Selective Calling & Tone Squelch

In a previous post we introduced the term squelch and how it was used in basic operation.  As mentioned there modern ham radio work (especially with repeaters) often involves other technologies that fall under a different category named selective calling, tone squelch being the most common form.

Selective calling is different from ordinary carrier or noise squelch.  In practice it is more of a security or channel sharing function.  Selective calling encompasses several similar technologies which largely do the same thing: prevent a transmitted signal from being received by other radios unless a particular code is entered by the sender.

From the Wikipedia article, “Selective calling is akin to the use of a lock on a door. A radio with carrier squelch is unlocked and will let any signal in. Selective calling locks out all signals except ones with the correct “key”, in this case a specific code.”

The most common form of selective calling in use by hams is a tone squelch system with the awkward name of continuous tone-coded squelch system (CTCSS).  Squelch in general is used to keep commercial and amateur radio repeaters from continually transmitting. Since a carrier squelch receiver cannot tell a valid carrier from a spurious signal (noise, etc.), CTCSS is often used as well, as it avoids false key-ups. Use of CTCSS is especially helpful where nearby repeaters may share the same frequency or in a high electrical noise or RF environment.


As the name implies, CTCSS sends out a continuous tone along with the transmit audio.  The tone is termed, “sub-audible”, although it is often a low audible frequency.  Most radio systems filter out these low frequencies so it is unlikely that you will hear the tone when listening to another ham unless you have an unusual radio and/or are wearing high fidelity headphones.

The CTCSS tone is selected by a repeater operator to avoid duplication with nearby repeaters on the same frequency.  In most cases, the objective is to reduce interference and not restrict legitimate access.  There are 100 established CTCSS frequencies but some are more commonly used than others.

CTCSS is often referred to as PL because it’s easier to say.  They mean the same thing but PL® (stands for Private Line) is a registered trademark of Motorola’s implementation and was the original employment of CTCSS.


Besides CTCSS, other forms of selective calling in use by hams include:

  • Selcall (mostly European)
  • Digital-Coded Squelch (DCS)

Why it matters to us

T2B04-2018Because various forms of selective calling prevent a signal from being re-transmitted (repeater) or received (simplex) without the proper code or tone, use of CTCSS or DCS is a possible reason other stations cannot receive you.  Especially on a repeater, if others cannot hear you it’s quite likely that you have the wrong code or your tone squelch is turned off.  How to know what the proper setting is?  Consult the repeater directory.

Some Useful Links

CTCSS – RadioReference

How radio “privacy tones” or CTCSS tones work – YouTube video


Get the Right Signal Tone – Ham Radio School

Hey, Why Can’t I Access the Repeater? – Ham Radio School

Differences between CTCSS and DCS – Retevis

CTCSS & Tone Burst Ham Radio Repeater Access – electronicsnotes

A Historical and Technical Overview of Tone Squelch Systems – WA6ILQ

Repeater Related Terminology – Repeater Builder group


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 Adj FT7900

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 selective calling or one specific variant known as 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