Originally Posted by
Wodrick
With a very few exceptions all Radio receivers are also, to a lesser extent, transmitters, the local oscillator, a fundamental part of a simple radio, will also radiate, to a greater or lesser extent depending on design, internal screening, and perhaps expence. I have seen quite expensive products that disgust the Radio Engineer in me ! Incidently this goes for anything with an oscillator in it, so computer clocks etc also qualify. That's why you don't use them.
That is all correct. Most scanners are not exactly the best designed bits of kit around. The main oscillator hasn't got much in the way of isolation to the aerial on the unit and it radiates through it. Usually about 10.7 MHZ or 21 Mhz away from the received signal, depending upon the design. Add to this yet another oscillator to convert the frequency down before demodulating.
The way that the oscillators produce the frequencies normally involve some sort of frequency synthesiser. That again involves another oscillator, usually in the form of a crystal oscillator.
So really a scanner can (and does) produce a heck of a lot of radio energy at all sorts of frequencies, some of which will be "in band" to a lot of the aircraft's equipment.
Hope that helps,
Astral (engineer & licenced Radio Amateur)