Interference on 856.5125 (AA trunk)

[Llwellyn]

Anyone else getting significant interference on this frequency when listening to AA County? It's so bad I can't make out the audio on the conversations, it's just heavily overmodulated voices in the background constantly. I'm having trouble pinning down the source (in order to acquire a notch filter). I'm having no trouble with any of the other channels, finally.

 

[troymail]

I'm not having any special problems with that freq -- AA County
just seems (to me) to be overall just weak today... not sure
why -- have to assume it's the weather.

 

[Llwellyn]

Here's a sound sample of the interference... first 3 transmissions are on 856.5125, the last 3 are on 854.5125 and another frequency.

http://www.llwellyn.com/noisesample.mp3

I'm missing better than half of the traffic I'm trying to catch due to this problem now. Very annoying!

 

[wadeless28]

Is your scanner stationary? Sounds like you might have a new cell tower in your area. Sounds like the same interference I get when I'm in close proximity to a cell tower.

I myself dont have that interference on my home scanner.

Or I could be way off. 8)

Mike

 

[Llwellyn]

Yeah, that's my base scanner. I haven't had it in the car yet on a different antenna setup to see if I still get it from the front yard area or mobile, I usually just use the Pro-92 or the 29 on simplex from the car.

I know there's a cell tower at 16th and Rt 2 in the shopping center; also, the new RINS/ITAC tower went up somewhere in Brooklyn Park but that shouldn't be an always on thing; they supposedly have to request the system to be repeater-enabled on a use-by-use basis before the repeaters are active.

I was just griping a bit, that's all.

 

[TinEar]

Llwellyn, it sounds to me very much like intermodulation being caused by proximity to a strong signal source. At one point in your recording you can just about make out the voice of the offender. It appears to just be swamping your receiver. If it's a single source, your only hope would be to get some kind of a notch filter to get rid of it. The problem is not simple. First, you need to identify the source and its frequency, filter it out, and then hope it's not a multi-source problem requiring a room full of filters. A directional antenna has the potential to help - assuming the noise source is not in the path of the desired signal you want to monitor. Of course, how many of us listen to a single signal source in a fixed direction? We generally need omnidirectional antennas which produce omnidirectional problems. I assume you've tried all the normal things like moving the radio to different locations in the house or my favorite which is tilting the whip in various directions which has the ability to maximize signal reception in one direction and attenuating signals in others.

 

[Llwellyn]

I did a little bit of digging in this... the interference is clear voice, I'm trying to narrow down what it might be. Fortunately, that means it's likely not cellular.

One thing that is puzzling, is while listening today trying to pin it down, I'm getting P25 digital traffic on this frequency. Did AA start using digital channels?

No one else for about 600 miles has a license for 856.5125 according to the database and the ULS.

 

[RyanXTS]

To the best of my knowledge the SOS channels are digital & encrypted on the AACo trunk. Maybe that was it?

 

[troymail]

AA County DOES use digital for a few things -- has for a while.

Some I-Call (private call) communications have been noted
in digital mode (usually police supervisor radios).

Police Narcotics (TG 32208) digital AND encrypted... it is a fairly
active TG too.

A reliable source told me in the past that the AA County system
only does encryption in digital mode... the Radio Reference database
info for AA County confirms what I suspected for some time now -
TG's above 32000 appear to be encrypted (look at Narcotics, Fire
PIO, Fire Marshall, etc.)

EDIT: I just listened to the sample you posted -- definately not
encryption.... the last section was clearly an AACo PD unit... some
of the earlier stuff sounded like weak distant users (I know you said
no one else is listed nearby but sometimes signals carry a long way).

Is this still going on?

 

[Llwellyn]

It is still going on. Let me tool around for a bit and get a new sample up (the previous sample was from normal scanning, and didn't have digital). I'll record a bit of open squelch and post it in a bit as an edit to this post.

Actually, strike that. I did some searching, and the strongest interference is actually on 856.5000.

It could be TV Audio from Channel 13 (215.7500) or Channel 2 (59.75) or Broadcast FM 107.1. They all line up within about 1Mhz of the base frequencies of 856.5 and 856.5125.

It could also be something else, but tonight all the interference has been music, with no voices. The other night, it was all voices, no music... I'm going to try getting an FM notch filter and see if I can't get it to behave after that. I'd hate to have to undo the new antenna setups since I gained so much other reception.

 

[troymail]

It sounded more like PTT transmissions rather than continous
broadcast like TV or radio... so, at least from the posted sample,
I'd rule them out.