Letter beacon

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Letter Beacon D on 5153.7 kHz

A Letter beacon (or single letter beacon) is a high frequency (HF) radio transmission of uncertain origin, which consists of only a single repeating Morse Code letter.

These transmissions sre often referred to as:

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



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)

Location of letter beacons

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, like the Ю beacon (22) which operated on 13627 and 13639 kHz in the early 1980s.

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) (24) 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 Callsign Location
A Astrakhan, RUS (tentative)
C RIW Moscow, RUS
D RCV Sevastopol, UKR
F RJS Vladivostok, RUS
K RCC Petropavlovsk-Kamchatsky, RUS
L Unknown, appeared on July 1, 2011
M RTS Magadan, RUS
P RMP Kaliningrad, RUS
S RIT 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
or Almaty, KAZ (25)

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. The identity of "P" beacon as an alias for RMP was first discovered by Ary Boender in 1995, when it transmitted RTTY weather forecast for Baltic Sea using callsign RMP on 3262 kHz [1].

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 kHz, 4558 kHz, 5154 kHz, 7509 kHz, 8495 kHz, 10872 kHz, 13528 kHz, 16332 kHz 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 cluster centered on 7509.0 kHz in the past transmitted on 7039.0 kHz, within the 40m international amateur radio allocation.

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

ID letter Regular callsign Channel Frequencies (kHz)
L (21) -0.8 5156.8, 7038.2, 7041.8(23), 8494.2, 8497.8
D RCV -0.3 3593.7, 4557.7, 5153.7, 7508.7, 8493.7, 10871.7, 13527.7, 16331.7, 20047.7
P RMP -0.2 3593.8, 4557.8, 5153.8, 7038.8, 7508.8, 8494.8, 10871.8, 10872.8 (21), 13527.8,
16331.8, 20047.8
S RIT -0.1 3593.9, 4557.9, 5153.9, 7508.9, 8494.9, 10871.9, 13527.9, 16331.9, 20047.9
C RIW 0.0 3594.0, 4558.0, 5154.0, 7039.0, 7509.0, 8495.0, 10872.0, 13528.0, 16332.0, 20048.0
A +0.1 3594.1, 4558.1, 5154.1, 7509.1, 8495.1, 10872.1 13528.1 16332.1 (10)
F RJS +0.2 5156.8, 7039.2, 8495.2, 10872.2, 13528.2, 16332.2
K RCC +0.3 4558.3, 5154.3, 7039.3, 8495.3, 10872.3, 16332.3
M RTS +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.
  • The cluster on 3594.0 kHz seems to be inactive in 2021.

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 kHz and 5465.9 kHz, operate exactly like the cluster beacons, sending only their single letter identifier. (20)

The majority of solitary beacons, and most notably "P" on various MF and HF frequencies, most of the time transmit their single-letter identifier in morse code. However, occasionally 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 communications 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 (20) 4325.9, 5465.9
V 3227.0, 3657.0, 4031.0, 4205.0, 4338.0, 4977.0, 5094.0, 5342.0,
5466.0, 5590.0, 7611.0, 7714.0, 7727.0
V (historic) 3335.0, 4028.0, 4108.0(18), 4150.0, 5141.0, 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
U 7830.5(27)
L 5156.8, 6917.4, 8497.8
W 8162.0, 8895.0

Channel marker "W", which appeared in 2012, is possibly related to a net for Russian Air Force Tu-95MS Bear H strategic bombers.

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 RTTY.(26)

ID letter Regular callsign Frequencies (kHz)
K 4005, 7906, 8144, 8158, 9043,
10571, 11555, 12150, 14477, 14967,
U 7569, 9057, 10216, 12185, 12238,

ENIGMA designation

Chris Midgley and Mike Gaufman of 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)

Purpose of letter beacons

The purpose and the applications of letter beacons are not yet known with certainty. Many theories and speculations have appeared in specialized bublications, 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).

Some High Frequency Beacons also transmit a single letter as identification. 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. ENIGMA 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. ENIGMA2000 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, pages 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://www.numbersoddities.nl/MX-profile.pdf , 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
  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.
  20. I doubt that "R" is a Navy service. The distance from Izhevsk (formerly Ustinov) to the nearset sea coasts is 1200 km. Only low frequencies are in use (3194//3321//4325) for 24 hrs operations. How could it reach ships in local daytime? Unlike clusters, it is not a propagation or navigation beacon. Unlike single "P" (RMP-Kaliningrad) naval station, it does not transmit any RTTY or CW messages. It is more similar to "The Pip" (3757;5448). In both cases, transmissions consist of a single letter (marker) in CW, and of short SSB voice messages, most probably for checking a readiness of network operators. I think that both "R" and "XP" are similar communications stations of Staffs of military districts (voenny okrug) of the Russian Army. The reliable coverage area would be up to 500 km, at least in local daytime.
  21. Cluster beacon "L" appeared on 7038.2 and 8494.2 kHz and was was first reported by the VERON Intruder Watch on June 29, 2011 at 1800 UTC. "L" was also active throughout July 2011. The first thoughts were that "L" transmitted from St Petersburg but so far there is no evidence to support this. It moved on July 23, 2011 to 7041.6 and 8497.9 kHz and again on July 24, 2011 to 7041.8 and 8497.8 kHz. (Ary Boender: "Number and Oddities", issue 166, pp.7). Cluster beacon "L" is not the solitary letter beacon "L" that transmitted in the past from Tirana, Albania, given that Albania is now a member of NATO. At the same time, the 10 MHz "P" beacon moved 1 kHz up, from 10871.8 to 10872.8 kHz.
  22. "SLHFB (Single letter high frequency beacons)", The SPEEDX reference guide to the Utilities, SPEEDX, 1984, page=K2.
  23. IARU R1 monitoring system: "Intruder News from: July 28th 2011"
  24. A couple of years ago I had the chance to speak to a Russian naval radio operator who works on a Russian frigate. He confirmed to me that the channel markers are military stations, mostly used by the navy. His English was poor so I could not ask for more detailed info, and as I don't speak Russian, you can imagine that it was a really nice conversation. I had to write down the morse characters of the call signs of the stations and he told me the locations. It was really fun, believe me :-) A couple of the markers that he mentioned have now disappeared but still exist as cluster beacons. The radio operator identified 'L' as St.Petersburg, 'P' as Kaliningrad (HQ of the Baltic Fleet), 'S' as Arkhangelsk and 'C' as Moscow (naval HQ). He knew that there were more stations but didn't know which ones because they were not relevant for his vessel. Ary Boender: "32nd edition of the N&O column / Spooks newsletter", December 24, 2000 [2].
  25. IARU R1 monitoring system, September 2016 [3]
  26. 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.
  27. ENIGMA-2000, Issue 109, November 2018, page 16.

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, pp. 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: "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.
  9. William I. Orr, W6SAI: "The Cluster Beacons Revisited; An Inside Look at Nine Puzzling Channels", Popular Communications, CQ Communications, Inc, ISSN 0733-3315, February 1985, pages 38-40.
  10. Enigma Control List [4], Enigma-2000, May 2005.
  11. Fritz Nusser: "Channel Markers and Cluster Beacons", Fascinatning Shortwaves (2001-2009)
  12. Alan Gale, G4TMV: "An introduction to beacon DXing", Version 1.0, December 2012, pp. 36-38.

External links

  1. Moscow's "C" Single Letter Cluster Beacon 7.390MHz 40mtr ham band, using a FT-817 HF tranceiver (Youtube video).
  2. Letter Beacon "D" on 5153.7 kHz using a Sangean 909 portable receiver (Youtube video).
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Wikipedia article: Letter_beacon

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