Marine VHF radio is installed on
all large ships and most motorized small craft. It is used for a wide
variety of purposes, including summoning
rescue
services and communicating with
harbours and
marinas,
and operates in the VHF frequency range, between 156 to 174
MHz. Although it is widely used for collision avoidance, its use for
this purpose is contentious and is strongly discouraged by some
countries, including the UK.
A marine VHF set is a combined transmitter
and receiver and only operates on standard, international frequencies
known as
channels. Channel 16 (156.8 MHz) is the international calling
and distress channel. Channel 9 can also be used in some places
as a secondary call and distress channel. Transmission power ranges
between 1 and 25 watts, giving a maximum range of up to about 60
nautical miles (111 km) between
aerials
mounted on tall ships and hills, and 5
nautical miles (9 km) between aerials mounted on small
boats at
sea-level.
Frequency modulation or (FM) is used.
Marine VHF mostly uses "simplex"
transmission, where communication can only take place in one direction
at a time. A transmit button on the set or microphone determines whether
it is operating as a transmitter or a receiver. The majority of
channels, however, are set aside for "duplex"
transmissions channels where communication can take place in both
directions simultaneously. Each duplex channel has two
frequency assignments. This is mainly because, in the days before mobile
phones and satcom's became widespread, the duplex channels could be used
to place calls on the public
telephone system for a fee via a marine operator. This
facility is still available in some areas, though its use has largely
died out. In US waters, Marine VHF radios can also receive
weather radio broadcasts, where they are available, on receive-only
channels wx1, wx2, etc.
