Radio picking up ATC traffic?
Discussion
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 narrow the corridor of unrestricted VFR airspace between Manchester and Liverpool airports but we still get the occasional airliner overhead (well above the VFR ceiling, which is just as well as there is a microlight airfield just down the road).
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 them receiving "Lancashire's most popular pop rock radio" instead of the ATC commands?
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 them receiving "Lancashire's most popular pop rock radio" instead of the ATC commands?
Some sort of weather anomaly perhaps? I was picking my daughter up from her grans the other evening and every radio station from 97mhz to 102 mhz was being being bled over by a foreign radio station which is most odd as they live in Gateshead! The weather was the only reason I could think of to explain this but how it occurs I don't know.
Most probably what we call an image response.
Most VHF radios convert the received signal to a lower frequency (typically 10,7 MHz) by mixing with a local oscillator. In this case the local osc will be 10,7 higher than the station your are listening to.
Mathematically: Local Osc 108,1 - 97,4 = 10,7 MHz
But : 118,8 - Local Osc 108,1 = 10,7 MHz also
So depending on how good the filtering is at VHF, the radio could also receive airband signals on 118,8 MHz.
Also the weekend of 6 - 7 july saw massive tropospheric propagation in NW Europe between UK and mainland.
Most VHF radios convert the received signal to a lower frequency (typically 10,7 MHz) by mixing with a local oscillator. In this case the local osc will be 10,7 higher than the station your are listening to.
Mathematically: Local Osc 108,1 - 97,4 = 10,7 MHz
But : 118,8 - Local Osc 108,1 = 10,7 MHz also
So depending on how good the filtering is at VHF, the radio could also receive airband signals on 118,8 MHz.
Also the weekend of 6 - 7 july saw massive tropospheric propagation in NW Europe between UK and mainland.
Seems very odd that you could make out any of the voice comms, given that airband VHF is AM, and you were listening to an FM station?
Are you sure they weren't just playing a back-to-back set of Saxon's "Strangers In The Night", Mike Oldfield's "Five Miles Out" and "Learning to Fly" by Pink Floyd?
I've experienced atmospheric oddities affect propagation, but never modulation.
Are you sure they weren't just playing a back-to-back set of Saxon's "Strangers In The Night", Mike Oldfield's "Five Miles Out" and "Learning to Fly" by Pink Floyd?
I've experienced atmospheric oddities affect propagation, but never modulation.
Propagation conditions, especially over water can be quite extremee at times. It doesn't matter whether the signal you try and receive is digital or analogue. Once the interfering signal becomes almost the same strength there will be audible/visible effects.
The one case when this may not is for WiFi type signals as these use Spread Spectrum transmission which is much more robust to interference.
On the 6 july the low power radio beacon near Ballymoney NI on 144 MHz was clearly received down here in the south of France.
The one case when this may not is for WiFi type signals as these use Spread Spectrum transmission which is much more robust to interference.
On the 6 july the low power radio beacon near Ballymoney NI on 144 MHz was clearly received down here in the south of France.
eharding said:
Seems very odd that you could make out any of the voice comms, given that airband VHF is AM, and you were listening to an FM station?
Depends on the FM detector circuit used by the radio. At school I retuned a FM portable to use the 'image response' and it worked perfectly on AM for the airband.Le TVR said:
eharding said:
Seems very odd that you could make out any of the voice comms, given that airband VHF is AM, and you were listening to an FM station?
Depends on the FM detector circuit used by the radio. At school I retuned a FM portable to use the 'image response' and it worked perfectly on AM for the airband.Eric Mc said:
In my plane spotting days back in the 70s it wasn't unusual to hear aircraft voice communications coming in on an ordinary domestic VHF radio.
Hmm, just in case you ever need to mention that a plane is a flat surface or woodworking tool again this week Eric Sorry mate but this one needed doing
I still have a couple of old AM/FM transistors (radios folks)
that capture all sorts of transmissions, from definite Air Band down to the droning old Black Country geezer with "Servicing to Alpha-Servicing to Alpha" all hours of day and night
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