KN6HSR/WRMM317
FRS and Tones
The last article was a general introduction to FRS, Family Radio Service. Now we’ll cover a few more details. First up is TONES!
PL, Private Line, Channel Guard, CTSCC, Tones, call them what you will, they are just different names for the same thing. It is not a form of encryption, your transmission is still able to be heard by anyone. It is a way to reduce hearing unwanted transmissions on the same channel you wish to operate on. Maybe your group wants to stay in touch while out for a hike, but doesn’t want to hear some fishermen chatting on the same channel. You could switch to another channel, where you might unfortunately have a similar problem. Instead use tones.
I think CTCSS is the best term to use as it describes what is happening. CTCSS stands for Continuous Tone Coded Squelch System. Let’s start from the end.
A Squelch System is a way to disable the audio on a receiver. Many radios have a squelch knob. As the squelch knob is turned clockwise, a stronger signal will be required to turn on or “open” the audio on the radio. It is normally adjusted so that the audio will turn on slightly above the ever present level of background noise. Sweet, no noise to bother your partner, assuring domestic tranquility.
Tone coded. Instead of a knob that sets the squelch level, a tone is used. If the tone is not present, it is like having the squelch knob turned all the way up, blocking all audio. If the tone is present, the audio is “opened”, or turned on. A tone is just that, a sound, in the low end of the hearing range, between 67 Hz and 254.1 Hz. This is Hz, not KHz or Mhz. 67 Hz is just above the annoying 60 cycle hum we sometimes hear, while 254.1 is a C4 for those musically inclined. But we don’t hear them on our HT!
What! Is my hearing aid malfunctioning? No. The radios are designed with an audio filter that blocks any sound below about 300 Hz. The tone is there, we just don’t know it.
Enter Continuous. The tone is there during the entire transmission, we just don’t hear it because all audio below 300 Hz is blocked. Hopefully the Continuous Tone Coded Squelch System makes some sense now.
You can use CTCSS on Transmit and/or Receive, sometimes called Encode and Decode. If you transmit on a different tone than the receiver is set for, the receiver won’t hear you. If you have your receiver tone set differently from the transmitter, you won’t hear the transmission. Simple as that. Your group operates on one tone, and the fishermen on another tone. You hope.
What can go wrong? There are 50 standard CTCSS tones, although a subset of 38 is sometimes used. This can lead to a problem. “I’m on tone 2, set your radio to tone 2.” If one radio has 50 tones, and the other has 38, the tone numbers may not match up. What you must do is specify the tone frequency, and you must know the correspondence between your tone # and the tone frequency. “I’m on tone 2, which is 71.9 Hz for me. Let’s use that tone.” is much better than just “use tone 2”. An example is in Table 1.
One final gotcha, or tip, is that if your tones are turned off, or tone 0, you will hear all transmissions on a given channel. This might be helpful to monitor for traffic, but you won’t know what tone is being used, if any, so your response might not be heard. And it won’t block any unwanted traffic.
This is written for novice FRS users, but everything here applies to GMRS and HAM radios.
73, SPARKY, KN6HSR