As you are a licensed radio amateur you probably already know about intermodulation.
If you have a cellular transmitter across the street at 700Mhz and you have a trunked system on the next building at 750MHz and your standard mic system usually are designed to handle weak signals from 10mW mic transmitters and focus on high sensitivity and not strong signal capability, it's difficult to get both, the two signals, together with all other signals in the air, reach a level in the mic receiver where it starts to create intermodulation products, IM.
The 700Mhz mix with the the 750MHz and creates a third signal inside the receiver that are a mathematical product of the two signals. It will create a signal at 650Mhz and a weaker one at 600Mhz and so on, not calculation with all other signals in the air.
Those IM signals exists only in the receiver and cannot be searched or scanned for using another receiver.
The only solution are to reduce the level of the signals coming from the antennas on the mic receivers diversity system to lower the level where IM issues doesn't exists.
Covering the antennas with aluminium foil are one solution. If the microphones then have some dead spots then uncover parts of the antennas until mic coverage are satisfactory but will still reduce enough of the total signal level to not create any problem in the receiver.
If you have, or can borrow, a good receiver like a scanner, you can set that to the exact frequency of the interfered mic and listen around in the room and also turn off that mic and listen in the scanner with the squelch fully open for any signals that could interfere on the exact frequency. The interfering signal would have to be strong to overpower the microphones transmitter.
If this is your first attempt of using SDR dongle receivers then it's a steep learning curve until you have it set up in a way that the result can be trusted. It's much easier using a standard dedicated scanner and listen to the signal by ear, if there in fact are a signal in the air to be found.
/Ubbe
