What is Intermodulation (in English)?

[Sammyboy]

Hi, another newbie here!

I have just bought a Maycom FR-100 scanner, which should arrive tomorrow (yipee!! ) and getting back into scanning after doing it in a rather ameture fashion when I was a teenager!

Basically, someone said to me in an email that the FR-100 suffers from 'intermodulation problems'. What exactly does this mean, and does the FR-100 indeed suffer from these problems?

I've done a few 'net searches on intermodulation problems and found some rather scientific answers that went a bit over my head, I'm looking for a plain English explanation of it is possible! From what I can gather, intermodulation basically is when neighbouring frequencies 'intrude' on the frequency you're listening to, so if you're listening to Approach on one airport, and another frequency for, say, ATC on a frequency close by will 'cut in' on your frequency. Is this about right?

Thanks!

[Andy]

That's basically right, Sammy.
Cheaper receivers (especially scanners) suffer from intermodulation. It happens when two strong signals (or even one strong and one weak) mix together in the first stages of a receiver and produce a third frequency. Here's an example:

You are listening to 171MHz for some thrilling taxi action and you keep getting bursts of noise and distorted audio. There's really nothing on the frequency, but your radio keeps telling you there is, so what's happening?
How about:
Strong AA transmissions on 86 MHz are causing your receiver to overload slightly, but normally you wouln't know anything about it - but... when GrotPizza comes on the air on 85MHz, the overloading bits start to mix the two frequencies together and make 171 MHz, and this phantom signal has the audio from both the frequencies.
There's also a 'difference' frequency created on 1 MHz, so you'd get the same phantom signal there.

Note that the interfering signals are a long way from the frequency you are listening on, so a scanner with good front-end selectivity would stop them getting in, but a cheap one with very broad input circuits would let them through and give you the problem above.
Hope this helps!

PS - don't worry too much about your Maycom. Unless it's very badly designed, you won't get serious probs from IMD unless you attach a big outdoor antenna.

[Sammyboy]

Hi Andy, thanks for the reply! Got the scanner now, and so far it's great, no problems or intermodulation at all! I will now probably be annoying my girlfriend by fiddling with it instead of talking to her! :p