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.
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.