Music on 120.40
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Music on 120.40
This may sound really stupid, but has anyone who operates into LHR heard any music on the Director frequency 120.40? I heard some briefly the other day and wasn't quite sure whether i was loosing it! Its only about 2-3 seconds. Please say its not just me! 
Cheer, Expedite.
Cheer, Expedite.
foxtrot xray
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FM Interference
While I am not familiar with the LHR problem, this is a common problem in the US and is usually..although not exclusively.. the result of what the techies call third-order intermodulation. This "interference" is actually generated in the front end of the airborne receiver when it is subject to the presence of strong FM broadcast signals (in the 88-108 Mhz FM band) and two of those staions have operating frequencies [f(1) , f(2)] which satisfy the relationship 2f(1) - f(2) = 120.4. For example, if you have an FM station on 106.5 and another on 99.5 MHz the resultant would be 120.5 MHz.(In the US the FM stations are assigned on a frequency plan that uses odd decimals ,ie., 88.1,88.3,88.5 ,etc. ,so the intermodulation product will fall on an "odd" frequency also. Your example of an "even" frequency suggests that this interference is a sideband of a larger problem which is appearing on a frequency 100kHz removed from 120.4. Newer receivers complying with ICAO standards having what is generally labelled "FM Imunity" are supposed to be less susceptible to this type of problem.
I'll bet a review of the FM broadcast frequencies in use around LHR will yield a pair which satisfies the above but if the problem is not universally experienced it could be indicative of a receiver problem.
Hope this helps.
I'll bet a review of the FM broadcast frequencies in use around LHR will yield a pair which satisfies the above but if the problem is not universally experienced it could be indicative of a receiver problem.
Hope this helps.
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Music at Merrill Field, ANC-1968
A310 Driver
Thanks for the thorough explanation; I learned to fly in Alaska and in the '60s and '70s at the exact same spot in the MRI Traffic Pattern, in a Cessna, broadcast would break through for a mere few seconds.
Of course in those days there were only about three FM stations in ANC so it was obvious that was the problem but no one was ever able to explain it as you did.
Thanks for the thorough explanation; I learned to fly in Alaska and in the '60s and '70s at the exact same spot in the MRI Traffic Pattern, in a Cessna, broadcast would break through for a mere few seconds.
Of course in those days there were only about three FM stations in ANC so it was obvious that was the problem but no one was ever able to explain it as you did.
Last edited by DeeTeeS; 5th Jan 2004 at 21:54.
I suggest that if the problem persists you report this to the LHR ATC Watch Manager, who, through their engineering department, can have the matter investigated by the UK Radio Authority.
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Interference cont'd...
When I worked at YTZ (Toronto Island Airport) there was a constant problem on ground 121.7. The transmitter for CHIN on 1540 MHz was located very close to the ground control antenna. Ground was always always picking up CHIN and it drove the engineers crazy. I used to see them over there couple of times a week. This was 17 years ago and I don't know if they solved the problem. CHIN put out 10,000 watts, I believe, and it was just too close to the airport to avoid rf getting into the system.
Oops - that should be 1540 Khz.
Oops - that should be 1540 Khz.
None but a blockhead
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There's a pirate FM transmitter called Unknown FM in south London (I think) broadcasting on 97.1 MHz. The transmitter's clearly faulty, as it's also been blocking an amateur radio repeater on 145.650 MHz - the third harmonic, one supposes, of the 48.55 MHz fundamental that's doubled to produce the 97.1 nominal output.
Not beyond the bounds of possibility that something that grotty could also be producing spurii on other frequencies.
R
Not beyond the bounds of possibility that something that grotty could also be producing spurii on other frequencies.
R
