Identifying Intermod/Overload/Other False Signals?

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FreqMeister

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Is there some way to help identify intermodulation and overload products from nearby cell towers, etc, without moving to a different location? I get a lot of garbage in the 440 - 470 Mhz range and it's hard to know the source. On a consistent signal I can check on another radio/scanner/antenna but on intermittent signals that's usually not practical.

Is in-home RFI (i.e. from cheap Chinese power adapters etc) much of an issue at VHF and UHF frequencies?
 

cmdrwill

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Is there some way to help identify intermodulation and overload products from nearby cell towers, etc, without moving to a different location? I get a lot of garbage in the 440 - 470 Mhz range and it's hard to know the source. On a consistent signal I can check on another radio/scanner/antenna but on intermittent signals that's usually not practical.

Is in-home RFI much of an issue at VHF and UHF frequencies?

You would need a Spectrum Analyzer.

In home RFI, yes in the VHF and UHF bands
 

RFI-EMI-GUY

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One of the SDR kits using a dangle would provide a cheap spectrum analyser. But they have their own strong signal limitations.

A tip. If you insert a 3 dB attenuator in the antenna line and the interference drops significantly, like 9 dB, the IM is occurring inside your receiver.

Sent from my SM-T350 using Tapatalk
 

FreqMeister

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One of the SDR kits using a dangle would provide a cheap spectrum analyser. But they have their own strong signal limitations.

A tip. If you insert a 3 dB attenuator in the antenna line and the interference drops significantly, like 9 dB, the IM is occurring inside your receiver.

Sent from my SM-T350 using Tapatalk
Did you mean a "dongle"? Please share more specifics? Basically, AFAIK, cheap spectrum analyzers generally suffer from limited spectrum viewing bandwidth with their USB audio interface which can only show a tiny fraction of the spectrum. Other products have poor RF performance, etc. I'm getting lots of fairly strong garbage in the UHF band even with a small rubber duck antenna. I don't want to further attenuate the signal when many of the signals I'm interested in monitoring are already somewhat weak and noisy.
 

Jim41

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Did you mean a "dongle"? Please share more specifics? Basically, AFAIK, cheap spectrum analyzers generally suffer from limited spectrum viewing bandwidth with their USB audio interface which can only show a tiny fraction of the spectrum. Other products have poor RF performance, etc. I'm getting lots of fairly strong garbage in the UHF band even with a small rubber duck antenna. I don't want to further attenuate the signal when many of the signals I'm interested in monitoring are already somewhat weak and noisy.
If you can identify the frequencies of the offending signals you can explore attenuating them using a filter or using a directional antenna like a yagi.

Jim41
 

RFI-EMI-GUY

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Did you mean a "dongle"? Please share more specifics? Basically, AFAIK, cheap spectrum analyzers generally suffer from limited spectrum viewing bandwidth with their USB audio interface which can only show a tiny fraction of the spectrum. Other products have poor RF performance, etc. I'm getting lots of fairly strong garbage in the UHF band even with a small rubber duck antenna. I don't want to further attenuate the signal when many of the signals I'm interested in monitoring are already somewhat weak and noisy.
Yeah, Dongle damn you autocollect.

Putting a 3 dB attenuator in line is strictly a diagnostic effort. If you insert the attenuator and the desired signal drops only the equivalent of 3 dB and the interferers drop 9 dB , you have identified a non linearity in your receiver. The solution is filtering, or a better receiver.

If the interferer drops 3 dB, the interference is external to your receiver.

There are so many SDR kits out there, that you would need to research what meets you requirements and budget.



Sent from my SM-T350 using Tapatalk
 
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zz0468

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One simple trick is to insert some attenuation into the antenna line. A set of bnc attenuators in 3db, 6db, 10db, and 20db values can be found at affordable prices. If you put in, say, a 10db pad in line with the antenna and the interference completely goes away, it's being generated in your receiver, likely from overloading.

Beyond that, you really need some sort of spectrum analyzer to identify what's actually happening.
 

FreqMeister

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One simple trick is to insert some attenuation into the antenna line. A set of bnc attenuators in 3db, 6db, 10db, and 20db values can be found at affordable prices. If you put in, say, a 10db pad in line with the antenna and the interference completely goes away, it's being generated in your receiver.

Beyond that, you really need some sort of spectrum analyzer to identify what's actually happening.
Thanks. I should probably try an external BNC attenuator. But, as I said previously, an attenuator isn't a long term solution as I have several already weak signals I want to monitor and an attenuator will render them unlistenable.
 

Ubbe

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If you get intermittent garbage it must be intermodulation problem. As explained it is easily checked by using an attenuator. I prefere a variable one that quickly can be adjusted compared to fixed ones.

With a variable one you can instantly tune and listen to those weak signals you want to receive and you will probably discover that the reception gets better with some attenuation. If not, try to use a FM trapfilter if you not already use one.

What receiver and antenna are you using?

In home interference are often constant and not intermittent.

/Ubbe
 

slicerwizard

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Did you mean a "dongle"? Please share more specifics? Basically, AFAIK, cheap spectrum analyzers generally suffer from limited spectrum viewing bandwidth with their USB audio interface which can only show a tiny fraction of the spectrum. Other products have poor RF performance, etc. I'm getting lots of fairly strong garbage in the UHF band even with a small rubber duck antenna.
If you can't identify "strong garbage" with a dongle, you're doing it wrong. At a whole $20-25 for one, get 'er done already.
 

lmrtek

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The "garbage" you hear may be trunking, digital, and telemetry signals as well as domestic rfi

Stay away from using preamps and large outside antennas unless you have a scanning receiver with a tight front end

A dongle receiver (SDR) is actually a spectrum analyzer so they can easily identify both in band and out of band interference sources

And since anything electronic near the antenna can cause rf interference at some frequency its important to keep the antenna well above and away from the house
 

GUNSABLAZIN

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Take your Yaesu outside with a homemade Foxhunt Yagi. That will find your RFI direction.
Does the Yaesu have a spectrum analyzer built in? I think the V6R does?
 
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