Today's land-mobile-radio communications systems have more features and functionality than ever before. In conventional radio, one of the most used and sometimes misunderstood features is CTCSS, which stands for Continuous Tone Coded Squelch System.

Different radio manufacturers give CTCSS different names. Motorola calls this feature PL or Private Line, Kenwood calls this feature QT or Quiet Tone, and GE/Ericsson/Harris calls this CG or Channel Guard. The amateur radio community simply calls this feature CTCSS. However, no matter what name you give CTCSS, it still functions the same.

CTCSS employs sub-audible tones — which are below the normal 300 Hz to 3,000 Hz voice range of the radio — and uses 67 Hz to 254 Hz tone modulation placed upon the carrier with the voice traffic. These tones are selected from an industry-wide accepted list of tones (see below) and generally are available in your radio's programming selection. The CTCSS function allows for encode and decode, whereby encode implies that a sub-audible tone is transmitted from the subscriber radio on the uplink and decode implies that a sub-audible tone is required to open the squelch on the subscriber radio receiver.

In the amateur-radio community, CTCSS encode is often referred to as “tone” in radio programming, while CTCSS decode is referred to as ”tone squelch.” In commercial radio programming, this same concept is referred to simply as PL, QT, CG (depending on the manufacturer) transmit and receive. You can have a different encode tone from the decode tone if your own transmitter is part of an interference problem.

CTCSS is used to access repeaters and networks, allows multiple users to communicate on the same frequency (channel) without receiving radio traffic from each other, and is employed to improve radio operation in high RF noise, interference and congested areas. The operation of a receiver without CTCSS employed is known as “open carrier squelch” or simply “carrier squelch.” Operating a receiver in open-carrier mode allows the radio operator to monitor and receive any traffic on that particular channel.

Normally, on a wideband system where the maximum deviation is 5.0 kHz, the CTCSS deviation on the encode side is 0.7 KHz. On a narrowband channel where the maximum deviation is 2.5 kHz, the CTCSS deviation is 0.3 kHz.

All conventional radio subscriber gear has a method for monitoring a given channel before transmitting. In generic terms this is called a "channel-monitor feature." Manufacturers are required to have this feature in place on their subscriber products and radio operators are required to use this feature and monitor the channel before transmitting, to ensure that they are not interfering with another user operating on the same frequency.

Portable radios have a monitor button, typically on the side of the radio body, for the user to press and momentarily determine whether another user is present on the channel. Mobile radios also can employ a button on the front control panel for monitoring; however, monitoring a channel with a mobile radio can be accomplished with the removal of the microphone from the hanger, provided that the radio programming enables the function to operate. This microphone function ensures that the radio operator is not distracted from operating the motor vehicle while using the radio.

Some other methods employed to aid the radio user in monitoring the channel before transmitting include the programming of the radio's LED indicators to illuminate while the channel is busy, even though audio is not heard from the speaker. Additionally, a channel-busy feature can be enabled during programming; this will inhibit the radio from transmitting until the channel is clear, which can be detrimental depending on how it is used. This feature is called “busy channel lockout.”

Ira Wiesenfeld, P.E., has been involved with commercial radio systems since 1966, and has experience with land-mobile-radio, paging and military communications systems. He holds an FCC general radiotelephone operator’s license and is the author of Wiring for Wireless Sites, as well as many articles in various magazines. Wiesenfeld can be reached at iwiesenfel@aol.com.

Christopher Dalton has designed, staged and implemented virtually every kind of LMR system in his two-decade-long career, including conventional, trunked, simulcast, Project 25, single-site and multisite. He holds an FCC general radiotelephone operator’s license. Dalton can be reached at cdalton@fairpoint.net.

Next: The pros and cons of CTCSS.

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Sidebar: CTCSS TONES

(All in Hz)

  • 67.0
  • 69.3 (or 69.4)
  • 71.9
  • 74.4
  • 77.0
  • 79.7
  • 82.5
  • 85.4
  • 88.5
  • 91.5
  • 94.8
  • 97.4
  • 100.0
  • 103.5
  • 107.2
  • 110.9
  • 114.8
  • 118.8
  • 123.0
  • 127.3
  • 131.8
  • 136.5
  • 141.3
  • 146.2
  • 151.4
  • 156.7
  • 162.2
  • 167.9
  • 173.8
  • 179.9
  • 186.2
  • 192.8
  • 203.5
  • 206.5
  • 210.7
  • 218.1
  • 225.7
  • 229.1
  • 233.6
  • 241.8
  • 250.3
  • 254.1

Note: Not every radio can do every one of these tones.