The mystery of the randomly changing CT codes

[Oldglide]

This started as what I thought was a reception problem, couldn't receive one of the local PD channels. After some investigation and a few suggestions I determined that the CT code had been changed from 103.5 to 67.0, but not on purpose. As I continued to monitor this channel the CT code changed daily sometimes in the middle of a conversation. Never on 103.5 but 67.0, 69.3, 71.9 and the list goes on. The reason for my concern has to do with public safety and the ability of the local PD to respond to any activity in my neighborhood, and there has been some, in a timely manner. After several attempts I finally contacted the local radio maintenance guy who is a member of our local FD. He was quite surprised in fact that this channel was even in use, appears to only be c2c, and that the CT code was randomly changing, "These radios are old, lots of different types and it looks like we've got a problem". He agreed that there was no way to manually change these codes. I invited him over to witness 1st hand what I found. At first the distortion was so bad the code search couldn't even identify the code. After a while he saw what was happening and even saw the code change in the middle of a conversation. He said he would let me know as much as he could about what he found. I would really like to know from a technical standpoint what happened. Stay tuned for more info as the story develops.

 

[SkipSanders]

Most likely? It's an interagency tactical channel, which is carrier squelch (they don't USE the CTCSS), but the various agencies involved still have their radios set to transmit (but not care about decoding) their own common CTCSS on the channel. They really should just transmit with no CTCSS, but it doesn't matter if they do.

 

[Avery93]

I have found my PSR-400 sometimes starts "detecting" random PL tones on transmissions that are actually carrier squelch. I'm not really sure why, but its probably caused by interference from nearby electronics combined with the GRE's sensitive PL decoder.

 

[ecps92]

What Freq and Agency?
What type of Scanner?

Many Scanners can and do False on Audio fluctuations...

This started as what I thought was a reception problem, couldn't receive one of the local PD channels. After some investigation and a few suggestions I determined that the CT code had been changed from 103.5 to 67.0, but not on purpose. As I continued to monitor this channel the CT code changed daily sometimes in the middle of a conversation. Never on 103.5 but 67.0, 69.3, 71.9 and the list goes on. The reason for my concern has to do with public safety and the ability of the local PD to respond to any activity in my neighborhood, and there has been some, in a timely manner. After several attempts I finally contacted the local radio maintenance guy who is a member of our local FD. He was quite surprised in fact that this channel was even in use, appears to only be c2c, and that the CT code was randomly changing, "These radios are old, lots of different types and it looks like we've got a problem". He agreed that there was no way to manually change these codes. I invited him over to witness 1st hand what I found. At first the distortion was so bad the code search couldn't even identify the code. After a while he saw what was happening and even saw the code change in the middle of a conversation. He said he would let me know as much as he could about what he found. I would really like to know from a technical standpoint what happened. Stay tuned for more info as the story develops.

 

[sfd119]

Yes, many mutual channels (esp firegrounds) use different PL tones to TX but they don't care about the RX.