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Faders   -   XF

Frequency:   (Many & Various)   Included;

2470 kHz	3188 kHz	3215 kHz	3382 kHz	4020 kHz	4062 kHz	4458 kHz	4477 kHz
4480 kHz	4496 kHz	4545 kHz	4560 kHz	5092 kHz	5105 kHz	5195 kHz	5311 kHz
5328 kHz	5398 kHz	5468 kHz	5787 kHz	6505 kHz	6796 kHz	6824 kHz	6848 kHz
6875 kHz	7384 kHz	7500 kHz	7658 kHz	7665 kHz	7813 kHz	7997 kHz	8185 kHz
9126 kHz	9138 kHz	9244 kHz	10139 kHz	10478 kHz	11100 kHz	11515 kHz	13431 kHz

So called because when listening to them, they appeared to fade away then return.   A favourite station of Chris from Enigma, these signals occupied a huge number of frequencies across the short wave bands.  Having no apparent respect for other users, these stations were found across Fixed, aeronautical, shipping and Amateur bands.

To listen to, the signals had a constant pulsed pitch, like a motor bike.  The reason for the fade was because each burst lasted 7.5 seconds, after which one of three things happened:-

a) Continued for 7.5 secs at full power - (Noticeable break between bursts).

b) Continued for 7.5 secs at reduced power.

c) Stopped completely - but if you kept listening, it would most likely reappear!

The signals were synchronised with the Time Standard, - as checked against a Radio-Controlled Clock or the telephone "Speaking Clock"- exactly on each minute you would find a corresponding burst start or finish.

Monitoring a fader frequency that had gone quiet, I noticed a single 7.5 sec burst every 15mins, exactly on the quarter-hour.  This would indicate the system was always operational, even when not transmitting.

  "Faders"  (59 kb)

Analysis by ENIGMA showed that Faders were actually a wide multi-channel FM, (Frequency Modulated), signal with peaks at:-

500 Hz   1100 Hz    1400 Hz    2000 Hz    2600 Hz    3400 Hz 

An audio spectrogram of the signal is both curious and revealing. The frequency of the signal can be seen to sweep linearly, from high to low, switching back to the high frequency and repeating the process continually. So it would seem that instead of the frequency peaks being fixed at the frequencies given above, they sweep high to low.

In order for a receiver to correctly receive and decode these sweeping frequencies, it would need to be exactly synchronised to the transmitter. This is where the time standard comes in, for provided both receiver and transmitter are locked to the same time standard, and sweeping the frequencies exactly in synch, decoding could easily be achieved.

Investigation indicated that some of the signals were transmitted from RAF Mildenhall, Suffolk, UK., and were almost certainly a secure digital communication system, with the weak fader signals being responding transmitters at other locations.

May have been part of a NATO secure data communication system known as LOCE - Linked Operational Intelligence Centres.

The faders, which had been heard on the shortwave bands for something like 30 years suddenly disappeared. No transmissions have been reported since May 2001.

Most likely these communications have gone over to satellite or been replaced by some other means of communication. Given the large number of these signals and their high usage, it just doesn't seem possible that they were simply redundant.

Any information welcome