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HF Antenna Tuners (current as of May 2009)
by Chuck Husick

HF-SSB radios that use automatic antenna couplers may generally be owner installed. In times past, when manual adjustment of fixed tuned couplers was required, only licensed radio operators could make the installation. Although owner installation is legal, unless you are familiar with working with antenna systems you will benefit by having the installation done by a professional.

The transmitter must be electrically matched to the antenna so that the antenna will absorb the transmitter power and radiate it into the atmosphere. This is most often accomplished with a microprocessor controlled automatic antenna coupler or "tuner". The coupler varies the "electrical length" of the vertical radiator wire by switching various capacitors or inductors contained in the coupler into the circuit between the radio and the antenna wire. This allows a wide range of operating frequencies to be served by a fixed length antenna. Some couplers provide a readout of the quality of the "match" achieved between the transmitter and the antenna. This is expressed in terms of standing wave ratio, SWR, a ratio of unity, 1.0, is optimum.

While the antenna system for the VHF radio is a relatively simple matter, the antenna system for effective short wave communication can be complex. As for the VHF antenna, the high frequency antenna must work equally well on all relative bearings For this reason most marine HF antennas are comprised of a vertical whip or a vertical or near vertical wire, usually about 22 feet long. From an electrical standpoint, the antenna must consist of both the upward vertical element and a corresponding downward projecting vertical element. The downward projecting element need not be an actual wire, it can be created by connecting the transmitter to the sea or to a large electrically conductive area such as a metal deck or a metal screen contained in a fiberglass cabin top. Making a proper connection to the sea or the deck creates a virtual antenna element extending downward for a distance equal to the length of the real antenna.

On vessels with steel or aluminum hulls the connection between the radio's antenna coupler ground and the sea is simple. However, currents at HF frequencies do not travel only along the surface of conductors, but also through the interior of the conductors. For this reason, the ground connection is usually made using copper tape about 2-3 inches wide and at least 0.003 inches thick. Fastening the copper ground strap to the hull will suffice.

Boats with wood or fiberglass hulls require a different solution to the grounding requirement. Fortunately, it is NOT necessary to copper sheath the exterior of the hull, nor is it necessary to install an exterior ground plate, other than the plate already installed as a part of the boat's lightning protection system. It is necessary to provide a significant area of metal in effective electrical contact with the sea. (About 100 square feet of contact area is desirable, although some claim that smaller areas will be sufficient for boats operating in salt water. Given the small cost of installing a reasonably large ground plane area it seems reasonable to do so.) Creating the ground connection is not difficult. At high frequencies, a layer of electrically conductive material, copper or bronze window screen, fastened to the inside of the hull in the bilge areas below the waterline, held in place with hot melt glue, will suffice. The fiberglass or wood hull will act as the dielectric in a capacitor and seemingly disappear, leaving the interior metal surface in contact with the sea. The conductive area need not be in one piece. Numerous small areas can be covered separately. All areas are joined together by soldering wide copper tape to them at numbers of places. As far as the radio frequency is concerned, the window screen is in the water.





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