Tone Squelch

A new Feature on 147.105: What Does it Mean to You?

As of May 20, 2008 I have implemented a new feature which may be of interest to some who use the repeater system. The repeater now transmits 103.5 Hz CTCSS tone. This article explains the potential benefits and discusses implementation for those who wish to take advantage of the new functionality.

Some background on tones and FM communications

Most repeaters have been tone access for years now. Requiring a sub-audible tone for access solves a lot of problems with noise and unwanted distant signals inadvertently keying up a repeater, while at the same time allowing the repeater receiver to be "wide open" to desired signals carrying the proper tone. The alternative at many sites would be to set the repeater squelch so tight that some user signals would be choppy or not into the machine at all. Tone activated repeaters make good sense; so much so that many frequency coordinators won't approve carrier access systems any more. Most repeater users appreciate a machine that sits quietly until it is needed, not keying up with blasts of noise or weak scratchy signals from a far away land.

So repeaters with tone access sit quietly until needed, but do our radios do the same? If using conventional carrier squelch, the answer is "sometimes". It all depends on the local noise environment and to a lesser degree, design of the radio's squelch circuit. In a world increasingly filled with things which pollute the radio frequency spectrum (other communications systems, computers, consumer electronics, alarm and security systems, to name just a few) it makes sense to protect all receivers in a communications system from unwanted noise or signals - or at least to have the capability of doing so. That means not just the repeater receiver, but user receivers too. In the commercial world this has been common practice for decades, and in many parts of the country hams are rapidly catching up. One simple way to do this is to use tone squelch in both directions: user radios transmit a tone to activate a repeater, and the repeater transmits a tone to activate user receivers. 147.105 now transmits a tone for this "other direction" tone squelch. It is up to each of us to decide whether this has any advantages for us.

Why this change?

When I had a high antenna at home, I was often hearing a distant repeater on 147.105 strong enough that I could not eliminate it with my radio's squelch. With low or indoor antennas I sometimes have noise from my own or neighbors' electronics "opening squelch" on my receiver. When mobile, I find myself turning the squelch up when driving through towns with lots of noise, only to turn it down again to hear the repeater when I am on the fringe of its coverage. I also hear that distant repeater when driving over hills or along a ridge line. Does any of this sound familiar? I'm sure I am not the only one who has ever wanted some way to eliminate some of these annoyances. Tone squelch will do exactly that.

Using tone squelch

If you don't need or want to use this feature, do nothing. Everything will continue to work just as it always has and you won't even know anything has changed (with one possible exception... see 'The only down side' below).

If you feel you would benefit from tone squelch, you will need to program your radio to look for the 103.5 Hz tone on the repeater. Virtually all amateur FM transceivers manufactured in the last 10 years have this feature, as do many older models. Some manufacturers call it "tone squelch" but the terminology may vary. On some radios the receive tone setting is completely separate from the transmit tone; you actually select 103.5 Hz in two different menu items, one for transmit and the other for receive. In others there may be only one menu option to set the tone, with another option to use that tone on transmit only, receive only, or both.

After programming, you can verify it worked with this test. Tune the radio to the 147.105 channel on which you programmed the receive tone squelch. Rotate the squelch knob on the radio fully counter-clockwise. You should hear silence instead of the usual hiss of an unsquelched radio. Now key up the repeater to be sure you can still receive it. Once programmed, your receiver will respond only to signals on that channel which carry the 103.5 Hz tone. Any noise or distant signal carrying no tone, or a different tone, will be ignored. This is a "per channel" setting just as the transmit tone, so it won't affect your other repeater or simplex channels.

The only down side

If this all sounds too good to be true, there is one down side. I guess at one time or another we have all hit the "reverse" button on our radio to call someone direct on the repeater output frequency. As an example, this often happens when you meet someone on the road and want to give them a quick call and invite them to switch to simplex, without going through the repeater to do so. But that might not work if the person you are calling has his receiver programmed for tone squelch. Why? Because his receiver is looking for 103.5 Hz from the repeater transmitter, and ignores all signals not carrying that tone. After hitting "reverse", some radios will continue to transmit the 103.5 Hz tone used to access the repeater; in that case it will work just as before. However, some radios transmit no tone when in "reverse". That will cause problems if the person you are trying to call "direct" is using tone squelch on his radio.

There is a way to ensure this always works, but it would require cooperation between users and some extra radio programming. How? Suppose you and the person you want to call direct on the repeater output had two channels in your radios programmed with 147.105... one channel would have the normal repeater settings, +600 offset, transmit tone 103.5, receive tone 103.5; the other channel would be 147.105 simplex, transmit tone 103.5, no receive tone. Switch to the simplex channel before making your "direct" call to the other station. You will be transmitting on the right frequency with the tone his radio is expecting to hear. He should then switch his radio to the simplex channel to answer. Is it worth the trouble? Users will have to decide and work this out among themselves. [I will be using tone squelch on my radios. I will have the 147.105 "special simplex" channel programmed as outlined above so as to be ready if I ever feel the need to use it]

"Flawed" tone squelch in some radios?

The way this should work (and the way it does work with most radios) is this: once receiver tone squelch is activated for a channel, the setting of the squelch knob on the radio becomes completely irrelevant for that channel. No matter where that control may be set, the radio remains quiet unless it detects a signal carrying the proper tone. When it does, it will receive that signal equally well no matter where the squelch knob is set. It is like the squelch is "wide open" even for very weak signals, as long as the proper tone is present. This is exactly what we want, as it ensures unwanted noise doesn't come blasting through when the repeater isn't in use, while letting us hear the repeater even if we happen to be in a fringe area where the signal is weak.

Some radios have a slightly different implementation of this feature. They will correctly ignore noise and signals not carrying the proper tone, but when a signal carrying the tone comes along they require it to also be strong enough to open the radio's carrier squelch (the one controlled by the setting of the squelch knob). This is a relatively minor annoyance, but if you have one of these radios and operate from areas where the repeater is weak you will need to make sure you don't have the squelch turned up too far.

Reverse burst supported

This section applies only to those repeater users who may be using commercial radios (Motorola, GE, etc.). The repeater does use "reverse burst" at end of transmission - that is, before the repeater carrier drops. If you are using a radio which supports reverse burst on receive, this will eliminate hearing a "squelch crash" when the repeater stops transmitting. Unfortunately there are no "made for ham" radios which support reverse burst, to the best of my knowledge.

Last update March 20, 2012

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