single-conductor wire, which acts as an antenna. The straight cylindrical single-conductor wire may be coated with a layer of insulation. 
     A liner surrounds the removable core. The liner defines a hollow interior with an inner diameter. The outer diameter of the removable core is sufficiently smaller than the inner diameter of the hollow interior such that the removable core is slidable with respect to the liner during installation of the removable core within the liner. The liner may further comprise a buoyancy jacket that is operable to provide that the antenna element is buoyant in water. 
     A pull cable may be secured to an outboard end of the removable core. The pull cable is sufficiently strong enough to pull the removable core into the liner and sufficiently long to pull the removable core into the liner from an outboard end of the liner. 
     A watertight cap is secured to the outboard end of the liner so that the pull cable is stored within the hollow interior of the liner inside of the watertight cap. 
     In one embodiment, the hollow interior is filled with air, which surrounds the removable core. In one embodiment, the core has a length in a range of from forty feet to sixty feet. 
     In another embodiment, the straight cylindrical single-conductor wire comprises a plurality of segments and may further comprise a plurality of capacitors, which interconnect the plurality of segments. The capacitors may be used to adjust the performance of the antenna or to optimize it near a desired frequency. 
     In another embodiment, the straight cylindrical single-conductor wire may or may not comprise a plurality of segments. If desired, any segments may be interconnected utilizing microelements such as capacitors, filters, amplifiers, sensors, and the like. 
     In another embodiment, the invention comprises a method for making an antenna element for use in a buoyant cable antenna for towing by a submarine. The method may comprise one or more steps such as, for example, providing a removable core, which comprises a straight cylindrical single conductor wire to act as an antenna, and providing a liner to surround the removable core. 
     Other steps may comprise providing that the liner define a hollow interior with an inner diameter. The removable core may comprise an outer diameter sufficiently smaller than the inner diameter of the hollow interior such that the removable core is slidable with respect to the liner during mounting of the core within the liner. The method may further comprise providing that the liner comprises a buoyancy jacket, which is operable to provide that the antenna element is buoyant in water. 
     In one possible embodiment, the method may comprise the steps of attaching a pull cable to an outboard end of the removable core, and providing that the pull cable is sufficiently strong enough and long enough to pull the removable core into the liner from an outboard end of the liner. 
     The method may further comprise securing a watertight cap to the outboard end of the liner such that the pull cable is stored within the hollow interior of the liner inside of the watertight cap. 
     The method may further comprise providing that the hollow interior be filled with air that surrounds the removable core. 
     The method may further comprise providing that the straight cylindrical single conductor wire comprises a plurality of segments and may further comprise interconnecting the plurality of segments using a plurality of capacitors. The method may further comprise utilizing the capacitors for adjusting the performance of the antenna or for optimizing it near a desired frequency. In more detail, the method may further comprise utilizing a length of the segments and a size of the capacitors for adjusting the peak gain to occur near a desired frequency. 
     In another embodiment, the method may further comprise providing that the straight cylindrical single conductor wire comprises a plurality of segments and may further comprise interconnecting the plurality of segments using a plurality of microelements. 
     The method may further comprise providing that said removable core has a length in a range of from forty feet to sixty feet. 
     In one preferred embodiment, the removable core is mounted to have sufficiently less strain applied thereto during towing than the liner whereby segments of the straight cylindrical single conductor wire can be connected utilizing relatively fragile elements such as microelements, as discussed hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts and wherein: 
         FIG. 1  is an elevational view, partially in section, showing a schematic for an antenna element for a buoyant cable assembly in accord with one possible embodiment of the present invention; 
         FIG. 1A  is an enlarged elevational view, partially in section, showing a schematic with capacitors connected in series with the antenna element for a buoyant cable assembly in accord with one possible embodiment of the present invention; 
         FIG. 2  is a schematic elevational view, partially in section, showing use of a buoyant cable assembly in accord with one possible embodiment of the present invention; and 
         FIG. 3  is a graph showing a comparison of frequency response, both calculated and actual, as compared to a prior art antenna element for a buoyant cable assembly in accord with one possible embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides a reconfigurable buoyant cable antenna (BCA). In one embodiment, the BCA has improved gain performance. The antenna element core can also be removed and replaced based on mission needs. Accordingly, the present invention provides an important improvement in BCA technology. 
     Referring now to the drawings and, more particularly to  FIG. 1  and  FIG. 1A , there is shown a schematic view of antenna element  10  for a buoyant cable antenna.  FIG. 1  and  FIG. 1A  show core  12  of antenna element  10 . Core  12  comprises straight cylindrical wire  14 , which may be a desired size wire, e.g. #18 AWG. Best shown in  FIG. 1A , foam insulating layer  16 , which may comprise a desired thickness, e.g., 0.177″ OD, is provided around straight cylindrical wire  14 . 
     In one embodiment, at desired intervals, which may be regular intervals, along the length of core  12 , wire  14  is interrupted into segments which are connected by electronic elements which may be microelements such as series capacitors  18 . Capacitors  18  may be ceramic capacitors. The product of this capacitance and the spacing is known as the “load factor” for the antenna and plays an important role in determining where in the HF band the antenna has its best performance. 
     In one possible embodiment, the length of core  12  may be in the range of about 40-60 feet. In one possible embodiment, string or pull cable  20 , which may be comprised of Kevlar, may be attached to core  12  at the outboard end, e.g., the outermost end thereof. In this embodiment, string or pull cable  20  may then be utilized to pull core  12  into liner  22 . Liner  22  may be polyethylene. There exists sufficient clearance  24  between core  12  and liner  22  to allow core  12  to easily slide in and out of liner  22  with a minimum of friction. This arrangement not only provides a way to remove core  12 , but also provides a means by which core  12  is strain-relieved and does not see any of the tensile force that liner  22  sees during towing as shown in  FIG. 2 , thereby protecting core  12  from damage, e.g., pulling capacitors  18  apart. 
     A portion of string  20  used to pull the core into liner  22  may be left inside the outermost end of liner  22  before installing watertight cap  26 . This is done to allow core  12  to be pulled back into liner  22  should it be necessary to remove core  12 . The other end  28  of liner  22  may be fitted with a watertight BCA connector  29  and then attached to a standard BCA coaxial feed  30 . 
     To remove core  12 , cap  26  on the outboard or outermost end is removed so that string  20  can be grasped and prevented from being pulled into liner  22  during the next step. Then inboard connector  29  is removed and core  12  is pulled out of liner  22 . This pulls some of string  20  back into the liner  22  but the free end is not pulled into the liner. The old core may then be cut from the string and the new core attached. String  20  may then be used to pull the new core into liner  22 . 
     During one test, a 51 foot long core was pulled into a liner by means of Kevlar string or pull cable  20 . String or pull cable  20  was originally pulled into liner  22  by means of a vacuum cleaner. However, other means, e.g., a long straight wire, could also be utilized to insert string  20  into liner  22 . 
     In a preferred embodiment, liner  22  comprises buoyancy jacket  32  extruded on an outer surface of liner  22  to give the antenna better speed-depth performance when being towed from a submerged submarine. 
       FIG. 2  shows antenna element  10  in buoyant cable antenna  46  which is deployed by submarine  48  utilizing deployment mechanism  42 . Buoyant cable antenna  46  may have various sections or elements such as signal lead-in and tow cable section  40 , which may comprise a non-buoyant portion. At the outboard end or outermost end of buoyant cable antenna  46  is buoyant single conductor antenna element  10 . In one embodiment, antenna element  10  may be about 50 feet long and/or be within a range of from forty to sixty feet long. However, antenna element  10  may be longer or shorter than this range if desired. 
       FIG. 3  illustrates the improved antenna gain for one embodiment of the present invention. Curve  50  shows a typical response for a 50 foot prior art antenna over a range of 5 MHz to 30 MHz. Curve  52  is the calculated gain of an antenna in accord with one embodiment of the present invention utilizing capacitors  18  and/or adjusting the length of core  12 . Curve  54  shows the measured gain of antenna in accordance with the present invention. In this embodiment, the peak gain at approximately 20 MHz is considerably higher than that of the prior art antenna core. By increasing the length of the antenna and adjusting the loading factor, the frequency at which peak gain occurs can be lowered. By shortening the antenna and adjusting the loading factor, the frequency at which peak gain occurs can be increased. The correct value of the loading factor depends on the antenna length and geometry. Thus, the above describe factors can be utilized to adjust the maximum gain to within the vicinity of the desired operating frequency or range of frequencies. 
     Accordingly, the present invention provides a means for re-use of antenna element  10  by allowing core  12  to be removed and replaced with a different core that is customized for a particular application. The preferred embodiment uses a core designed for substantially improved gain performance. In addition, the fact that the core now “floats” mechanically inside liner  22  provides a means for strain relief of the core and components thereof. In the prior art BCA, the core is integrated into the jacket and so must be able to bear the mechanical strain seen while being towed, precluding the use of miniaturized electrical elements in the core assembly. 
     As discussed above, the preferred embodiment of antenna element  10  comprises a buoyancy jacket  32  over liner  22 , wherein liner  22  defines a hollow center that houses core  12 . In another embodiment, the jacket could be made of several layers including an inner mechanical strain-bearing layer followed by the buoyancy layer, and finally a surface finish layer. Various types of cores could also be used. In the above-described invention, core  12  may consist of a periodic structure comprising insulated wire segments and capacitors. This core may be replaced with a single uninterrupted conductor, or one with integrated electronics, e.g. amplifiers, filters, sensors, and the like. 
     Many additional changes in the details, components, steps, and organization of the system, herein described and illustrated to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention. It is therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.