Installation; Selecting The Antenna Location - JRC JSS-296 Service Manual

3. INSTALLATION

3.1 SELECTING THE ANTENNA LOCATION

The antenna is the electrical conductor that radiates the RF (Radio Frequency) energy from the transmitter and
picks up radio signals from other stations for the receiver. There are many antenna types and configurations.
The most common are the vertical whip (10 meters in length) and the long wires (10 to 20 meters in length).
The vertical whip is most often used aboard motor vessels, tugs, tankers, and fishing boats. The long wire is
found predominantly aboard in the from of an insulated side stay or spring stay, insulated back stay, or triadic
stay.
There are several important factors that must be considered when installing the antenna system. The antenna
must be as unobstructed as possible and the antenna must be separated from any other antenna system,
structure, metal stay, or guy wire. LORAN and OMEGA antennas should be as far away as possible. At least
10 meters is the recommended minimum distance. Even at these distanced, there will be some detuning and
directivity to the radiation pattern. The maximum separation possible is preferable.
Should a major portion of the antenna be secured alongside a metal superstructure, a shift in the antenna
characteristics causing poor radiation efficiency and difficulty in tuning is expected.
Proper high voltage insulators must be used for the antenna and lead wire and must never be painted or sand
blasted.
When planning the installation of the antenna, you must remember that the antenna itself is only a small part of
the total antenna system. Of utmost importance is the grounding system (counterpoise) which forms the
"other half" of the antenna. The radiation efficiency of an antenna is directly proportional to the effective area
of the counterpoise. The larger counterpoise (more area covered) the lower the radiation resistance and the
higher the radiation efficiency. Counterpoise systems are no problem on large metal-hulled vessels. The
larger the vessel, the better the counterpoise. On small and non-metal hulled vessels, however, the
counterpoise requires much more thought and planning. In general, a surface of at least 30 square meters is
recommended in addition to bonding all large metal objects together as a part of the counterpoise system.
Figure 3-1 (a) shows the effect of an improper counterpoise system with long grounding straps. This has the
effect of including the radio, the antenna coupler, the microphone, and even the operator as part of the radiating
system. This not only is inefficient but also is potentially harmful to the operator. The microphone may be
"hot" and may "sting" or burn the lips. Figure 3-1 (b) illustration shows a proper system where the antenna
actually begins at the counterpoise that keeps the radio (and operator) at "ground" potential.
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