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CTCSS - Continuous Tone Controlled Squelch System - is now used on all UK repeaters. This requires transmission by the user of a continuous sub-audible tone and in return the repeater transmits the same tone when in talkthrough mode. most repeaters (including GB3FR and GB3SO) do not transmit the tone when idents are sent during idle periods.

The tones are allocated geographically in such a way that two repeaters which are on the same frequency and within possible range of each other will use different tones. As far as is practicable, all repeaters within the same geographical area but on different frequencies are allocated the same tone. The cw ident of all repeaters includes a letter after the callsign to indicate the tone in use as follows:

Tone A = 67.0Hz

Tone B = 71.9Hz Tone C = 77.0Hz
Tone D = 82.5Hz Tone E = 88.5Hz Tone F = 94.8Hz
Tone G = 103.5Hz Tone H = 110.9Hz Tone J = 118.8Hz

GB3FR and GB3SO both use tone B - 71.9 Hz.

Implementing CTCSS

Almost all modern commercial rigs have CTCSS fitted, and it is very easy to add to older rigs, or ex-pmr equipment. Commercial CTCSS boards are readily available or you can build your own according to this design by Adrian Knott G6KSN.