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  and ENIGMA-2000  designation.
These radio transmissions were discovered in the late 1960s. 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. There is indirect evidence that this particular transmitter was located in Cuba. 
In 1982 there were also reports, supposedly based on HF direction finding by the US military, that beacon “K” transmitting on 9043 kHz was located at , near the city of Khabarovsk in the USSR.  A few years later, it was 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. 
According to Schimmel, in 1986 the Federal Communications Commission (FCC) released the following HF direction finding results for single letter beacons, all of which indicate locations in the USSR: 
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".
A recent source (2006) regarding locations was published on the Web by Ary Boender. 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 (inactive) 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. Kaliningrad officially uses the ITU registered callsign RMP.
The single letter beacons can be 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 (December 2007) active, according to published listeners’ reports.
A group of radio beacons with single-letter identifiers ("C", "D", "M", "S", "P", "A", "M" and "K") have been regularly reported near 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.
ID letter Frequencies (kHz) D 3593.7, 4557.7, 5153.7, 7038.7, 8494.7, 10871.7, 13527.7, 16331.7, 20047.7 P 3593.8, 4557.8, 5153.8, 7038.8, 8494.8, 10871.8, 13527.8, 16331.8, 20047.8 S 3593.9, 4557.9, 5153.9, 7038.9, 8494.9, 10871.9, 13527.9, 16331.9, 20047.9 C 3594.0, 4558.0, 5154.0, 7039.0, 8495.0, 10872.0, 13528.0, 16332.0, 20048.0 A 3595.1, 4558.1, 5154.1, 7039.1, 8495.1, 10872.1, 13528.1, 16332.1 F 7039.2, 8495.2, 10872.2, 16332.2 K 5154.3, 7039.3, 8495.3, 10872.3, 13528.3 M 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 all of them 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.
However, the majority of solitary beacons, most notably "P" on various MF and HF frequencies, transmit their single-letter identifier in Morse code. 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" , 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 call-sign RIW, which is allocated to a Russian naval communication station in Khiva, Uzbekistan.
There are also a few oddities, transmitting signals with poor modulation and irregular timing, like "V" on 5342 and 6430.7 kHz.
The following call-signs and frequencies of solitary beacons and markers have been reported recently (September 2007 to September 2009) in Numbers&Oddities newsletter, Utility DX Forum and ENIGMA-2000:
ID letter Frequencies (kHz) R 4325.9, 5465.9 V 3658.0, 5141, 5342, 6430.7, 6809, 7027.5, 8103.5, 10202 P  420, 583, 3167, 3291, 3327, 3699.5, 3837, 4031, 4043, 4079 C 8000
This group includes 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" is designated F1A. The use of FSK indicated that the transmitter was suitable for FSK data transmissions, like Radioteletype.
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: 
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 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)
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.  Some stations of this family, in particular the “U” beacon, have been implicated in deliberate jamming.
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.
Connolly also links "P" channel marker with communications facilities at the Russian naval base of Kaliningrad . "P" transmissions carrying Russian Navy "XXX" (flash priority) morse code messages with callsigns RPM and RDL further support this view.
A few aero navigation Non Directional 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).
Notes & references
- ^ 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 1990s. They issued a printed bulletin every few months, which was mailed to subscribers. The 18 in total issues of the ENIGMA bulletin contained a lot of interesting information about odd and mysterious high frequency transmissions, contributed from radio listeners worldwide. The reception logs were organized, collated and analyzed by the core ENIGMA staff. The coverage was mostly about the so-called Numbers stations, but there was also some coverage of letter beacons. Those bulletins were not archived in libraries and it is difficult to obtain copies today. The major achievement of ENIGMA was the systematic classification and naming of the numerous radio oddities, which were previously described by different and conflicting names in various publications. The original ENIGMA group was disbanded in 2000. In his farewell letter, ENIGMA co-founder Mike G. wrote that all ENIGMA publications and research material would be deposited at the British Library (Boston Spa).
- ^ 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.
- ^ Anonymous (1984). "SLHFB (Single letter high frequency beacons)", The SPEEDX reference guide to the Utilities. SPEEDX. p. K1.
- ^ Anonymous (1984). "SLHFB (Single letter high frequency beacons)", The SPEEDX reference guide to the Utilities. SPEEDX. pp. K7–K10.
- ^ a b Schimmel, D.W. (1994). The underground frequency guide. HighText Publications, Inc.. pp. 78–83. ISBN 1-878707-17-5.
- ^ a b William I. Orr, W6SAI (December 1984). "High Frequency Single-Letter Beacons (SLBs); Part 1: The K- and U-Beacons, The Search Goes On". Popular Communications (CQ Communications): 28–31. ISSN 0733-3315.
- ^ a b Anonymous (January 2000). "Station News". ENIGMA Newsletter (ENIGMA) (18): 15.
- ^ Ary Boender (2006-09-02). "Channel Markers & Cluster Beacons". http://www.numbersoddities.nl. Retrieved 2008-01-18.
- ^ a b Robert Connolly (January 2008). "Maritime matters: Why we hear more signals from the Russian Navy?". Radio User (PW Publishing Ltd) 3 (1): 32. ISSN 1748-8117.
- ^ Ary Boender (December 2008). "Numbers & Oddities, issue 135" (ZIP). http://www.numbersoddities.nl/2008.zip. Retrieved 2009-02-12.
- ^ a b Poundstone, Willian (1983). Big Secrets. New York: Quill. pp. 191–193. ISBN 0688048307.
- ^ Ary Boender (July 2009). "Numbers & Oddities, issue 142" (ZIP). http://www.numbersoddities.nl/2009.zip. Retrieved 2009-10-04. [dead link]
- ^ Some transmission are in FSK morse code (F1A) instead of CW (A1A), but other beacon characteristics classify it as a solitary "P" beacon.
- ^ Anonymous (1995-01-16). "Station Naming". ENIGMA Newsletter (ENIGMA) (7): 11–12.
- ^ "ENIGMA Control List, Number 23" (PDF). ENIGMA-2000. October 2007. http://www.ominous-valve.com/ecl23.pdf. Retrieved 2008-01-19.
- ^ Pleikys, Rimantas (1998). Jamming. Vilnius Lithuania: Rimantas Pleikys. http://www.zilionis.com/jamming/jamminge.htm.
- Schimmel, D.W. (1994). The underground frequency guide. HighText Publications, Inc.. ISBN 1-878707-17-5. OCLC 31628702.
- Harry L. Helms, W5HLH (1981). How to tune the secret shortwave spectrum. Tab Books, Inc.. pp. 141–143. ISBN 0-8306-1185-1.
- Anonymous (1984). "SLHFB (Single letter high frequency beacons)", The SPEEDX reference guide to the Utilities. SPEEDX.
- Spooks mailing list.
- 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.
- Mike G. (January 1998). "Single letter cluster beacons". ENIGMA Newsletter (ENIGMA) (14): 31–33.
- Simon Mason (January 1999). "New revelations about single letter transmissions (MX)". ENIGMA Newsletter (ENIGMA) (16): 39–40.
- William I. Orr, W6SAI (December 1984). "High Frequency Single-Letter Beacons (SLBs); Part 1: The K- and U-Beacons, The Search Goes On". Popular Communications (CQ Communications, Inc): 28–31. ISSN 0733-3315.
- William I. Orr, W6SAI (January 1985). "Those Mysterious High Frequency Single-Letter Beacons (SLBs); Part 2: The “Cluster Beacons” – A Soviet Riddle!". Popular Communications (CQ Communications, Inc): 22–24. ISSN 0733-3315.
- William I. Orr, W6SAI (February 1985). "The Cluster Beacons Revisited; An Inside Look at Nine Puzzling Channels". Popular Communications (CQ Communications, Inc): 38–40. ISSN 0733-3315.
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