Squelch Explained: Carrier, Tone and Noise Squelch on Ham Radios

Anybody who has turned on an FM radio with nobody on the channel knows why squelch exists. Without it, the loudspeaker produces a loud, steady hiss of noise, and the operator has to turn the volume down and up each time that a station speaks. The squelch circuit solves this by muting the audio until a real signal arrives. It is one of the simplest functions on a VHF or UHF radio, and yet many operators do not know how it decides when to open, why there are several kinds, what the numbers on the menu mean and why the weakest signals are sometimes cut off by a setting that seemed reasonable. This guide explains how squelch works, compares the types, gives a simple method for setting the threshold and lists the common faults and their cures.

What a Squelch Does

A squelch is a switch on the audio path that is controlled by a detector. When the detector says “there is a signal”, the switch opens and you hear the audio. When it says “no signal”, the switch closes and the radio is silent. The Wikipedia article on squelch describes it as a circuit that suppresses the output of a receiver in the absence of a sufficiently strong desired input signal. The important word is “decides”: the radio does not know what is a signal and what is noise, so it uses an indirect test, and different tests give different squelch types.

The squelch is also a convenience that helps to save power. A handheld that is silent most of the time draws much less current than one whose audio amplifier drives a loudspeaker all day, and for that reason a good squelch is part of the battery life of a portable radio. On a scanner or a radio that monitors several channels the squelch is the trigger that makes the scan stop on a busy frequency.

Noise Squelch: How an FM Receiver Hears the Silence

The most common type in FM radios is the noise squelch, sometimes called carrier squelch. It uses a property of FM. When there is no signal, the receiver output contains a large amount of wideband noise, including high frequency components well above the voice range, say 6 kHz and higher. When a strong signal arrives, the limiter and the discriminator suppress the noise, an effect called quieting or capture, which is explained in the article on the capture effect, and the high frequency noise falls to a low level. The squelch circuit takes the output of the discriminator, passes it through a filter that keeps only the high frequencies, and rectifies it. The resulting voltage is high when there is only noise and low when there is a signal. A comparator, set by the squelch control, opens the audio when the voltage drops below the threshold.

It is a clever method, because it requires no signal level measurement at all, and it responds to the quality of the signal and not to its strength. Its weakness is that it needs a certain amount of quieting before it opens. A very weak signal that does not reduce the noise enough will not open the squelch, though your ears could copy it. This is the reason why a tight setting makes you miss distant stations.

The Types Compared

The S meter squelch is used mostly on HF transceivers and some FM sets. On SSB and CW the noise squelch does not work well, because there is no steady carrier that quiets the noise, so radios use a level-based squelch that opens when the signal is stronger than a set value. It follows that the setting has to be adjusted for each band and each condition, since the level of the background noise changes. The tone and code squelches solve a different problem: they do not tell you whether there is a signal, but whether it is addressed to you.

Tone and Code Squelch

A tone squelch opens only for a signal that carries the right sub-audible tone, as used on repeaters. Our guide to CTCSS tones and DCS codes gives the lists of tones and codes and shows how to find the one for your local repeater. The description in the article on CTCSS gives the system a more formal definition. For the operator, the most important thing to know is that tone squelch is not a privacy feature, it only filters. A station without the tone does not open the squelch of your radio, but the signal is on the channel and can be heard by anybody who turns the tone squelch off. Use it to keep the channel quiet when other users or distant interference are present, and always remember to switch to carrier squelch or to use the monitor button when you want to check whether the channel is really free before you transmit.

The digital voice systems have their own codes. A DMR radio has a colour code, D-STAR has a call sign squelch, and System Fusion can use a digital group ID. The aim is the same as for the analog tones, to open the audio only for the signals meant for you. The comparison in our guide to DMR, D-STAR and System Fusion explains how they work.

Setting the Threshold

The squelch control on a handheld is usually a menu with numbered levels from 0 to 9 or a knob on a mobile radio. A good setting is a compromise between quiet and sensitivity. The procedure is simple.

1. Turn the squelch down so that the audio opens and you hear the noise on an empty channel, at a comfortable volume.
2. Turn it up slowly until the noise just disappears. This is the threshold, the point at which the circuit starts to close.
3. Add one step or a small turn to make the setting stable. This gives a margin so that small variations in the noise do not open it.
4. Test it with a weak signal or a repeater at the limit of its range. If the first words are cut off or the audio chops, you set it too tight, so lower it one step.

A level that is a bit too high is the cause of many complaints about the “poor range” of a handheld. It is not the antenna or the power, but the squelch that does not open for a signal that is perfectly readable. If you want to check whether a signal is hiding below the threshold, press the monitor button, which opens the squelch temporarily, and listen. A very weak signal is usually noisy, but a message that is still understandable shows that you could lower the setting. The article on VHF and UHF operating basics describes how range and settings interact on a typical contact.

Common Problems and Cures

• The squelch does not close and the hiss never stops. The threshold is too low, or there is a strong interfering signal on the channel. Raise the setting, and look for a nearby source of noise if the problem persists.
• The squelch opens at random. Noise bursts from electrical equipment, a poor antenna connection or a nearby transmitter can trigger it. A tone squelch reduces the effect, because only the correct tone opens the audio.
• The first syllable is cut off. The squelch takes a moment to open, particularly with a tone or code squelch that must decode the tone before it opens. Ask the other station to pause a moment after pressing the push to talk, and check the setting of the repeater tone.
• The radio is silent but others hear the repeater. You have the wrong tone, or you have set a tone squelch for receive that the repeater does not send. Switch the receive tone off, and try again.
• A loud burst of noise at the end of each transmission. This is the squelch tail. Many repeaters send a short reverse tone burst to close the squelch of the receivers smoothly, and a radio with a squelch tail eliminator will mute the burst.
• Audio chops on weak stations. The squelch hovers at its threshold and opens and closes. Lower the level a step, or use the monitor and accept some noise.

Squelch and Repeaters: A Short Story

Consider a typical evening on a local repeater. You are driving on the edge of its coverage and the signal comes and goes. With a tight squelch you hear nothing for stretches of the road, and you assume that you are out of range, but the monitor button shows a readable voice under the noise. With a loose squelch you hear a continuous noise between the words, and the radio is tiring to listen to. The best result usually comes from a moderate setting plus a tone squelch that matches the repeater, which keeps the speaker quiet for the interference and opens quickly for the repeater. When you reach a hill with a clear view, the signal is strong, and even a tight setting opens at once. The lesson is that the squelch does not change the radio, only the point at which you decide that a signal is worth hearing, and the choice depends on the conditions and on your own patience for noise.

Squelch in Your Scanner and Your Receiver Software

The same ideas appear in scanners and in software defined radio programs. A scanner uses the squelch to decide when to stop on a channel, and a poor setting makes it stop on every burst of noise or skip weak stations. Many receiver programs offer a level squelch in dB that is easy to read on a waterfall: set it just above the noise floor, and the audio opens only for signals that rise above it. For voice, also look for features that add a short hang time, so that the squelch does not close between words. On a handheld that you use on a trip, a good habit is to set the squelch at a moderate level at home and to check it at the new place, because the noise changes between a city and a mountaintop. The advice in our guide to the first handheld radio lists the features worth checking.

The Bottom Line

Squelch mutes the receiver when there is no signal. Noise squelch uses the drop in high-frequency noise that a real FM signal causes, signal-strength squelch uses the received level and tone and code squelches open only for a signal with the right sub-audible tone, digital code or colour code. To set it, raise the control until the hiss just stops and add one step, then check with a weak signal and the monitor button. Too tight a setting cuts off weak stations, and too loose a one leaves the hiss on. Tone squelch is a filter, not a privacy feature, and the monitor button is the way to check that a channel is really free.

Frequently Asked Questions

What is squelch on a radio?

It is a circuit that mutes the audio when there is no signal, so you do not hear the hiss of noise on an empty channel.

How should I set the squelch level?

Turn it up until the noise just stops, then add one step. Check with a weak station and lower it if the audio is clipped or chopped.

What is the difference between carrier squelch and tone squelch?

Carrier squelch opens for any signal that quiets the noise. Tone squelch opens only for signals that carry the correct sub-audible tone.

Does tone squelch make my conversation private?

No. It only keeps your receiver silent for signals without the tone. Anybody with the tone squelch off can hear the conversation.

Why can I not hear weak stations on my handheld?

The squelch may be set too high. Lower it a step or use the monitor button to see if there is a signal under the threshold.

Does squelch work on SSB?

The noise squelch does not work well, since there is no steady carrier. HF radios use a level-based squelch or leave it off.