Monday, July 18, 2016

CTCSS (Continuous Tone Coded Squelch Systems)

1.01 - the very basics  (Jim, KC9HI)

CTCSS    (Continuous Tone Coded Squelch Systems) 
aka "Tone Access" or "PL"

Many repeaters have a LOCK on their receiver to keep unwanted interference from getting in (Lock = receive CTCSS tone). If it does, your radio needs to transmit the matching KEY to enter. (Key = transmit CTCSS tone).

If you put a LOCK on YOUR radio's receiver, the repeater now needs to transmit the matching KEY to enter your receiver as well.

Very important - Not all repeaters that require a KEY to unLOCK its receiver will transmit a KEY back to the user. If it doesn't transmit a KEY, and you have programmed your receiver with a LOCK, you will not hear the repeater because it cannot get in without a KEY.

Never LOCK your receiver unless you are absolutely sure the repeater is sending the matching KEY to YOUR radio as well.


Enc = Encode tone (Key) on the transmitter
Dec = Decode tone (Lock) on the receiver


What is a CTCSS (PL) Tone?

A CTCSS is a 'Tone' sent along with your voice when you transmit.
The transmit tone is referred to as the Encode tone.
The receive tone  is referred to as the Decode tone.
It is Sub-Audible. You can't hear it, but a receiver or repeater can.
The 'Tone' on your TX must match the tone required by the RX on the other end.

CTCSS is part of a repeater's receiver that allows only signals with the proper "Tone" to access or open the receiver.  This allows distant repeaters using a different CTCSS to co-exist on the same frequency.

Some, but not all, repeaters will transmit a CTCSS Tone back.

If the repeater you are trying to listen to is not transmitting a tone, but you have programmed your receiver to require one, you will not hear that repeater.

For this reason, DO NOT initially program your radio's receiver to require a tone. Program your transmitter ONLY to match the repeater you are trying to access.


Avoid using CTCSS on simplex channels.  You must retain the ability to hear others currently using the same frequency to avoid interfering with existing conversations.

Adding these tones in no way guarantees privacy or security of any type. It is used to block out reception of other signals using the same frequency.  Any receiver that does not have CTCSS/DCS tones activated on their radio's receiver will be able to hear your transmissions.

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