Summer 2003 Issue

Tone—To Go

By Gary Pearce, KN4AQ

It’s summer vacation season. Hams are on the road, taking their VHF/UHF FM radios with them. As they travel, to their dismay they discover that they can’t use many of the repeaters they pass on the road—even repeaters they hear on the air that are being used by local hams. The reason is because the repeaters are tone-guarded, and the mobile ham doesn’t know the tone.

Does the FM guru have a solution? Maybe, but first I think I’d better do the tone primer that all FM columnists must do at least once. I’ve been holding off, waiting to let the CQ VHF subscription base increase. We’re up, so it’s time. After this quarter’s column, I’ll try to let the subject go for a long time and look at other stuff.

What is “Tone”?

You’re reading CQ VHF, so there’s a pretty good chance that you are very familiar with tone squelch. However, I remember how baffled I was when I first encountered it in the early 1970s. What was this mysterious technology that allowed the signals of only certain select hams into a repeater? I wasn’t the only one in the dark. Lots of hams use tone squelch today without understanding it. Many operators haven’t figured out how to make it work and they are stuck on carrier-access repeaters. Therefore, here’s the tone primer.

First, the very simple explanation: Tone is a system that lets a receiver accept certain radio signals on a channel, rejecting other signals on that same channel.

Now I’ll get slightly more complex: Tone squelch is based on a low-frequency, low-volume pure sine-wave tone that is added to your voice, which is sent from your transmitter, and detected at a receiver to make the receiver do something—such as turn on a speaker.

To wrap your mind around just what that tone is, here’s my third explanation: Tone squelch is like someone sitting in the back seat of your car, blowing a soft, very low note on a clarinet while you’re talking. Really! The clarinet player holds the right note, and some receiver out there says, “Hey, that signal’s for me!” No tone and the receiver pretends that there’s nothing out there. (The receiver is a fan of clarinet music.)

This rose goes by many names. The generic name is Continuously Tone Coded Squelch System, or CTCSS for (not very) short. The name and the initials are both awkward mouthfuls, so I just call it tone (or tone squelch when I’m feeling formal). It is probably best known as PL, the initials of the Motorola trade name Private Line. Almost all the old-timers call it PL, although there are a few holdouts for the name applied by other companies, such as GE’s Channel Guard (CG). If I were a really old timer, I could tell you the name used by RCA, E.F. Johnson, and a handful of other legendary brands of commercial FM radios. Write to me if you feel the need to remind me. It is also sometimes called sub-audible tone, or sub-tone.

Okay, now that we’re on a first-name basis, just what is tone squelch? How does it work? What can it do for you, and what can’t it do?

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