V.H.F. A.T.C. crosstalk.

[FlyGooseFly!]

Any real Teckies out in the radio shack? We have two cheap and decrepit radios set up on standard internal or even broken antennas that receive aircraft traffic when set on 98.80 Mhz and up to 105.40Mhz in Buckinghamshire, England - why is this? As full and answer as possible please.

[HEATHROW DIRECTOR]

It's probably due to undesirable mixing going on in your receiver, or simple overload if there is a ground station or aircraft nearby. The intermediate frequency of your radio is probably 10.7 mhz... if you double that and add it to the frequencies mentioned you get 120.2mhz and 126.8, both of which are in the civil airband.

I expect someone else will give you a scientific explanation.

[DenhamPPL]

I think Heathrow Directors explanation was pretty damn good. Earlier FM radios were never particularily good at suppressing interference from other bands especially if they themselves were not in good condition.

Antenna's on radios are normally a particular length for a reason - the aerial needs to be "tuned" to the frequency it is intended to listen on for best reception and interference suppression. If the antenna on an fm radio is broken (ie the top of the telescopic bit) then the antenna is going to be shorter and therefore more likely to pick up interference from higher bands (ie airband) which utilize shorter antenna's for their particular frequencies. Coupled with "manual" tuning (ie not digital but "elastic band" technology) and you've a very good chance of receiving transmissions from aircraft nearby. I used this to my advantage when I was a schoolboy and used to listen to the airband on a normal FM broadcast radio when I was a "spotter";)

When I used to work for a London radio station we used to get quite a few complaints from listeners about reception. Most of those complaining lived under the approach path for either 27 Left or Right and had cheapo trannies tuned in to us rather than the newer more swish digital tuners (with ribbon antennas).

No doubt some swot will come on and give you the techie reason for 10.7 intermediate crystal frequencies in old transistor radios but its Sunday evening and my brain is too fuddled with good red wine to go into much more detail:D

Hope this helps!

Regards

Andy

[chevvron]

We have a Roberts portable which sometimes picks up aircraft calling Bournemouth on 119.475 when we are tuned to 102.2 (Smooth FM)