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Near to my location we have IAF(Indian Air Force - Tambaram) training center is there, While hearing FM radio at a at frequency of 106.4MHZ I can hear the cockpit commands/voices daily when these IAF flights cross by. How come it is possible? Is it due to FM radio i am having? I cant hear the same interference in mobile FM.

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  • \$\begingroup\$ I used to modify FM radios to pick up aircraft communications. Just a little tweaking of the demodulator and a little change to the tuning range. Couple of turns on the right places inside the radio - didn't even have to change any parts. \$\endgroup\$ – JRE Feb 27 '18 at 16:08
  • \$\begingroup\$ The mobile receiver probably has better "image rejection" -- i.e., a more selective front end. \$\endgroup\$ – Dave Tweed Feb 27 '18 at 17:06
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While hearing FM radio at a at frequency of 106.4MHZ I can hear the cockpit commands/voices daily when these IAF flights cross by.

The VHF airband (in most countries as far as I know) uses frequencies between 108 and 137 MHz and although these are AM modulated they are nicely positioned in the spectrum to be picked up by a standard FM radio that has an IF of 10.7 MHz. This picture may help: -

enter image description here

So, you have your FM band and you have a "wanted" channel that you need to convert to an intermediate frequency of 10.7 MHz. You position your local oscillator so that it is 10.7 MHz higher than the wanted channel and mix-down the wanted signal to 10.7 MHz for band-pass filtering and subsequently demodulation. However, you could also mix down an unwanted channel that is 10.7 MHz higher than your local oscillator.

So, 106.4 MHz plus an IF offset of 10.7 MHz is 117.1 MHz - this would be your local oscillator frequency and an unwanted image that is 10.7 MHz higher (at 127.8 MHz) could also produce an IF of 10.7 MHz. Given that FM demodulators can also amplitude demodulate a weak AM signal this is a possible culprit.

Back when I was a teenager I could pick up the cops on my FM receiver although they were using a frequency and modulation that I should not theoretically be able to receive.

I cant hear the same interference in mobile FM.

What frequency(ies) does it use?

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    \$\begingroup\$ Direct conversion is unlikely. More probably, aircraft using \$106.4 + 2\cdot10.7 = 127.8\$ MHz are coming in as an image. \$\endgroup\$ – Dave Tweed Feb 27 '18 at 16:58
  • \$\begingroup\$ @DaveTweed oops you are quite right. \$\endgroup\$ – Andy aka Feb 27 '18 at 16:59
  • \$\begingroup\$ Same FM station 106.4MHz in mobile i am not getting interference. \$\endgroup\$ – vikenesh vicky Feb 28 '18 at 0:51
  • \$\begingroup\$ The mobile phone may use a local oscillator frequency that is 10.7 MHz lower than 106.4 MHz. The L.O. can be either side of a receive station and still down-convert correctly via its mixer. \$\endgroup\$ – Andy aka Feb 28 '18 at 8:43
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Harmonics in IF mixer , with high power interference demands better RF for 10.7MHz IF filters . Examine the carrier frequency difference with LO. You may find the intermod (IM) products cause capture problems on cheap FM radios.

I recall being inspired in Grade 2 in Flin Flon in early 60’s when I could hear RCMP radio on TV when nearest broadcast I could get on a hot summer day was Texas

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