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  CTCSS doesn't fix anything!
(It just hides it)

By Mike Morris WA6ILQ
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A posting on the repeater-builder mailing list said:
I can tell you that the same tones on different frequencies inside the same site can cause a problem. My 2-meter repeater was on 151.4, the same tone as the local high band fire department, and also on the VHF community repeater.
When a combination of the units with 151.4 came up, I had intermod on my 2-meter machine. Also at times there was noise on the fire channel that we could tell disappeared when the 2-meter dropped along with the community repeater. Luckily I own the tower so I was able to move my 2-meter repeater to 123.0 and it happened that my private channel on the community repeater was also 151.4 which I also changed. Now I try to make sure that every PL inside my site is different. Since there are NO two PL's the same, the problem went away. Our Motorola Tech told me this is common at tower sites using the same PL on different frequencies.

Well, I can guarantee that changing tone frequencies did not solve the RF mix. The fire department radio tech probably determined where the RF mix was and fixed it.

Too many people think that changing the tone (or adding tone to an existing carrier squelch channel) can fix a problem. The use of any selective access system (tone or digital) doesn't solve interference issues, it just covers them up so that you don't hear them. Uninformed people think that if they can't hear the interference then it's not there.

If CTCSS, also known as "Private Line" (PL), "Channel Guard" (CG), or any one of a half-dozen other names, is a new topic to you I really suggest that you go read the article titled "A Historical and Technical Overview of Tone Squelch Systems", then come back here.

The Big Misunderstanding:

CTCSS does not prevent or cure RF interference, it just adds selective earplugs. If two RF signals are on the same frequency at the same time, there will still be interference (even if one of the signals is a mix product, radiated noise from a cheap switching supply, excessive deviation on the adjacent channel (sometimes called "spillover"), or simple RF grunge). If the signals are close to the same amplitude, the user will hear a heterodyne or beat note. If there is 6dB or more of difference then ther is no beat note.

The misunderstanding comes from the fact that if a CTCSS decoder is being used on the receiver and the grunge / unwanted signal has no tone (or a different tone), the receiver will not unsquelch. As said above, uninformed people think that if they can't hear the interference then it's not there. Sorry, but it's still there. If your receiver is hiding behind a tone decoder then you just have electronic earplugs in place.

To diagnose an interference problem you need to listen in carrier squelch mode (it is my personal belief that every repeater, even if it normally lives in CTCSS mode, should have a carrier squelch mode that can be commanded from the input just to allow diagnosis of interference problems). I can't tell you how many times hearing several seconds of a mix has allowed me to determine what transmitters are mixing. If the fates are being good to you that day, you might even hear a dispatcher's voice, or wonder of wonders, a callsign.

One of the more useful tools for finding problems is an all-mode receiver that covers the frequency of interest - one that includes AM, SSB, Narrow FM, and Wide (broadcast) FM. A land mobile receiver (i.e. one with filters for 5 kHz deviation) can hide a problem. Years ago we had a spur from a broadcast transmitter and only by chance luck did we find it early - and only because a user had a scanner with the feature that allowed the user to specify the signal type. He punched up the repeater input, put the scanner in wide FM mode, and heard the FM station - on 440 MHz! The deviation was multiplied, but it was identifiable enough to allow you to match the audio as you tuned across the FM band on an adjacent broadcast FM receiver.
Sometimes the SSB mode of the multimide receiver helps identify the type of interference. Sometimes you need the narrow CW filters to nail it down. And you really want to check the harmonic and subharmonic frequencies. I've found that with some types of noise you need to go down in frequency to the lowest point you can find an identifiable signal. And on others, you need to go up in frequency. Since harmonics go down in RF power level as you go up in frequency, you can get closer (physically) to the emitter. I once used an AOR scanner in a shielded metal box to find an interfering signal - it was a lawn sprinkler controller. The interference was being received by a high band land mobile base station, and it took looking for it on the 9th harmonic to get close enough to identify which of 7 controllers (side by side, mounted on a wall) it was.

Back to CTCSS:
CTCSS is frequently used to preclude nuisance kerchunks in high RF environments as well as helping to solve interference problems. 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. Some repeaters may also generate a subaudible tone on the repeater output so that repeater user's radios that are capable of decoding the tone will not hear other signals on the channel that would otherwise open their squelch.

A classic case of "not getting it":   (from a ham club newsletter)
The existing PL is 179.9 Hz and the new PL will most likely be 151.4 Hz barring any unforeseen issues. The reason for the change is that 179.9 Hz is a third harmonic of 60 Hz and there is a lot of electrical noise at the site from a utility pole that not only causes issues to weak signals but falses the PL decoder as well.

The correct solution? Get the interference from the utility pole fixed! Usually it's a bad splice in the cabling to the transformer, but occasionally it's the transformer itself. By your own admission the interference is preventing your system from hearing your weak users!
If it's a shared radio site, then the problem is probably affecting other systems as well!
To fix it, first, call your site manager. He should be informed about any issues that affect more than one tenant. If the dirty power transformer is a recurring problem he may already have a trouble ticket open at the electric utility. If not, he'll probably know someone in their two-way shop, then call him to find out who is the proper person in the power line repair department to call. Or he might discover that the 2-way shop can issue a repair order themselves.
And you need to leave the 179.9 Hz tone in place until it's fixed, perhaps as a remotely enable-able secondary tone.

In closing, a couple of paragraphs from Wikipedia:   (but Wikipedia pages change, what you see if you go there may be different)

Family Radio Service (FRS), PMR446 and other "bubble pack" radios often use from 10 to 38 different CTCSS tones (the number depends on the manufacturer), usually erroneously called "sub-channels", or "privacy codes" in the sales literature. While these do not add to the available number of conversations which can take place at once in a given area, they do reduce annoying interference experienced by users. However they do NOT afford any privacy, no matter what the sales literature says. A receiver with the tone squelch turned off (i.e. in carrier squelch mode) hears everything.

It is a bad idea to use any coded squelch system to hide interference issues in systems with life-safety or public-safety uses such as police, fire, search and rescue or ambulance company dispatching. Adding tone or digital squelch to a radio system doesn't solve interference issues, it just covers them up. The presence of interfering signals should be corrected rather than masked. Interfering signals masked by tone squelch will produce apparently random missed messages. The intermittent nature of interfering signals will make the problem difficult to reproduce and troubleshoot. Users will not understand why they cannot hear a call, and will lose confidence in their radio system. In a worst-case scenario in a life safety environment a missed message, or a misunderstood message, will result in one or more deaths.

Contact Information:

The author can be contacted at: his-callsign // at // repeater-builder // dot // com.

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