CTCSSState of the art repeater access system
Until recent years, most Amateur repeaters operated with Carrier Squelch. The repeater would key up and transmit anytime a carrier appeared on the input. Many repeaters still use this system. Now, more and more open repeaters are using an access system that utilizes a specific sub-audible tone transmitted continuously by the user during the entire transmission. This system is called CTCSS.
CTCSS stands for Continuous Tone Coded Squelch System. The term, PL is often used by radio amateurs for this same system. PL stands for Private Line and both are the © property of Motorola Inc.
Although there are various variations, with the most common system, a single low frequency tone, low enough to be filtered out without disturbing voice audio is transmitted continuously by the repeater user at a very low level along with the voice audio. This tone opens the repeater's squelch, instead of a carrier. There are many advantages to operating a repeater with this system, but there is also some common misunderstanding as to what CTCSS will and will not do.
With Carrier squelch, a repeater responds to anything on the input frequency. Besides intended signals, this includes unintended signals, intermod, and strong static. This can make the repeater noisy and annoying to monitor during idle periods. It is possible that a user, even with emergency or priority traffic will be unable to get anybody to respond, particularly during noisy periods because the noise interferes with other activities while monitoring.
A repeater that is configured to respond only to signals containing a specific CTCSS tone will only respond to user signals and will ignore other signals. The carrier squelch can be set to a much lower threshold, or not used at all, allowing the repeater to respond to weaker signals. The repeater will be much quieter during idle periods, making monitoring much more pleasant and will not cause the repeater to dominate a scanner during idle periods allowing more efficient scanning of other frequencies. It also will reduce “time outs” because the timer will not be kept running in-between transmissions by unwanted signals.
When two or more signals appear on the input of a FM receiver, the stronger signal dominates the receiver masking the weaker one(s). This is called capture effect. This becomes a problem if repeaters are spaced too closely. If an unintended signal is stronger than an intended signal on a repeater input, the intended signal would not be heard. For example, a mobile or even moderate fixed station working another repeater can override a weaker signal from a handheld transceiver even it is nearer to the intended repeater, blocking it from the receiver. Even if the repeater uses CTCSS the repeater still will not hear the intended signal. It will not retransmit the unwanted signal, but it will also not hear the intended one. For this reason, CTCSS will not allow spacing of repeaters significantly closer than would be possible with carrier squelch. This is a major misunderstanding about CTCSS by the Amateur Community.
The Indiana Repeater Council has adopted a voluntary CTCSS frequency plan shown at the right. This plan avoids a situation where two repeaters on the same frequency use the same tone and the users inadvertently key up both repeaters.
The Indiana Repeater Council encourages, but does not require the use of CTCSS. Since the benefit is only to the repeater system using it, we believe it should be up to the individual repeater operator to weigh the benefits and make the choice based on the individual situation.
There are other types of CTCSS systems not covered in this article. Many private repeaters use a digital CTCSS system using a digital signal. CTCSS can also be transmitted on the repeater output to allow people with receivers capable of CTCSS decode to monitor with less interference.
Soon, all Amateur repeaters will use this superior method of access.