Cell Tower Interference / SDR Trunk

[davejl2001]

I have been monitoring a P25 Phase II LSM 700MHZ system for a few years now with a Airspy R2 with no issues. The system is in the range of 769-774 MHZ. Recently a new cell tower was installed very close to my house (1000 feet). I’m now having problems with audio distortion ever since the new tower was turned on. I have played around with the gain with no luck. I’m using the latest version of SDR Trunk 0.6.1 final with my gain set at 15. My power receive level is within the range of 45dB-60dB and the control channel holds sold with no loss. Any recommendations or types of filters would be greatly appreciated. Thanks

 

[KJ4DHF]

There was someone on another forum at 1 time making SAW filters for this problem.

 

[rabbit108]

I have been monitoring a P25 Phase II LSM 700MHZ system for a few years now with a Airspy R2 with no issues. The system is in the range of 769-774 MHZ. Recently a new cell tower was installed very close to my house (1000 feet). I’m now having problems with audio distortion ever since the new tower was turned on. I have played around with the gain with no luck. I’m using the latest version of SDR Trunk 0.6.1 final with my gain set at 15. My power receive level is within the range of 45dB-60dB and the coudesntrol channel holds sold with no loss. Any recommendations or types of filters would be greatly appreciated. Thanks

You will need to specify the frequency range and type for a custom filter. Since you already have an R2, I suggest you download a copy of SDRSharp v1784 which includes the latest version of Spectrum Spy software.
https://airspy.com/downloads/sdrsharp-x86-dotnet4.zip



This what I get at my location with my R2 and no amp:

 

[davejl2001]

You will need to specify the frequency range and type for a custom filter. Since you already have an R2, I suggest you download a copy of SDRSharp v1784 which includes the latest version of Spectrum Spy software.
https://airspy.com/downloads/sdrsharp-x86-dotnet4.zip

View attachment 175596

This what I get at my location with my R2 and no amp:

View attachment 175597

Already tried something similar to this in SDR Trunk, but still receiving distortion in the 769-774 MHZ range.

 

[rabbit108]

Already tried something similar to this in SDR Trunk with no luck. Still getting distortion

I already tried something similar to this in SDR Trunk, but still getting distortion in the 769-774 MHZ range.

If the distortion is caused by overload from a strong local signal, you must determine the frequency of that signal to order a custom filter. If the distortion is not caused by overload, then you have a problem with SDR Trunk.

If you right click on the scan in Spectrum Spy, a yellow line will collect peaks from intermittent signals. The longer you let the scanner run, the more peaks you will collect.



If you are not able to run a Spectrum Spy scan, I am not able to help you any further. I don't do trunking.

 

[maus92]

LTE / cellular interference will be wideband aka 5-10 MHz per channel just below the talk-out frequencies for the 700 MHz public safety band (Band 14). Ironically it's FirstNet. A filter would need to block below 769 MHz as it is not a specific frequency. There is a band of uplink frequencies just above 775 MHz, but that would come from subscribers aka low power and unlikely to affect you.

 

[WB5UOM]

Might be interesting to see what it looks like if you zoomed in on the 769-774 area you mentioned

 

[davejl2001]

Here it is.

 

[WB5UOM]

Is any of that what you are trying to monitor?

 

[davejl2001]

Yes.

 

[wtp]

you might have to move.
one thing i used to do when going near a tower here is to point the antenna at the tower.
that is the null point for antennas.
best reception is on the broad side.
we now try to avoid going down the road and tower.

 

[maus92]

IDK, lots of usable signals in the slice of spectrum.

 

[WB5UOM]

Thats what I was kinda thinking, op have you tried a different software?
maybe. just a thought

 

[davejl2001]

IDK, lots of usable signals in the slice of spectrum.

When I increase the gain, the distortion gets worse.

 

[maus92]

When I increase the gain, the distortion gets worse.

Which makes sense. But as I said, lots of usable signals in that waterfall shot that should not require boosting gain, but you'll have to be specific about what system/frequencies you are having trouble receiving.

 

[rabbit108]

Dave said

the control channel holds sold {solid} with no loss.

He also said

When I increase the gain, the distortion gets worse.

Which makes sense.

Both make sense. They are classic symptoms of Desensitization (AKA Desense) from a strong signal on another frequency that causes fundamental overload and spurious signals (spurs) that damage the weaker signals you are trying to receive. The spurs raise the noise floor, which reduces the SNR of the weaker signals.
Desensitization (telecommunications) - Wikipedia

But as I said, lots of usable signals in that waterfall shot that should not require boosting gain,

Boosting the gain can't repair the damaged weaker signals.

but you'll have to be specific about what system/frequencies you are having trouble receiving.

Dave did specify system/frequencies in his first post

I have been monitoring a P25 Phase II LSM 700MHZ system for a few years now with a Airspy R2 with no issues. The system is in the range of 769-774 MHZ.

I suggested to Dave that he use the Spectrum Spy software that is available for his R2. It acts as a substitute spectrum analyzer at much less cost than a real SA. He said that he had done something "similar" with the P25 software that didn't help. Finally, after a request from @WB5UOM, Dave posted a scan:



Well, it is "similar," but it doesn't show a possible strong signal on another frequency that might be causing interference, only his received signals. Also the freqs are difficult to read and there are no horizontal lines indicating relative strength.

Dave stated in his first post:

Recently a new cell tower was installed very close to my house (1000 feet). I’m now having problems with audio distortion ever since the new tower was turned on. I have played around with the gain with no luck.

and maus92 stated in post #6:

LTE / cellular interference will be wideband aka 5-10 MHz per channel just below the talk-out frequencies for the 700 MHz public safety band (Band 14). Ironically it's FirstNet. A filter would need to block below 769 MHz as it is not a specific frequency. There is a band of uplink frequencies just above 775 MHz, but that would come from subscribers aka low power and unlikely to affect you.

So, I think it is entirely possible that a strong signal below 769 MHz or the new cell tower are causing interference. But, that isn't sufficient information to order the correct custom filter. I would want to verify what signal is causing the interference with further scans and tests with a battery operated laptop and portable antenna at other locations as suggested by @wpt in post #11. Like, what frequencies are being used by the new cell tower?

This is what those frequencies look like at my location:

 

[maus92]

A high pass filter that blocks below 768 MHz would be sufficient to deal with Band 14 / LTE interference as there is no public safety usage between that and the UHF-T band. Specific as to what systems is difficult to receive means, specifically, what system is troublesome. Perhaps there is another issue affecting reception.

 

[rabbit108]

A high pass filter that blocks below 768 MHz would be sufficient to deal with Band 14 / LTE interference as there is no public safety usage between that and the UHF-T band.

Yes, that high pass filter would probably help Dave IF his 720-820 MHz Spectrum Spy scan looks like my 720-820 MHz scan:



but he hasn't posted his 720-820 MHz Spectrum Spy scan yet.

Specific as to what systems is difficult to receive means, specifically, what system is troublesome.

That is correct. A custom filter for Dave must have the proper specs to match HIS problem.

Perhaps there is another issue affecting reception.

That is possible. The transmitter causing his problem might even be in the GHz range, which would require a low pass design. There is a Homedale app for his computer that will show scans of the WiFi bands.
Homedale::WLAN Monitor

Custom filters are available from Dale at PAR Electronics:
PAR Electronics | Filters for the commercial 2 way market, MATV, FM broadcast, laboratory, marine industry, amateur radio, scanner and short wave listening enthusiasts

 

[maus92]

I don't know how to be more clear. When I ask about what specific system, that is what I mean. Perhaps the target system's operators have made changes to transmitters / ERP, site placement, antenna patterns, frequencies, programming, etc. Having a comprehensive set of data that includes the parameters that are missing will help to identify the problems and guide a solution.

 

[rabbit108]

I don't know how to be more clear. When I ask about what specific system, that is what I mean. Perhaps the target system's operators have made changes to transmitters / ERP, site placement, antenna patterns, frequencies, programming, etc. Having a comprehensive set of data that includes the parameters that are missing will help to identify the problems and guide a solution.

I agree; you made it clear. Dave gave us his system parameters in his first post:

I have been monitoring a P25 Phase II LSM 700MHZ system for a few years now with a Airspy R2 with no issues. The system is in the range of 769-774 MHZ. Recently a new cell tower was installed very close to my house (1000 feet). I’m now having problems with audio distortion ever since the new tower was turned on.

What is needed now is the freq of the transmitter causing the problem to specify the filter specs. Dave is the only one able to provide that missing information for HIS location.