Why was my radio picking up ATC conversations?
 

Evening all, a strange thing happened earlier. I was working in the garage this evening with the radio set to Rock FM (97.4 MHz). I live in the narrow corridor of unrestricted VFR airspace between Manchester and Liverpool airports but we still get the occasional airliner overhead above VFR ceiling.
Now, in my limited experience of several light aircraft/gliding airfields, I thought all uk civil aviation radio traffic was contained around 127.xxx MHz? So why on earth were the songs on my radio being periodically interrupted with things like "This is [callsign] on final approach 10 miles out", "Roger this is [callsign] bearing onto heading xxx" and "Copy that tower, thankyou and have good evening"? It seemed to happen every 5 minutes or so, and I only ever got one half of the conversation (presumably the transmissions from the aircraft overflying). And on a similar point, if I could pick up their conversations when I was supposed to be listening to a radio station, what was stopping the pilots receiving "Lancashire's most popular pop rock radio" instead of the ATC commands?

It's all to do with harmonics :cool:

An excellent answer by a Ppruner from a few years ago (MM 43)

:D

A great many years ago I was working at an ATCRU when a gentleman from the GPO (yes, it was that long ago!) demanded to check all our transmitter equipment to make sure it was within 'spec'.

"What's the problem?" we asked. It appeared that someone in a nearby village had reported us to the GPO as some of our transmissions were breaking through on his TV and interfering with 'Ena Sharples'!!

The GPO chappie confirmed that all our kit was OK, so we asked him what was going to happen now. He gaily announced that he would nip round to the complainant ...

... threaten him with a summons for illegally intercepting ATC communications and tell him to get another set!! :{

Interestingly, I was considering posting on the same subject.
I live in Stevenage, Hertfordshire the frequency is 97.3 (LBC).
Interference varies from very clear to garbled. Happens any time of day any type of weather (I beleive atmospheric conditions can sometimes affect?) any time of year. Only on that frequency & only on that radio.

The explanations and reasons are given above but it comes down to the fact that domestic radio sets are not manufactured to a high standard and are susceptible to "breakthrough" from stations they are not tuned to for a variety of reasons. I am a radio amateur and, for our sins, we become involved with neighbours having problems with breakthrough from our transmitters. As well as radios, one neighbour of mine used to hear me on his hi-fi record player. When I asked if his hi-fi was designed to receive radio transmissions he got the gist.

Occasionally used to get an initial call to Bournemouth Approach (119.475 mhz) when listening to Smooth 102.2 mhz. Don't get it nowadays as we only listen to Talksport on 1089 khz.

LL INT North used to be quite regular on Virgin FM in Staines in the mid 90's. rubbish lyrics though...

That explanation is on the right lines, but a bit confused.

Every radio since the 1930s - and by every, I mean most - is a superhet. It works by converting the range of frequencies on the dial to a single frequency before amplifying, filtering and turning it into audio. (The invention of this idea was like the invention of standard containers in shipping - suddenly, all the hard work could be got right just once, no matter what was being shipped.)

It's very easy to convert a range of frequencies to a single frequency. You just arrange to generate your own very weak radio signal at a fixed offset from the one you want to receive, and then mix your signal and the received signal together. At that point, you get a lot of different stuff out - but mostly, you get a signal at the difference between the two frequencies.

Change your local frequency, which is easy to do, and you change the frequency you're picking up. The difference between the two is what you then amplify, filter and turn into audio - which is quite hard work, electronically, but made much easier if you just have to do it for the one frequency.

So, if you want to pick up signals between 88 and 108 MHz, which you do for normal FM, you create a signal inside your radio between 98.7 and 109.5 - always 10.7 MHz above your wanted signal. IThen, you do all the hard work on the fixed frequency of 10.7 MHz.

Tune into 97.4 on the dial, you're tuning your local radio signal to 108.1. The difference is 10.7. But there's also a difference of 10.7 between 108.1 and 118.8 - which is a standard aeronautical voice frequency (used by Glasgow Tower, I believe). If the radio isn't good at rejecting frequencies above 108 MHz (and cheap ones aren't), then you'll get both your wanted FM radio station and any aircraft on 21.4 MHz above that station, at the same time.

It gets worse. Broadcast FM signals are quite wide and it's not uncommon for FM radios to receive 200 kHz (0.2 MHz) or more either side of the central frequency at once. So when you're listening to 97.4 you can pick up everything from 118.7 to 118.9 MHz. Manchester Tower is on 118.625, which is a bit outside that range - but certainly not outside the bounds of probability that a cheap FM radio will pick up a nearby aircraft transmitting on that frequency.

(Incidentally, that's a major reason why you're not allowed to use radio receivers in the cabin on flights - FM radios generate signals that fall within the VOR band, and that could be bad....)

At around 108.0 I once picked up, from my then residence in Bury, Lancashire, on a 'stereogram', A TWA overflight near Pole Hill VOR. Very rich tone. Would have made a lovely air-band radio. No 8.33 of course.

Thank you for the explanation SLF. It makes our FM immune radios seem pretty sophisticated compared to a bog standard radio in the kitchen. I shall be mighty wary of Rock FM when telling my ILS traffic to contact tower on 118decimal625!

Mind you it's the taxis breaking through on the UHF frequencies that can be a real pain.

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