Abstract:
A grounding device for water vehicles. The device includes an elongated electrical conductive section. The device also includes an elongated rope for increasing water resistance positioned in an adjacent relation to the elongated electrical conductive section. The device is of particular usefulness in grounding RF transmitters on high speed water vehicles having electrically insulative hulls.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein was made under Contract No. N66001-00-C-0018 with the Government of the United States of America and may be manufactured and used by and for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to water vehicles, and more particularly to methods and apparatus for the electrical grounding of water vehicles. 
     (2) Brief Description of Prior Developments 
     Many high speed water vehicles are built with hulls comprised of nonconductive materials such as composite fiberglass and resin materials and wood. Electrical grounding of on board RF transmitters and on board electrical equipment can, therefore, require particular adaptations on the hull which may be expensive and time consuming to install. For example, in some cases a partial metal liner is installed on nonconductive hulls to facilitate grounding. 
     In other situations, the difficulty in grounding water vehicles with nonconductive hulls may limit communications capabilities on such vehicles. For example, many smaller high speed water vehicles with nonconductive hulls make use of VHF RF transmitters even though such transmitters are limited in terms of range. Although the extended range HF RF transmitters may be desirable for many such vessels, grounding requirements may limit the use of HF RF transmitters on such vessels. 
     Referring to FIG. 1, a prior art arrangement for grounding an RF transmitter is shown in which a boat  10  equipped with a transmitter  12  and an outrigger  14  is moving in a forward direction  16 . The outrigger mounting  14  includes a lower horizontal beam  18  and an upper angled beam  20 . There is a lower pulley  22  on the lower horizontal beam  18  and an upper pulley  24  mounted on the outer end of the upper-angle beam  20 . An RF ground wire shown generally at numeral  26  extends from the RF transmitter  12 . This RF ground wire  26  includes an outward length  28  which extends from the RF transmitter  12  to the upper pulley  24 . From the pulley  24  to the lower pulley  22  the RF ground wire  26  includes a vertical length  30 . Extending in a rearward direction  32  which is generally opposite from the forward direction of the boat  10  there is a rearward length  24  of the RF ground wire  26 . At the terminal end of this rearward length  34  there is a rearward terminal weight  36  which is usually able to maintain the rearward length  34  of the RF ground wire  26  in a body of water  38  at low speeds. At higher boat speeds, however, the rearward terminal weight  36  and the rearward length  34  of the RF ground wire  26  will often begin moving in an outward and upward skipping motion  40  so that the rearward length will move out of the water as at  34 ′ and the rearward terminal weight will similarly move to  36 ′. 
     The velocity of the water vehicle containing the RF transmitter source is “speed limited”. Typically, under normal wave and weather conditions the water vehicle velocity range is only 4 to 7 knots. Under more adverse wave and weather conditions the water vehicle velocity must be reduced to maintain continuous contact with the RF ground conductor and water. The speed limitation of the water vehicle is created by a combination of the forward velocity of the water vehicle and the drag forces imposed on the weighted RF ground conductor increase causing the weighted RF ground conductor to lose continuous contact with the water. The loss of continuous contact or “skipping action” of the RF ground conductor wire causes loss of signal strength, increases interference with other electronics on the water vehicle, and can cause data collection loss during antenna calibration. 
     An improved method and apparatus for grounding electrical apparatus on water vehicles is, therefore, needed. 
     SUMMARY OF INVENTION 
     The present invention is a high speed RF ground device for water vehicles. The device includes an elongated electrical conductive section and an elongated rope for increasing water resistance positioned in an adjacent relation to the elongated electrical conductive section. 
     The present invention also encompasses a method for grounding an RF circuit on a water vehicle which includes the step of connecting an elongated grounding devise which includes an electrical conductive section and an elongated rope for increasing water resistance section to the RF circuit. The water vehicle is then caused to move in a forward direction. The elongating grounding device is then positioned to extend rearwardly from the water vehicle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is further described with reference to the accompanying drawings in which: 
     FIG. 1 is a perspective view of a boat with a prior art apparatus for grounding an RF transmitter; 
     FIG. 2 is a perspective view of a preferred embodiment of the RF ground device of the present invention; 
     FIG. 3 is an enlarged view of the area encircled  3  in FIG. 2; and 
     FIG. 4 is a perspective view of a boat on which the RF ground device shown in FIGS. 2 and 3 are used to ground an RF transmitter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 2 and 3, the grounding device is shown generally at  42 . This grounding device includes a marine rope  44  which may be comprised of synthetic fibers, natural fibers or a mixture of natural and synthetic fibers. Referring particularly to FIG. 3, the rope  44  includes transverse spaces as at space  46  known as wicking which allows the rope to be water permeable. Referring again to FIGS. 2 and 3, the grounding device  42  also includes a conductive RF ground wire  48 . There are also a plurality of insulated sleeves as at sleeves  50  and  52  which circumferentaly surround both the marine rope  44  and RF ground wire  48  and hold them together as an integral unit. 
     Referring to FIG. 4, the use of the grounding device  42  to ground an RF transmitter on a boat is illustrated. A boat  54  is equipped with an RF  56  transmitter with a tuner  57  and is in motion in a forward direction  58 . In an opposed aft direction  60  an RF ground wire  62  extends from the RF transmitter  56  to the tuner  57  and another wire  63  extends from the tuner  57  through ground wire  63  to a connection  64  from where the wire rope composite device shown generally at  66  extends rearwardly into a body of water  68 . Preferably the ground wire  62  extends in an aft direction approximately midway between the port side  70  and the starboard side  72  and over the stern  74  of the boat  54 . In addition to mounting the device over the stern, mounting may also be through the hull and below the water line. A hydroplane device such as those used to keep fishing line submerged during trolling may also be attached at or near the aft terminal end of the rope  44 . A suitable hydroplane device is commercially available from Nekton Corp. of Danville, N.H. under the trademark “Z-WING”. 
     Water vehicles using the RF ground are not ordinarily speed limited and can traverse waypoints with as much speed as safety allows during the normal operation of this invention or under adverse wave or weather conditions. The constant RF ground plane results in a stronger more consistent signal strength, reduces interference with electronics on the water vehicle. Other advantages are the reduction in data collection storage requirements, significant reduction in time required for RF antenna calibration, and decreased water vehicle rental labor costs. 
     Test 
     The construction of the RF ground wire-rope device used during a test conducted in salt water in the Chesapeake Bay near Dam Neck, Va. was a marine synthetic type rope under water conditions of 1 to 3 foot seas. The marine synthetic rope was approximately ¾ inch in thickness and approximately 20 feet in length. The RF ground wire was wrapped along the length of the rope and secured to the rope using plastic tie-wraps forming a wire-rope. The wire-rope RF ground was jury-rigged mounted over the stern of the water vehicle and placed in the water. Water vehicle velocities of 30 knots or more were attained using this RF ground wire-rope device while maintaining constant RF signal-ground conductivity. The testing of the RF ground wire-rope device was used for beamforming calibration of an HFSWR receive antennas. The successful use of the wire-rope weighted device was attributed to the increased surface area contact of the RF wire-rope ground, flexibility of the rope material and the physical properties of water. Plastic sleeves tie-wraps were used to secure the wire and rope together which enabled the wire-rope to bend as a unit, shape itself to various wave conditions, and also allowed for the physical properties of water to be taken advantage of. 
     The physical properties of water (surface tension, cohesion and adhesion) allow the water (and its dissolved substances) to move through spaces (wicking) of the rope material and adhere to the solid materials used in the wire-rope device. This wire-to-water, wire-to-rope, and rope-to-water contact results in maximum continuous RF ground contact. Preferably, the characteristics of the spaces (wicking) will be selected to optimize performance for particular physical properties of the water in which use is anticipated. It will be appreciated that once the rope becomes permeated with water, particularly salt water, the overall conductivity of the device will be increased. 
     The mounting method described above was an expedient way to implement the RF ground wire-rope device for this test. Those of ordinary skill in the art will appreciate that various other mounting methods using downriggers, outriggers, or temporarily attached to water vehicles hull (under the water line) are acceptable within the limitations of the present invention. The only adjustment to the wire-rope construction is establishing the proper wire-rope length required for the mounting method to be used for the water vehicle. Also, the conductive RF ground wire could alternatively be on the inside of the rope or sleeve. 
     It will be appreciated that the RF ground of the present invention allows RF transmitters mounted on water vehicles to be efficiently and cost effectively grounded through a wide range of vehicle speeds and water and weather conditions. 
     It will also be appreciated that especially when used without the hydrodynamic depressor, the grounding device described above will ordinarily not become entangled with buoys or lobster or crab traps or debris in the water. 
     While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating therefrom. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.