In many of my ham radio related posts, I make mention of PL Tones, CTCSS, and sometimes just refer to it as a tone. I have come to realize that I have never really taken the time to explain what these things are.

CTCSS stands for Continuous Tone Coded Squelch System
PL is a term that was created by Motorola and stands for Private Line
(both of the above are basically the same thing and the terms are often used interchangeably)

These tones are a sub audible tone that is injected into the signal just before it leaves the transmitter. The tone settings on radios usually have two basic settings: Tone and Tone Squelch (most of the newer radios also have a Digital Coded Squelch) for the user to choose from. I have never used or played with the Digital Coded Squelch so cannot say for sure, but it probably works similar to the Tone Squelch setting.

First, with the Tone setting turned on, the radio will add the sub audible tone that you have selected with each transmission. This is important because it is often necessary to use this setting to make use of repeaters on the VHF and UHF bands. In this setting, the receive function of the radio is not affected and the radio receives as if the tone were not turned on.

With the Tone Squelch setting, the radio will continue to add the tone when you transmit, but it will also listen for the tone when receiving. If the tone is not heard from the transmitting radio, your radio will remain muted and you will not hear whatever traffic is happening on that frequency.

As an interesting side-note, this is the system that is used by FRS radios when they advertise having “38 Privacy Codes” within each of their 15 or 22 or however many channels. The thing is, these “Privacy Codes” don’t really make it private because anyone who has them turned off and is on that channel will hear any traffic on that channel.

Ok, that explains what PL Tones are and how they work, but what’s the point and why are they used? Well, it depends on the situation, but there are 2 main reasons that they are used. The first reason would be to have multiple users on the same frequency or use the same frequency for multiple purposes.

For an example of the first part of that, think of the FRS radios that I mentioned before. While it doesn’t necessarily offer any additional privacy or security, you and your friends can be on the same channel as someone else but not have to hear what they are saying and you would still be able to hear your friends. In order for this to work, both radios (your’s and your friend’s) would need to be using a “Tone Squelch” implementation of CTCSS tones.

An example of using the same frequency for multiple purposes would be when using VoiceAlert on APRS. When using this APRS Radar, the CTCSS tones are used to determine what signals are within simplex range and the ones that are not do not have the tone on them.

The other reason for tones to be used, and the reason that is most likely in ham radio implementations, is to limit interference in high RF environments and decrease the amount of static bursts that are heard. This is especially important on repeaters where noise coming into the repeater would then be re-transmitted and heard by all users on the system. Along these same lines, many repeaters also generate a PL tone on the repeater output allowing users to use “Tone Squelch” rather than just “Tone” so that they will not hear other interference sources on the frequency that would otherwise open the squelch on the their radio.