Category Archives: intermodulation

Radio Rusty Bolt?

Not had too much time over the last week or so to do much radio wise, but the other day while scanning around in the vhf low band found an odd signal on one of my scanners at 70.860MHz. This is the part of the spectrum that was used by the fire brigade until the introduction of the airwave system. Strangely it was a WFM signal and after a few minutes of music identified itself as BBC Radio Nottinghamshire!

Where I live isn’t brilliant for radio reception, but the BBC station is one of the stronger due to a nearby local ‘fill-in’ transmitter so I suspected it was breakthrough or some product of intermodulation. However I wasn’t sure if it wasn’t some ‘cordless headphones’ or other form of AV link or baby alarm given the proliferation of Chinese cheap and cheerful FM modules available that can operate at this frequency. That now seems very unlikely as the signal has been present continuously for days now.

To try to rule out any internally generated image I tried some of my other scanners, with different antennas and found the signal on them too. However one of them did exhibit some pager breakthrough as can be seen in the following video.

It therefore does suggest to it being an intermodulation product, but exactly what is causing it I am unsure. We have cast iron guttering on the house which has been recently repainted, so could this be caused by a rusty bolt that has been disturbed now acting as a diode?

Very curious…