Repeater Offset & CTCSS Calculator
The offset is a duplexer problem, not a convention — 2 m has the tightest fractional separation of any band, and it is the busiest.
The offset as a fraction of the band
What each CTCSS tone costs you in decode time
The offset is a filtering problem, not a convention
The natural assumption is that repeater offsets scale with the band — bigger frequency, bigger offset, roughly in proportion. They do not. Measured as a fraction of the carrier the six bands span 6.6 to one, from 0.411 per cent on two metres to 2.697 on 33 centimetres. Ranking the bands by megahertz of offset and by fraction of carrier gives two different orders, which is the tell that no single rule is being followed.
What decides it is the duplexer. A repeater listens and transmits at the same instant on two frequencies through one antenna, so the filter between them has to reject its own transmitter by something like 85 to 90 dB — and cavity rejection depends on how far apart the two frequencies are as a fraction, not on the raw megahertz. Two metres has the tightest fractional separation of any band and is also the busiest, which is exactly why a two metre duplexer is a rack of large cavities while a 70 centimetre one, with 2.73 times the fractional room, fits in a shoebox.
Two smaller things fall out of the same measurement. A low CTCSS tone is slower to decode, because decode time is cycles over frequency: about 119 milliseconds at 67.0 Hz against 31 at 254.1, and a spoken syllable is only 150 to 250 milliseconds long — so the lowest tones really do clip the front of the first word. And the tone set does not avoid its own harmonics, despite looking carefully spaced: 97.4 Hz doubles onto 194.8 with 192.8 sitting 1.04 per cent away, which is closer than a plain geometric series over the same range would put them. Decoders have to measure the period rather than trust the spacing.
How to use
- Enter the repeater output — the frequency you listen on.
- The conventional direction comes from which segment that output sits in.
- Check the tone table before choosing a low CTCSS tone for a busy machine.
- Treat your frequency coordinator directory as authoritative, not this page.
Frequently asked questions
What is a repeater offset?
It is the gap between the frequency a repeater transmits on and the one it listens on. You listen on its output and transmit on its input, and your radio shifts automatically once the offset is set. The standard figures are 600 kilohertz on two metres, 5 megahertz on 70 centimetres, 1.6 on 1.25 metres, 1 megahertz on 6 metres, 25 on 33 centimetres and 12 on 23 centimetres.
Why is the offset different on each band?
Because a repeater transmits and receives at the same instant through one antenna, and the duplexer between them has to reject its own transmitter by something like 85 to 90 decibels. Cavity rejection depends on how far apart the frequencies are as a fraction of the carrier rather than in megahertz, so each band needs enough fractional room for a duplexer that can actually be built.
Does the offset scale with the band?
No, and it is the natural assumption. Measured as a fraction of the carrier the six bands span 6.6 to one — 0.411 per cent on two metres against 2.697 on 33 centimetres. Ranking the bands by megahertz of offset and by fraction of carrier produces two different orders, which is the tell that no single proportional rule is being followed.
Which band is hardest to build a repeater on?
Two metres, on the fractional measure that actually matters, and it is also the busiest repeater band. Its 600 kilohertz offset is only 0.411 per cent of the carrier, where 70 centimetres gets 2.73 times that fractional room. This is why a two metre duplexer is a rack of large cavities while a 70 centimetre one fits in a shoebox, for the same isolation figure.
How do I know whether a repeater is plus or minus?
From which segment its output sits in. On two metres, 145.11 to 145.49 and 146.61 up to 147.00 are conventionally minus, and 147.00 to 147.39 is plus. On 70 centimetres 440 to 445 is usually plus and 447 to 450 usually minus, but that one genuinely varies by region — some areas run the opposite convention on the same output.
Is CTCSS the same as privacy?
No, and the trade name "Private Line" has misled people for decades. CTCSS is a squelch code — a sub-audible tone that tells the repeater a signal is meant for it, and tells your radio which signals to unmute. Anyone with a scanner, or with tone squelch simply switched off, hears every word you say. It reduces nuisance traffic on your speaker; it conceals nothing.
What does a CTCSS tone actually do for a repeater?
It stops the machine keying up on things that were never trying to reach it. A repeater with a tone required will not open on distant co-channel signals, on intermodulation products from nearby transmitters, or on the background noise a badly sited receiver hears all day. That is why so many formerly open machines now require a tone, and it is about interference rather than exclusion.
Does the CTCSS tone I choose affect anything audible?
Yes, though it is rarely mentioned. Decode time is cycles divided by frequency, so a typical eight-cycle decoder needs about 119 milliseconds at 67.0 hertz and 31 at 254.1 — a spread of 3.8 to one. A spoken syllable runs 150 to 250 milliseconds, so the lowest tones really do clip the front of the first word while the receiver is still deciding.
Are CTCSS tones spaced to avoid harmonics?
They are not, despite looking carefully arranged. 97.4 hertz doubles to 194.8, and 192.8 sits only 1.04 per cent away — closer than the 1.65 per cent a plain geometric series over the same range would give. So a decoder cannot reject a harmonic by spacing alone and has to measure the period of the incoming tone instead.
Why are some CTCSS tones so close together?
Because four of them — 69.3, 206.5, 229.1 and 254.1 — were added later into a set that had already been spaced out, and every one of the five tightest adjacent pairs in the full 42-tone list involves one of those four. The original sequence has no pair closer than 2.67 per cent; with the infill tones the tightest pair is 1.47.
What is the reverse function for?
It makes your radio listen on the repeater input, so you hear the other station directly rather than through the machine. Two uses: if you can hear them plainly on reverse you are both close enough to work simplex and could free the repeater for people who need it, and if you cannot hear a station at all on reverse you know the path runs through the repeater rather than between you.
Can I rely on these offsets everywhere?
Treat them as the conventional pairing rather than as a listing. Band plans are set by regional frequency coordinators and do vary, particularly on 70 centimetres and 6 metres where the offset direction differs between areas for the same output frequency. Your local coordinator directory is the authority, and a repeater there may also carry a non-standard offset for a good reason.
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