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Calculating radio range, VHF, aerial height and distress calls
When calculating the range, remember that VHF, distress calls included,
only reaches as far as the radio horizon:
2.5 times the sum of the square roots of both aerial heights, in nautical miles.
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VHF radio follows the line of sight. A masthead aerial fourteen metres up reaches a coast radio station thirty metres up out to a good twenty nautical miles, while a handheld set at head height barely manages five. Transmitting power makes little difference; height makes all of it.
For a distress call this means that anyone out of range will not be heard on VHF. That is why an EPIRB and satellite communications should be carried as well on a long passage, since neither depends on the horizon.
The radio installation counts as equipment and should be included in the sum insured. More important, though, is permission to operate it: marine VHF requires a radio operator's certificate. Transmitting without one is a regulatory offence. Your watersports legal expenses insurance would step in, but the problem is easily avoided.
Frequently asked questions
How far does a handheld radio reach?
At head height, barely more than five nautical miles.
It is the aerial height that decides, not the transmitting power.
Do I need a radio operator's certificate?
For marine VHF, yes.
Operating without one is a regulatory offence.
Worked out for your type of boat
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