How Can I Determine If Logged Frequencies Are Legitimate Or Inter-Modulation/Mixing/Front End Overload?

[JASII]

As posted above, I am using a Uniden SDS100 connected to a laptop computer running software that logs active frequencies and PL/DL/Color Code/NAC/RAN, etc.

Is there any way to determine which are legitimate frequencies and which are a result of intermod, mixing and front-end overload?

 

[prcguy]

Turn on the attenuator and if it’s IMD generated in the receiver it should go away. IMD is non linear so for every 1dB you attenuate a signal going into the device making IMD, the IMD will reduce by 2dB. So if your radio attenuator is 10dB, the legit signals will go down 10dB but the IMD will go down 20dB or probably disappear as you might bring the receiver out of IMD land.

 

[popnokick]

For strong persistent signals, first tune to the frequency in question. Then disconnect the antenna. If it goes away it’s most likely NOT a birdie or something internal to the receiver. And is instead coming in via the antenna.

 

[nd5y]

Aside from the technical aspects of receiver operation (IMD, etc.) It's very important to know band plans and which types of services use which frequencies, and what the channel spacing and allocations in different bands are.

 

[Ubbe]

Is there any way to determine which are legitimate frequencies and which are a result of intermod, mixing and front-end overload?

If already logged then not much can be done. But usually it will sound weird from analog frequencies and digital will not decode, if you also do audio recordings.

To verify you'll need scanners of different brands, as the same brand usually behave the same from overload issues, or a real bombproof receiver, like a Pro2006.

SDS scanners are a real nightmare when it comes to RF performance and have a huge amount of mixing products and false frequencies, so not suitable to find new unknown frequencies, if not verifying by programming another scanner with the found frequencies and double check.

SDS100.mp4

drive.google.com

/Ubbe

 

[Whiskey3JMC]

Is there any way to determine which are legitimate frequencies and which are a result of intermod, mixing and front-end overload?

You can always invest in a RR premium subscription then utilize the Frequency Data search to see what's been ID'ed in the RRDB and the FCC Data search for what hasn't. maprad.io provides a nice radius search option but it helps to have GPS coordinates for the area you're searching. Listen in to said frequency(ies) in question for a while to see if they beep their callsign at some point. Helps to at least know and log the trailing digits to help you pinpoint the station. These should help you determine if the station is real or an image of another station