Abstract:
An amphibious antenna for providing Near Vertical Incidence Skywave (NVIS) communication when grounded to a conductive fluid. The amphibious antenna has a support member for supporting a helix. The helix includes a first helical arm that is not insulated and grounded, when in use, through a conductive fluid into which the antenna is placed, and a second helical arm that is insulated from the conductive fluid.

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. 

   BACKGROUND OF THE INVENTION 
   (1) Technical Field of the Invention 
   The present invention relates to antennas and more particularly, to amphibious antennas for providing Near Vertical Incidence Skywave (NVIS) communication. 
   (2) Description of the Prior Art 
   Tactical communications in the frequency range of 2–30 MHz take advantage of ionospheric propagation effects to gather or disseminate intelligence over large distances. In the 2–12 MHz range, one mode of ionospheric propagation (i.e., Near Vertical Incidence Skywave (NVIS)) is used for distances shorter than long haul ionospheric skip (less than 800 km), but longer than the “radio horizon” distance at these frequencies (greater than 40 km). 
   Antennas used for NVIS communications are typically large resonant wire structures of various forms that include inverted Vees or horizontal dipole arrays. Depending on the frequency of operation, the beam patterns of these antennas are distinguished by a lobe that points directly over head (zenith) in order to affect NVIS mode communications. 
   There is a need for NVIS communication capabilities over sea as well as over land. Moreover, there is a need for an antenna structure that is collapsible, compact, and portable. 
   SUMMARY OF THE INVENTION 
   The present invention is a novel amphibious antenna for use in or over sea or on land. The antenna having a first helical arm that is insulated and a second helical arm that is un-insulated. The un-insulated helical arm providing a ground to a conductive fluid. The antenna provides Near Vertical Incidence Skywave (NVIS) communication as well as some line-of-sight capability over land or sea when connected to a suitable manpack transceiver. Further, when the second helical arm of the antenna is placed in or near a conducting interface, such as sea water, the electromagnetic boundary conditions are such that cancellation of the radiation fields at low angles, relative to the horizon, is minimized. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will be better understood in view of the following description of the invention taken together with the drawings wherein: 
       FIG. 1  is a side view of an antenna according to the present invention; 
       FIG. 2  is electrical schematic of the antenna shown in  FIG. 1  showing one helical arm shorted to sea water and one insulated helical arm, wherein the insulation over the second helical arm is not shown; and 
       FIG. 3  is a collapsible antenna having a helix wherein the size of the exposed helix is exaggerated. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  is an antenna  10  having a hollow, insulating support member or core  18  for supporting helices  13  according to the present invention. The antenna  10  has at least two helical “arms”  11 ,  12 . The first helical arm  11  is exposed and not insulated, while the second helical arm  12  is insulated by insulation  14 . The insulation may be selected from any suitable material; however, in the preferred embodiment, fiberglass, or light weight plastic is used. The first helical arm  11  that is exposed is typically made from a conductive, non-corrosive metal, such as stainless steel. The second helical arm  12  may be made from a conductive material, that may be the same material as used for the first helical arm  11 . However, because the second helical arm  12  is protected from corrosion by the insulation  14 , the material chosen may not be non-corrosive, for example copper or brass. 
   The support member  18  of the antenna  10  is preferably constructed from a lightweight insulating material, such as plastic. In a preferred embodiment, the support member is approximately 12 inches in diameter and 10 to 12 feet in length. In the preferred embodiment, the helical arms  11 ,  12  are comprised of wide straps or ribbon shaped conductors instead of thin wire to allow enough surface for a good electrical connection to sea water, while simultaneously allowing for wide impedance bandwidth. 
   In use, a user places the antenna  10  in sea water. When the antenna  10  is deployed in sea water, the first helical arm  11  that is exposed and in contact with sea water provides the ground for the second helical arm or insulated portion  12  of the antenna  10 . 
   When the antenna  10  is deployed over sea water, the first helical arm  11  that is exposed behaves as a grounding electrode for the second helical arm or insulated portion  12  of the antenna, allowing the antenna  10  to behave as a slow-wave transmission line antenna. The antenna is a slow-wave structure because the phase velocity along the axial direction of the antenna is smaller than the velocity in the direction occupied by the helical conductor; a function of a helical pitch angle. When the second helical arm  12  is placed in, on or near a conducting interface, such as sea water, the electromagnetic boundary conditions are such that cancellation of the radiation fields at low angles, relative to the horizon, is minimized. The second helical arm  12  formed by the connection to sea water has a broad beam pattern that extends over a considerable portion of the hemisphere, including zenith, permitting NVIS capability. The transmission lines for the antenna  10  (not shown) may be preferably attached to the first and second helical arms  11 ,  12  by running the lines through the support member  18  and drilling a hole through the support member  18  wherein the lines may be attached directly to the first and second helical arms  11 ,  12 . 
     FIG. 2  is an electrical schematic of an equivalent antenna over sea water of the antenna  10  shown in  FIG. 1 . The details for the antenna feed have been omitted for clarity. When the antenna  10  is used over land, the helical arms  11 ,  12  are open circuited, forming a slow-wave dipole antenna with a pattern similar to that of the grounded helical transmission line antenna. The resulting wide beam pattern in both modes (ungrounded and grounded) permits NVIS communication as well as some line-of-sight capability over land or sea. 
   Referring to  FIGS. 1 and 3  an alternative embodiment of the antenna  10 , comprises the antenna  10  being collapsible in length. The support member  18  is made up of a series of non-conducting cylindrical shells  17  of varying size for mechanical support with mechanical stops (not shown) that keep the shells from coming apart. The helical arms  11 ,  12  are wound in the appropriate manner for its function (i.e., over the smaller diameter shells for support or within the larger shell assembly for insulation). When not in use, the antenna  10  is collapsible by pushing ends  15 ,  16  of the antenna  10  toward each other or by compressing the antenna  10  flat, like an accordion. When the antenna  10  is required for operation, the ends  15 ,  16  are moved away from each other or the antenna  10  is stretched open and manually deployed. In a preferred embodiment, the antenna  10  would comprise a length of about 15 feet when deployed and a length of approximately one-quarter to one-third of the deployed length when collapsed. 
   In summary, the antenna  10  according to the present invention is collapsible (in one embodiment), compact, lightweight, and manually deployed. The antenna  10  has dual mode (grounded and ungrounded). 
   The antenna  10  in the collapsible embodiment allows a user to carry the collapsed antenna  10  on his/her back. When the antenna  10  is needed for use, the user moves the ends  15 ,  16  of the antenna  10  away from each other, thereby manually deploying the antenna  10 . In one embodiment, the antenna  10  is placed in seawater and powered up for use. 
   When the antenna  10  is needed but sea water is not available or when the antenna  10  cannot be submerged in sea water, the user moves the ends  15 ,  16  of the collapsed antenna  10  away from each other, thereby manually deploying the antenna  10 . The antenna  10  is then used over land or sea water. The antenna  10  uses a slow-wave structure to enable performance over land and the sea. The antenna  10  is unique in that it uses exposed and insulated conducting arms or helical arms  11 ,  12  to affect a hybrid radiator for use over land or the sea. 
   After the antenna  10  is used in or over sea water, or over land, the antenna  10  is collapsible by pushing the ends  15 ,  16  of the antenna  10  toward each other or by compressing the antenna  10  flat. The antenna  10  is compacted into a flat package, which a user can easily carry. 
   In an alternative embodiment wherein portability is not required, the antenna  10  may be integrated directly into a sea-craft, such as a raft or Zodiac. The antenna  10  may be made part of a floatation collar. Further, the antenna  10  can be placed into sea water during use and retracted when not in use. Alternatively, the antenna  10  can be used over sea water. Modifications and substitutions by one of ordinary skill in the art are considered to be within the scope of the present invention, which is not to be limited except by the following claims.