Transmitter power


The maximum power that can be legitimately transmitted by a marine VHF radio is 25 watts. That doesn’t sound very much, compared with something like a domestic light bulb, but in the right conditions and from a tall enough aerial, it may be good for a range of sixty miles or more.


For short range communications, there is no need to use so much power: perfectly satisfactory results can be achieved with much less. Of course, this reduces the drain on the battery, but the really big advantage of using less power is that by reducing the range over which one vessel’s transmissions can be heard, it allows others, a few miles away, to use the same frequency.


For this reason, all marine VHF radios must be able to transmit on a reduced power setting of 1 watt or less. With a good aerial, at least a couple of metres above the waterline, even this may achieve good communication over distances of 5-10 miles.


Portable VHF sets could, in law, transmit at up to 25 watts, but in practice they never do. In most cases, their maximum output is between 3 watts and 6 watts, but they are still required to have a 1 watt low power facility.


Channel selector


In order for one radio to receive the signals transmitted from another, the transmitter and receiver have to be tuned to the same frequency as each other. VHF frequencies, however, involve long and unmemorable sequences of numbers, such as 156.375 Mhz. To simplify things, some frequencies have been given international channel numbers (see page 25). 156.375MHz, for instance, is known as Channel 67.


 


Being able to change channel quickly and accurately is so important that on most radios, the channel selector is the most prominent control. It‘s usually in the form of a rotary knob or up and down keys, and matched by prominent numbers on the display, but in some cases — generally the more up-market models — it may be a numbered keyboard.

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