Letter beacon

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Letter Beacon D on 5153.7 kHz

"Letter beacons" are radio transmissions of uncertain origin, which consist of only a single repeating Morse Code letter.

They are also often referred to as:

  • SLB, or "Single Letter Beacons"
  • SLHFB, or "Single Letter High Frequency Beacons"
  • SLHFM, or "Single Letter High Frequency Markers"
  • Cluster beacons
  • MX — an ENIGMA (1) and ENIGMA-2000 (2)designation.


Location of letter beacons

The letter beacon radio transmissions were discovered in the late 1960s but were known only to a few specialized DXers. Their presence became known to the wider amateur radio community in 1978, when beacon “W” started transmitting on 3584 kHz, in the 80 meters band. SPEEDX published indirect evidence that this particular transmitter was located in Cuba. (3)

In 1982 SPEEDX reported, supposedly on the basis of HF direction finding by the US military, that beacon “K” transmitting on 9043 kHz was located at 48° 30' N - 134° 58' E, near the city of Khabarovsk in the USSR. (4), (5) A few years later, W. Orr, W6SAI, suggested that the "K" beacons were actually located at Petropavlovsk on the Kamchatka Peninsula and the "U" beacons were located at the Barents Sea coast, between Murmansk and Amderma. (6)

According to Schimmel, in 1986 the FCC released the following HF direction finding results for single letter beacons, all of which indicate locations in the USSR: (5)

ID letter Location
C Moscow, RUS
D Odessa, UKR
O Moscow, RUS
P Kaliningrad, RUS
S Arkhangelsk, RUS
U Between Murmansk & Amderma, RUS
Z Mukachevo, UKR

The link with the USSR and, more recently, Russia is further supported by the existence of single letter beacons transmitting letters existing only in the Cyrillic morse code alphabet.

The ENIGMA group also accepted these locations for cluster beacons "C", "D", "P" and "S", adding Vladivostok for beacon "F". (7) A recent source (2006) regarding locations was published on the Web by Ary Boender.(8) This publication also contains an extensive list of frequencies of letter beacons, both current and historical. The following locations are stated for cluster beacons:

ID letter Location
A Astrakhan, RUS (tentative)
C Moscow, RUS
D Sevastopol, UKR
F Vladivostok, RUS
K Petropavlovsk-Kamchatsky, RUS
M Magadan, RUS
P Kaliningrad, RUS
S Severomorsk, RUS

For solitary beacons and markers, Boender suggests these locations:

ID letter Location
L Tirana, ALB (defunct)
R Izhevsk (Ustinov), RUS
P Kaliningrad, RUS
V Khiva, UZB

Transmissions of the "P" beacon in December 2007, even on medium frequency (420 and 583 kHz) indicate the Russian Naval Base of Kaliningrad as a possible source. (9) Kaliningrad officially uses the ITU registered callsign RMP.

Types of letter beacons

The single letter beacons are currently classified in two groups, the "Cluster beacons" and the "Channel markers". A beacon "P" exists in both groups. A third group, the FSK beacons, is now extinct. The following sections list the beacons currently (July 2010) active, according to published listeners’ reports.

Cluster beacons

A group of radio beacons with single-letter identifiers ("C", "D", "M", "S", "P", "A", "M" and "K") have been regularly reported in small spectrum segments centered around 3594, 4558, 5154, 7039, 8495, 10872, 13528, 16332 and 20048 kHz. The term "cluster beacons" is frequently used for them, as these beacons transmit in parallel on frequencies only 0.1 kHz apart. These beacons transmit only their single-letter identifier in standard CW (A1A) using morse code.

The following callsigns and frequencies of cluster beacons have been reported recently in Numbers&Oddities newsletter, Utility DX Forum and ENIGMA-2000:

ID letter Channel Frequencies (kHz)
D -0.3 3593.7, 4557.7, 5153.7, 7038.7, 8493.7, 10871.7, 13527.7, 16331.7, 20047.7
P -0.2 3593.8, 4557.8, 5153.8, 7038.8, 8494.8, 10871.8, 13527.8, 16331.8, 20047.8
S -0.1 3593.9, 4557.9, 5153.9, 7038.9, 8494.9, 10871.9, 13527.9, 16331.9, 20047.9
C 0.0 3594.0, 4558.0, 5154.0, 7039.0, 8495.0, 10872.0, 13528.0, 16332.0, 20048.0
A +0.1 3594.1, 4558.1, 5154.1, 7039.1, 8495.1, 10872.1 13528.1 16332.1 (10)
F +0.2 7039.2, 10872.2, 16332.2
K +0.3 5154.3, 7039.3, 8495.3, 16332.3
M +0.4 5154.4, 7039.4, 8495.4, 10872.4, 13528.4, 16332.4

Occasionally some cluster beacons (especially "F" and "M") have been reported transmitting on frequencies different from their regular channel for short periods.

Solitary beacons and channel markers

A second family of letter beacons includes those operating outside the clusters. For this reason they are often called "Solitary beacons" or "Solitaires". These beacons also transmit their single-letter identifier in standard CW (A1A) using morse code.

A few solitary beacons, like "R" on 4325.9 and 5465.9 kHz, operate exactly like the cluster beacons, sending only their single letter identifier.

The majority of solitary beacons, however, and most notably "P" on various MF and HF frequencies, most of the time they are active transmit their single-letter identifier in morse code. However, sometimes the routine transmission is interrupted and brief messages are sent in fast morse code or in an FSK digital mode. Therefore, the proper term for these beacon-like single-letter transmissions is "channel markers" (6) (15), as their purpose is to occupy and identify a particular HF transmission channel when no traffic is transmitted. There is no evidence that the cluster beacon "P" and the solitary beacon "P" are directly related.

It was reported in Numbers and Oddities, issue 142, that beacon C on 8000 kHz also transmitted messages under the regular callsign RIW, which is allocated to a Russian naval communicatios station in Khiva, Uzbekistan. (11)

There are also a few oddities, transmitting signals with poor modulation and irregular timing, like "V" on 5342 and 6430.7 kHz.

The following callsigns and frequencies of solitary beacons and markers have been reported recently in Numbers&Oddities newsletter, Utility DX Forum and ENIGMA-2000:

ID letter Regular callsign Frequencies (kHz)
R 4325.9, 5465.9
V 3658.0, 4108.0(18), 4150.0, 5141.0, 5342, 6430.7, 6498.0, 6809, 7027.5, 8103.5, 10202
P (12) RMP 420, 448, 474, 490(17), 583,
3167, 3291, 3327, 3699.5, 3837, 4031, 4043, 4079
C RIW 8000
L inactive
W inactive

FSK beacons

This group included the "K" and "U" beacons, which are no longer active. They transmitted their morse code single letter identification by shifting the frequency of the carrier by approximately 1000 Hz. This mode of "FSK-CW" has the ITU designation F1A. The use of FSK indicated that the transmitter was suitable for FSK data transmissions, like Radioteletype.

ID letter Regular callsign Frequencies (kHz)
K 9043

ENIGMA designation

ENIGMA devised a naming scheme for all stations in their sphere of interest. In the original scheme, the following identifications were issued to letter beacons: (13)

ENIGMA ID Description
MX Cluster beacons
MXV Irregular “V” beacons, not in clusters
MXS Solitaires: letter beacons out of cluster bands
MXF FSK beacons (K, U), no longer active in 1995

ENIGMA-2000, the internet based ENIGMA successor group, revised the original ENIGMA designators. The current designations for letter beacons are the following (since 2007):(14)

ENIGMA ID Description
MX Solitary HF single letter beacons
MXI Single letter beacons in clusters
MXII FSK beacons (K, U), no longer active
MXV Irregular “V” transmissions
MXP Letter beacons also sending messages
MXIII (deleted, merged with MX)
MXIV (deleted, merged with MX)

Applications of letter beacons

The purpose of the letter beacons is not known yet with certainty. Many theories have appeared in specialized publications but none is based on documentary evidence. They have been postulated to be radio propagation beacons, channel markers, used in tracking satellites, or used for civil defense purposes. (15) Some stations of this family, in particular the “U” beacon, have been implicated in deliberate jamming. (16)

Today the radio propagation beacon theory is generally accepted for the cluster beacons. According to ENIGMA the cluster beacons are used by the Russian Navy (and especially the submarine branch) to find the most suitable radio frequency for contact based on current radio propagation conditions. (7)

Connolly also links "P" channel marker with communications facilities at the Russian naval base of Kaliningrad. (9) "P" transmissions carrying Russian Navy "XXX" (flash priority) morse code messages with callsigns RPM and RDL further support this view.

Similar systems

QSL card from a USCG beacon with signle letter ID

A few aero navigation Non Directional Beacons (NDBs) and marine beacons also transmit single letter identification codes. They can be easily distinguished from Letter beacons as they transmit in the allocated low frequency and medium frequency bands, most of them are listed in appropriate aviation handbooks and their transmission mode is A2A (full carrier with audio modulation).

On September 7, 2010 a beacon was heard on 9111.7 kHz at 1546 UTC. It sent a slow marker "A", which did not sound like a Russian beacon (MX). It sounded like somebody was playing with the key sending letters "A" and "M". Transmission lasted untill at least 1630 UTC. (19)

See also

Notes & References

  1. E.N.I.G.M.A. stands for "European Numbers Information Gathering and Monitoring Association". It was a unique association of radio listeners based in the United Kingdom and operated during the 1990’s.
  2. ENIGMA-2000 is an internet based community with the same general interests as the old ENIGMA association and with wider coverage of general Intelligence matters. This group produces a regular newsletter and maintains the old ENIGMA station naming scheme. ENIGMA-2000 shows less interest in letter beacons than its predecessor.
  3. "SLHFB (Single letter high frequency beacons)", The SPEEDX reference guide to the Utilities, SPEEDX, 1984, page=K1.
  4. "SLHFB (Single letter high frequency beacons)", The SPEEDX reference guide to the Utilities, SPEEDX, 1984, page=K7-K10.
  5. Schimmel, D.W.: "The underground frequency guide", HighText Publications, Inc., ISBN 1-878707-17-5, 1994, pages=78–83.
  6. William I. Orr, W6SAI: "High Frequency Single-Letter Beacons (SLBs); Part 1: The K- and U-Beacons, The Search Goes On", Popular Communications, ISSN 0733-3315, December 1984, pages 28-31.
  7. "Station News", ENIGMA Newsletter, issue 18, January 2008, page 15.
  8. Ary Boender: "Channel Markers & Cluster Beacons", http://home.luna.nl/~ary/chmarker.htm , September 2006.
  9. Robert Connolly: "Maritime matters: Why we hear more signals from the Russian Navy?", Radio User, ISSN 1748-8117, PW Publishing Ltd, Issue 3.1, January 2008, page 32.
  10. Ary Boender: "Numbers & Oddities" issue 135, http://www.ary.luna.nl/2008.zip , December 2008.
  11. Ary Boender: "Numbers & Oddities" issue 142, http://www.ary.luna.nl/no142.zip , July 2009.
  12. Some transmission are in FSK morse code (F1A) instead of CW (A1A), but other beacon characteristics classify it as a solitary "P" beacon.
  13. "Station Naming", ENIGMA Newsletter, issue 7, January 1995
  14. ENIGMA Control List, Number 23, ENIGMA-2000, October 2007, http://www.ominous-valve.com/ecl23.pdf
  15. Poundstone Willian: "Big Secrets", Quill, New York, 1983, ISBN 0688048307, pages 191-193
  16. Pleikys Rimantas: Jamming, Rimantas Pleikys, Vilnius 1998, http://www.zilionis.com/jamming/jamminge.htm
  17. Robert Connolly: "DGPS, Single Letter Beacons and NDB changes", Radio User, ISSN 1748-8117, PW Publishing Ltd, Issue 5.10, October 2010, page 49.
  18. Ary Boender: "Numbers & Oddities" issue 159, http://www.numbersoddities.nl/n&o-159.pdf, December 2010.
  19. Ary Boender: "Numbers and Oddities", issue 156, September 2010, pp. 15.

Further reading

  1. Schimmel, D.W.: "The underground frequency guide", HighText Publications, Inc., ISBN 1-878707-17-5, 1994.
  2. Harry L. Helms, W5HLH: How to tune the secret shortwave spectrum, Tab Books, Inc., ISBN 0-8306-1185-1, 1981, pages=141–143.
  3. "SLHFB (Single letter high frequency beacons)", The SPEEDX reference guide to the Utilities, SPEEDX, 1984.
  4. Spooks mailing list.
  5. Numbers and Oddities: Ary Boender compiles this monthly bulletin with reception reports of various mysterious transmissions and makes it available for download at his personal web site.
  6. Mike G.: "Single letter cluster beacons", ENIGMA Newsletter #14, January 1998, pages 31-33.
  7. Simon Mason: "New revelations about single letter transmissions (MX)", ENIGMA Newsletter #16, January 1999, pages 39-40.
  8. William I. Orr, W6SAI: "High Frequency Single-Letter Beacons (SLBs); Part 1: The K- and U-Beacons, The Search Goes On", Popular Communications,CQ Communications, Inc, ISSN 0733-3315, December 1984, pages 28-31.
  9. William I. Orr, W6SAI: "Those Mysterious High Frequency Single-Letter Beacons (SLBs); Part 2: The Cluster Beacons – A Soviet Riddle!", Popular Communications,CQ Communications, Inc, ISSN 0733-3315, January 1985, pages 22-24.
  10. William I. Orr, W6SAI: "TThe Cluster Beacons Revisited; An Inside Look at Nine Puzzling Channels", Popular Communications, CQ Communications, Inc, ISSN 0733-3315, February 1985, pages 38-40.
  11. Enigma Control List [1], Enigma-2000, May 2005.
  12. Fritz Nusser: "Channel Markers and Cluster Beacons", Fascinatning Shortwaves (2001-2009)
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Wikipedia article: Letter_beacon

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