CTCSS tones and DTMF generator

What CTCSS does

CTCSS (continuous tone-coded squelch system) adds a low-frequency tone, between 67 and 254 Hz, to everything you transmit. A repeater or receiver set to the same tone only opens its squelch when it hears that tone, so it ignores other signals on the channel. Your radio filters the tone out of the received audio, so you don't hear it. Motorola's name for it, PL (Private Line), is also common.

UK repeater tones

UK analogue repeaters need a CTCSS tone for access. To stop neighbouring repeaters on the same channel being opened together, the country is divided into areas, each with a letter code from A to J for its tone. A is 67.0 Hz, B 71.9 Hz, C 77.0 Hz, D 82.5 Hz, E 88.5 Hz, F 94.8 Hz, G 103.5 Hz, H 110.9 Hz and J 118.8 Hz. Repeater lists give each repeater's tone or letter, and a few repeaters use other tones. Set your radio to transmit the tone (often called TONE or ENC mode). Setting it to decode as well (TSQ) keeps your radio quiet until the repeater transmits.

The older access method, a short 1750 Hz toneburst, is still accepted by many repeaters in addition to CTCSS but can't be the only method. The toneburst should last about 200 to 400 ms. The button above plays a 300 ms burst, and holding a handheld's microphone to your speaker will work in a pinch.

DTMF

DTMF (dual-tone multi-frequency) is the system used by telephone keypads. Each key sends two tones at once, one from a low group (697 to 941 Hz) and one from a high group (1209 to 1633 Hz). Radio amateurs use DTMF to control repeaters and internet gateways such as EchoLink and AllStar, for example to connect to a node or a reflector. Most handhelds have a DTMF keypad, and the A to D keys are often available too.

Before using DTMF on a repeater, listen to check it's not in use, and follow the repeater or gateway's own instructions for its command codes.

Questions