Very High Frequency (VHF) and Ultra High Frequency (UHF) don’t bend over hills, around headlands, or over the horizon to any great extent, nor are they bounced back by the ionosphere, so they are used for local broadcasting, television, and communications.


The maximum range at which you can expect a VHF signal to be received depends on the height of the transmitting and receiving antennas.


The distance to an antenna’s VHF horizon (in miles) is approximately three times the square root of its height in metres.


Two antennas will be in within range of each other when their horizons overlap:


So a motor cruiser, with its antenna at 4m, would have a radio horizon of 6 miles, while a sailing boat, with its antenna at 16m, would have a radio horizon of 12 miles. They should be able to contact each other at a range of about 6+12=18 miles.


Modulation


A constant, unchanging series of radio waves is of little practical use: it is the radio equivalent of a continuous single-note whistle, or a steady white light. In order to carry a message, it has to be varied in some way. This is known as “modulation”, and in its simplest form consists of nothing more than switching the transmission on and off.


Simple on-off keying can be used to send messages by Morse code or something similar, but more sophisticated forms of modulation are required to transmit sounds such as speech.


 


The first stage in the process is to convert sound — the physical vibration of the air — into electricity. This is the job of a microphone.

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