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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
I've just been playing about with the Twente SDR web interface and was listening to a few interesting signals, when I realised that I can hear muffled audio in the background on the Time from NPL signal at 60kHz, (formerly the MSF rugby clock signal.) The audio sounds English but I had to close the bandwidth right down to avoid interference from other nearby VLF signals. I'm almost certain that the audio is that of BBC radio 4 LW that broadcasts on 198kHz, because if you adjust the waterfall so that the spectrum of both transmissions is on the screen at the same time, you can hear the muffled audio on 60kHz stop when there are gaps in the BBC R4 198kHz transmissions spectrum. Like where there is a gap between programs, or when the announcer pauses for breathe!
This audio crosstalk seems like a strange thing to happen because BBC R4 transmitter is analogue and located at Droitwitch in Worcestershire. and the 60kHz MSF transmitter is digital and located in Anthorn in Cumbria. How could audio from one of these be ending up getting modulated onto the other one's transmission? And so prominently without anyone noticing!
I've read about the Gorky Luxemborg effect where the non-linearity of the actual air above high-powered radio transmitters results in the signal strength of any radio waves passing through it picking up the modulation of the high power transmitter it passes over. But neither of these stations is particularly high powered, and they're not located particularly close together. And I had always assumed this effect was very small. This amplitude modulation is very noticeable!
The other explanation I thought of was intermodulation from overloading causing non-linearity in the receiver. But both of these stations are a long way from Twente in Holland, aren't particularly powerful, and the SDR's antenna is tiny, so I very much doubt that it is being overloaded causing intermod in the receiver.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
Very odd. To find it, go to , set the SDR to 60 kHz and the bandwidth down to 1 kHz, click AM and turn up the volume. Now I went on a quest to see if this was repeated on any other WebSDR in public operation and found that, although I could pick up the NPL MSF signal on a couple of them, I couldn't hear anything over the noise. The most promising were the Farnam SDR : and Eindhoven: sites, although they don't seem to have anything like the signal strength at that frequency compared to the one at Twente.
I have a feeling that it's not to do with intermodulation with MSF, but with something else, and I seem to get a slightly greater volume at 59.5 kHz. The radio 4 transmitter is, after all, the most powerful long range transmitter currently in operation at 500 kW. I tried to see if I could get radio 4 at any other frequencies but I found there was too much noise, so it could be that radio 4 appears in other unexpected places in the electromagnetic spectrum, but we can't hear it over the noise floor.
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BBC Radio 4 audio appearing on 60kHz time from NPL
