$250 For the First One Who Finds The Digital Interference on 159.840 in Northern Ca

We have been trying to track down the semi-strong interference to our commercial repeater input in the Grass Valley area that is some kind of digital modulation. It has been going on for a couple of years now.

The first time I heard it I thought one of our customers' mic had gotten stuck in a Detroit Diesel truck. But it is some kind of digital modulation. Sometimes it is strong, others weak and sometimes nonexistent. No real pattern established. When it is strong, it is usually 1/2 second pulses every 3 seconds but sometimes it comes in as a long continuous transmission like a voice.

It is not P25, I know what that sounds like. It does not sound "exactly" like any digital mode I have heard samples of.

I have not been able to locate any really likely license in the FCC ULS though we typically get interference on various frequencies from the Plumas County area, Stockton area, and possibly Santa Cruz. A friend in Fairfield listened locally on a high antenna and could not hear it during a time I know it was active.

250 bucks to the first one to correctly identify who/what it is. I will gladly answer questions for more detail and try to get a recording of it but for now I am posting what I have to go on.

Thanks for looking and let the hunt begin
Birken

No reward needed, but did you see this?

159.840 Search Results

Can you post a clip of the sound? This might assist others in identifying the source.

If you get interference from Santa Cruz, then this is a possibility...

WQPU454 (LOCKHEED MARTIN CORPORATION) FCC Callsign Details

It's licensed for TRBO, which would match the sound you mention, And I'd bet it's a Capacity Plus system, which could also match the "pulse" noise.

If others cannot hear it on different equipment not co-located at the site (such as your friend with the high antenna) then it might not be fundamental to that frequency. In other words, it could be an intermodulation mixing product caused by any number of things internal and external to the primary site receiver.

I assume you have had RF technicians at the site that have already ruled this out? Because if not, you may be looking in the wrong "places" (not the tuned receiver frequency).

-Mike

BirkenVogt said:

When it is strong, it is usually 1/2 second pulses every 3 seconds but sometimes it comes in as a long continuous transmission like a voice.

Click to expand...

I agree with Kevin. This describes Capacity+ distinctly.

I've got a remote receiver on top of one of our buildings in Santa Cruz. I just popped in 159.840 and have been listening for a few minutes and have heard nothing. I can try again on Monday and listen for a bit.

I'm not hearing anything at all on that frequency, and the Lockheed site is above us. I should be able to hear it no issues.

Stay tuned. I will report times of day when it comes on.

I have not heard it since I posted this but it seems like a morning/daytime thing and it may be propagational. I will give any updates and hopefully a recording when I can.

Mototrbo Capacity Plus sounds very much like TRBO voice but in short 1/2 second bursts every 3 to 5 seconds as you said. Check out the video below of conventional Mototrbo voice. Each time the guy keys up the XPR portable in this video you hear low pitched slow data, then a medium speed data and finally a higher speed unkey message. If it were Capacity Plus, it would sound like the beginning (low pitch/low speed) data in very short, frequent intervals. Let us know.

https://www.youtube.com/watch?v=ltUQc-5MTWE

Could be several people. Skipp May in Dixon comes to mind, he has a small FB6 operation and has been playing with Cap+ also Norden is running Cap+ and is just getting started for the season, so they might be more active now. You should do a ULS search and plot your neighbors.

Chances are they are ALL outside your radius by a long shot. Good luck with that.

I'll add my (professional) opinion...

As others have pointed out, it does sound like a MOTOTRBO Capacity Plus system. It could also be a Linked Capacity Plus system (multi-site Capacity Plus).

And, as others have pointed out, assuming that this system is always on-the-air, the way you describe hearing it at times, with a very strong signal, and not hearing it for pretty long stretches at other times, it sounds like the system is some distance away and propagation (such as 'ducting') is what is responsible for being able to hear it when you do hear it.

This time of year is one of the worst (or the best, if your a ham operator) for such propagation. We've had complaints from one of our customers, over the last few weeks, of a problem that now appears to be interference from a very distant station that they normally would never hear.

And of course, if this interference is being caused by station(s) that are normally out of range of your normal coverage area, and you're hearing them due to 'better' propagation of these signals, then their is nothing that can really be done to reduce or eliminate this interference, that won't effect your system in a negative manner (other than move to higher frequencies, such as UHF, where the signals don't propagate through 'ducting' as easily as they do at VHF frequencies).

John Rayfield, Jr.
Rayfield Communications

Hello,

A Capacity Plus system likely has more than one frequency at a site. Each frequency supports two voice channels. Capacity Plus uses a rest channel that sends out the 1/2 second bursts every three or four seconds. When one talkgroup becomes active, the rest channel moves to the other channel on the same frequency. As long as only one talkgroup is active, the system will stay on the same frequency. If two or more talkgroups become active, then the rest channel moves to another frequency at the site and the original frequency will become silent when the talkgroups stop transmitting.

I suspect only one frequency at the other site is causing your interference when that frequency is used as a rest channel.

73 Eric

I think I managed to attach a sound clip of two of the bursts, ignore the low frequency tone after the first one. I will try to do better next time but this was around 5 minutes ago

Here is a longer clip of the actual sound, sorry the level is so low

If you feel up to it, you may want to consider running DSD (or a derivative) and listening to the audio. Alternatively, you could run DMRdecode (using a computer and discriminator equipped scanner) to determine the proper configuration to program into a MotoTrbo radio and use that to listen to the users. TT

Definitely sounds like Mototrbo Capacity Plus to me.