Mystery Signals of the Short Wave

Dedicated to the more unusual, strange, bizarre and apparently meaningless signals on the short wave bands !

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The Twenty Minute Idler

Frequency: (Various)

Known Frequencies;
4049 kHz 4301 kHz 4705 kHz (4705.5 kHz)
5305 kHz (5305.5 kHz) 5307.5 kHz (5308 kHz)
6801 kHz

Current Frequencies;
5305 kHz (Re-discovered Dec 2013)
4301 kHz (Re-discovered Nov 2014)

This station was originally reported in January 1998 by ENIGMA on 5305 kHz, transmitting a carrier for exactly 20 minutes every hour, on the hour, 24 hours a day.

Originally named "The Blank Carrier", it was in fact a modulated signal which sounds like a low burbling hum.  Examination of the signal showed that it consisted of two distinct tones, peaking 250 Hz apart.

This would indicate a simple two-frequency FSK, (Frequency Shift Keying),  system.

This type of FSK transmission, called RTTY (RadioTeleTYpe), used by both amateur and commercial operators, transmit two closely spaced signals,(i.e. frequency-shifted,) less than 1 kHz apart, to provide the Mark - Space signals for sending data over the radio. In the receiver, an injected beat frequency converts these two signals into audio tones which, using the Baudot code are then displayed as ASCII symbols. Various speeds and frequency-shifts are used.

The "Twenty Minute Idler" used an unusual frequency-shift of 250 Hz.

  "The Twenty Minute Idler"  (47 kb)

Rimitas Pleikys in Lithuania noted on 24 March 1999, that there were three closely spaced signals operating on 5305.5,   5307.5   &  5308 kHz. Each was operating independently, transmitting from 2000 UTC for 20 minutes,  40 minutes &  60 minutes respectively.   The signal strength of each signal also varied, the strongest being 5305.5 kHz giving a hefty S5 in Lithuania.  Here in Southern England, only 5305 kHz had been monitored by me, and this was giving only an S3 reading here.

On 7 April 1999 Rimitas further reported signals on 4049 kHz,   4301 kHz,   4705 kHz,   & 6801 kHz   as well as 5305 kHz.

In England, in 1999, 4049 kHz, 4301 kHz, 5305 kHz and 6801 kHz could be clearly heard from early evening, transmitting for exactly twenty minutes.

    Analysis of the "20 Min Idler" signal

20 Min Idler Spectrum

When I updated this page in 2008, I was surprised to note that very little had changed except that the 4049 kHz frequency had possibly been dropped - it was not heard by me despite monitoring the channel on various days and times.

The other three frequencies heard in the UK during 1999, 4301 kHz, 5305 kHz and 6801 kHz, were still clearly transmitting the idler tone for 20 minutes every hour, and probably due to conditions prevailing at the time were clearly audible in South East England during most of the 24 hour period.

I monitored the "Twenty Minute Idler" at least two or three times a week on 6801kHz, as it was 1kHz removed from a CW numbers station I regularly logged.

During August 2010 I noticed the station was suddenly missing from its regular hourly schedule. This wasn't in itself unusual as it had occasionally disappeared before, presumably for maintenance or repairs, but transmissions had always resumed within a few days.

This time, however, the station failed to reappear and as it was still missing in Nov 2010, it was assumed that the station had permanently ceased transmissions.

Update: Jan 2014

In Jan 2014, I was surprised to receive an email from a British radio amateur, Matt G7OBR, who told me that he thought he had heard a similar sounding signal on 5305kHz, which ceased at exactly 20 mins past the hour.

After monitoring the freq for some days, it was clear that this was indeed the same signal, and despite some poor conditions, I did manage to log the station hourly at times from around 1600z to 0300z on 5305kHz always starting exactly on the hour.

There was, however, one noticeable difference. While most times the signal did cut off at exactly H+20 mins, there were occasions where it continued - sometimes for five or more minutes, even for another twenty minutes before ceasing.

Prior to 2010, the station was always known to conform to an exact twenty minute transmission. So if we assume this is an automated process, why now should the times occasionally vary?

Checks on the other known frequencies failed to find any trace of the signal, 5305kHz being the only remaining active frequency.

Update: Dec 2014

Following the re-acivation of the 5305kHz frequency, reported by Matt G7OBR in Jan 2014, I carried out checks at intervals on all the previously known frequencies - but nothing was found, so I left the station to its own devices for a while.

When I repeated checks on November 14, I found 5305kHz was now transmitting in parallel with another of the old frequencies, 4301kHz. Further checks on the two frequencies confirmed this, & it was noted that there was a very small time delay between the two transmissions - as if the two transmitters were being switched in sequence, either manually or automatically.

Although it is possible that both frequencies were active in Jan 2014, perhaps conditions preventing the 4301kHz transmission from being heard, this seems unlikely. There was no evidence of any other frequency being active despite checks made at different times of the day & night. It would appear that for whatever reason this previously dormant system is now being brought back online.

It will be interesting to monitor this station throughtout 2015 & to see if it continues, & whether any further frequencies are activated.

I can only think that this is some form of a back-up network ready to spring into life should whatever satellite, land-line or radio link fail, or in times of emergency. It does seem to be a forgotten relic of the Cold War era, and throughout all the time, from its discovery in 1998 it has never been heard to send any message, callsign or any other transmission other than the constant RTTY idler tones.

Sincere thanks to Rimitas Pleikys from Lithuania for his work on this signal, and for allowing it to be published here.

Thanks also to Matt G7OBR who rediscovered the signal in Dec 2013.

Update Jan 2015