CTCSS is a system whereby a (not really) sub-audible tone is encoded into the transmit audio of a radio and a receiver on another radio on that frequency will only open up (decode) when it picks up that specific tone. While the tones are called “sub-audible”, most people can hear them as a hum in their radio’s speaker when monitoring a radio or repeater that is encoding such a tone.
Higher frequency tones are more easily heard. Most radios capable of CTCSS have at least 38 tones between 67.0 Hz and 250.3 Hz, with some having even more tones over a wider frequency range. For instance, you may keep in contact with friends on 146.535 MHz Simplex. You or your friends may experience intermittent bursts of noise which open your regular Carrier Squelch (CSQ) with enough frequency to be really annoying, whether you are getting those bursts of noise on a base, mobile, or HT. If the operator being thus annoyed turns their squelch up enough to eliminate the noise, they might reduce or also eliminate the ability to hear friends who are calling who are below the squelch threshold.
In some cases, the noise bursts may be so strong that the Carrier Squelch will not silence them. These are where CTCSS really shines. Let’s say that instead of running your squelch up really high and risking missing calls or not eliminating the noise, you and your friends agree to set your radios to encode a certain tone on that channel when you transmit. For Simplex purposes, I would recommend a fairly high tone, say 203.5. The reason I recommend higher tones for Simplex is so that a ham trying to join the conversation will be more likely to hear and notice that a tone is in use in case he wants to join in. Noticing that everyone is using a tone, he won’t think your group is being stuck up if he calls and doesn’t get a response. If he knows you are using a tone, he can either guess at what it is (a skill that CAN be learned) or use his radio’s “Tone Scan” feature. Of course, popular local repeater tones may also be used.
Getting back to use of the tones, now that everyone is encoding that tone (“T” on most FM ham radios), if any member of the group is experiencing annoying bursts of noise, intermod, etc, he can turn on his radio’s decoder (“TSQ” on most FM ham radios) and his receiver will be silent until someone transmitting that specific tone keys their mic. While this feature will not stop interference from very strong noise sources, it will keep your receiver quiet until one of your group transmits.
CTCSS can be a great “quick fix” for all kind of noise, which may override your normal Carrier Squelch. One new ham was enjoying the local repeater when they held their bi-weekly nets, but a computer or other device in his home was breaking his Carrier Squelch on the repeater frequency, even with his squelch set as high as it would go. Since he was receiving that repeater full scale and that repeater’s trustee wisely encoded the same tone on its output that was required to access it on the input (more on this later), I suggested that he activate his CTCSS decoder (“TSQ”) on that channel. That way he could leave his base on that repeater channel and not be annoyed by the noise that was overpowering his regular squelch on just that one frequency. Problem solved!
CTCSS originally was developed for commercial Land Mobile FM radio systems such as police and business band, to keep the end-users from being constantly bombarded with chatter from other agencies or businesses who were on the same frequency. With growing numbers of FM repeaters on ham bands, it was soon put to use to keep users in distant towns from keying up the local repeater in addition to the one in their area during band openings or when operating from higher elevations. It excels in this purpose.
Back in the mid- 1990’s, quite a few FM ham radios might have come with only Carrier Squelch capability, with the option to add a tone board to allow your radio to encode a tone and allow access to repeaters that required a CTCSS tone to access them. Even more radios came with no CTCSS decode board to allow you to decode tones and keep the speaker on your radio silent. Again, such boards were available in many cases as an expensive “add-on”. Thankfully, for the last ten or fifteen years, just about all modern bases, mobiles, and HT’s, for FM ham radio use come equipped to encode (“T”) or encode and decode (“TSQ”) CTCSS tones.
One of my pet peeves with regard to repeaters is how many repeater owners and/or trustees will set up a repeater to require a tone on the input to key up a repeater, but fail to encode that same tone on the output for the convenience of the end-users. In cases where older equipment is being used and finding or purchasing a tone encoder might be difficult or expensive, I can understand why a repeater owner or trustee might not choose to encode a tone on the output. Just about all repeater equipment of newer vintage is programmable (just like your mobile or HT) for encoding and decoding tones. In those cases, setting up a repeater to encode a tone on the output involves a couple of minutes or literally just seconds of extra effort. That little bit of extra effort enables the users (supposedly the reason they go to the trouble and expense to put a repeater up to begin with) to turn on their decoders (“TSQ”) and put their carrier squelches on a minimum setting while screening out annoying noise from computers, car electronics, gas pumps, and more.
Encoding on the output also allows travelers and newcomers to find out what the access tone required to get into the repeater is, via the use of their radio’s “Tone Scan” feature. Setting the repeater up to encode on the output (IMHO) shows superior engineering and pride in the repeater by the owner or trustee, and something quite the opposite in those who won’t spend the extra few seconds or minutes to set up encode on the output, if such is enabled by a few key presses.
Of course I have heard numerous outlandish and illogical “reasons” for not encoding a tone on the output. “We don’t want our users to turn on their tone squelch because they might interfere with a distant repeater.” (Uhhh… really?) “Not encoding a tone on the output makes it easier for users to program the repeater in their radios.” (Uhhh, no… It increases the chances that they won’t hear the repeater, say if they program “TSQ” instead of “T”.) One of my (not so) favorites is, “the repeater passes tones.” This is almost as bad as no tone at all, maybe worse. Different radios have different tolerances and tightness with regard to the tones they emit. Running that through the repeater’s audio sections can cause variances in the tone reaching the end user’s radio, often resulting in some people opening up your decoder properly, some chopping in and out, and others not opening your decoder at all. “Passing tones” through the repeater rarely if ever works as well for the end-user as properly encoding a steady tone on the repeater’s output.
One of the biggest misconceptions I hear (too regularly) and usually from veteran hams is, “they PL’d such-and-such repeater. That means it is now a closed repeater and they don’t want us to use it anymore.” NOPE! Repeaters are PL’d to eliminate or reduce incidences of the repeater being keyed by users of another machine a long way off, during band openings or similar circumstances.
Rarely is simply adding CTCSS to gain access done with intent to discourage people from using it. Quite the opposite, it is done to make using the repeater a more enjoyable experience for everyone. On that note I will add that some of the modern, menu-driven radios can be challenging to program, especially for some of our senior veteran hams. If you know one of these old timers who have been shut out of a repeater that was recently PL’d, please offer to help them re-program their radio. That is one of the many rewards of the hobby, helping others.
Hopefully this has helped some to better understand CTCSS and how useful it is and can be on FM. As I noted elsewhere in this article, it IS fifty-plus year-old technology, not some evil combination of witchcraft and nuclear science, the slightest error in use of which might destroy the world as we know it.