Originally Posted by
skiddyiom
Astral, good explainantion.
Isn't it something to do with harmonics? The reciever transmitting several harmonics to a receding degree. It's light years since I did any radio theory (radio's had valves then
) so I may be totally wrong. But this would also explain why mobile phones aren't permitted, as the harmonics generated may coincide with the frequencies used by equipment on the aircraft.
skiddy
Thanks - I do try
My explaination was somewhat an over simplification of what I would normally write. Most scanners and VHF / UHF equipment convert the incoming signal down in frequency at least twice. Scanners normally are double conversion superhet units. The sad part is that the filters on the front end of scanners (that would attenuate any signals getting out of the scanner) is very wide and virtually useless at stopping things getting out.
The oscillators apart from having the main fundemental frequency offset by whatever the first IF frequency is. They also produce odd or even harmonics upwards from the main frequency. So if you were say listening on 120 Mhz and the first IF was 10.7 Mhz the first oscillator would be working at 109.3 or 130.7 Mhz. Plus of course harmonics at multiples of it.
The same applies for any system generating clocks for computers, mobile phones, games consules etc. Manufacturers producing equipment for the professional market take great care to ensure that the radiation from whatever they are making is within strict limits and are checked with great care that they do not radiate anything where it could cause problems.
The reason I mention the above. Is that anything fitted into an aircraft has to pass strict tests and a long procedure to be approved. Which explains to some extent the high costs and the slowness to bring in new technology into aviation. Personally I wouldn't have it any other way. It pays my mortgage!
Astral