Squelch is a funny word that is familiar to many of us without understanding what it really means. Hard to improve on the definition beyond that in the Technician license exam question:
Most useful when using voice modes (phone), squelch makes radio operation more bearable by turning off the audio when there is no valid signal. Without squelch our radios would be cranking out a lot of unwanted background noise.
The reality of both AM and FM radio (which we covered recently) is that there is electrical noise in the bands from many sources, natural and man-made. This noise is often randomized so that it appears as hiss or fuzz (white noise) from radio receivers. A squelch circuit mutes receiver audio to block the noise when there is no real signal. Squelch acts as a noise gate which closes for random noise and opens when a real signal (such as modulated voice) appears on frequency. How the squelch circuit determines what is a valid signal and what is noise varies; there are a few common techniques (refer to algorithm link in references below).
Squelch is built into more expensive broadcast receivers. It may also be found on AM transceivers (particularly CB radios) and is a feature of nearly every FM amateur rig.
While less prone to electrical noise, FM technology is susceptible to a lack of true signal. Traditional FM receivers use an LC tuning circuit that generates ‘hiss’ with no signal present. You may have noticed this while tuning between stations on your broadcast FM radio. Since hams commonly use VHF/UHF FM transceivers for local chat and EmComm work, the squelch feature of our rigs is of particular interest.
Squelch setting is important because if you make it too tight you may not hear a weak signal; too loose and you get constant noise. General good practice (at least a starting point) is to turn down the squelch until you hear background noise (hiss), then increase the threshold until the noise goes away, then just a little more.
Squelch threshold is always settable on your FM transceiver. There may be a knob to turn as in the example below, or it may be through keypad menu.
Squelch tail is a common term that is related to all this. The tail is the brief “pfffft” sound heard when another station stops transmitting. Audio example on YouTube here. It is the natural presence of noise during the delay between the time a signal drops and the squelch kicks in. On a repeater it will often followed by an audible beep or similar courtesy tone.
So far we have discussed the basic squelch feature of common ham radio transceivers. There are other squelch techniques in common use with repeaters such as CTCSS, DCS, and PL. These fall into a separate category called Tone Squelch, mainly because they have a different purpose and functionality. We may discuss this in a future post.
Some Useful Links
Squelch setting – W3ATB blog
Squelch algorithms – PA3FWM site
Squelch Tail – DAP-COM reference