Operation Manual S15
Page
47
As the range of VHF signals are governed by line of sight, the antenna
should be placed as high as possible, while remaining clear of any
metallic objects that could influence the resonance of the antenna.
The most popular antennae for marine use are 1m (3ft 3in) long. On sail
boats these are usually mounted at the masthead, where the length of
the antenna keeps it clear from the navigation lights and wind vanes etc.
This type of antenna can also be mounted on the cabin roof or radar arch
on powerboats.
Longer whip antennae are recommended for larger boats. These radiate
the same total power as smaller antennae, but concentrate it into a
narrower beam, which is advantageous on a tall mast at extreme range
where concentrating the available power into a narrow horizontal beam
becomes more important. However, if the antenna is not vertical when
transmitting, the beam will be angled either too high or too low (Fig E).
Here the wider beam of the shorter antenna will be more universally
effective, although the signal will be weaker (Fig F):
Therefore vessels with a large heel angle (small sailboats) would be
better choosing a short masthead antenna. Your local agent should be
able to provide specific advice on antenna choice for the vessel it is to be
fitted to.
The antenna coaxial cable and any connectors used must be rated at 50
Ω . Under no circumstances should standard domestic TV cable and
connectors be used. Incorrectly rated cabling and connectors could result
in power not reaching the antenna, but also power could be reflected
back into the radio, damaging it in the process.
Fig F
Fig E
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