User's Manual Part 2

Fig 12.10 - Effect of heel on range of longer whip antenna
Here the wider beam of the shorter marine antenna will be
more universally effective, although the signal will be weaker
(Fig 12.11, p. 103).
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.
12.3 Antenna installation
The most important factor in the performance of the radio will
be the quality and positioning of the antenna. Most recorded
problems with VHF radios are related to poor antenna siting,
faulty cabling, poor quality cable joints and low voltage supply.
Even a VHF as technically advanced as the RS81/RS82 cannot
compensate for these factors. Therefore, if replacing an existing
VHF installation, it is important that the antenna is thoroughly
checked for any faults or damage before use.
As the range of VHF signals are governed by line of sight (see
section 13.3), the antenna should be placed as high as possible,
while remaining clear of any metallic objects that could influ-
ence its resonance.
The most popular antennae for marine use are 1m (3ft 3in)
long. On sailboats these are usually mounted on the masthead,
where the length of the antenna keeps it clear from the naviga-
tion lights and windvanes, for example. This type of antenna
can also be mounted on the cockpit roof or powerboat garages.
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 (see the effect of
heel on range, Fig 12.10).
RS81/RS82 VHF
102
E04572