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 48°30’N 134°58’E? / ?48.5°N 134.967°E? / 48.5; 134.967, 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.

Cluster Beacons

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.

The following callsigns and frequencies of cluster beacons have been reported recently (September 2007 to November 2008) in Numbers&Oddities newsletter, Utility DX Forum and ENIGMA-2000:

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, 16332.1
F 7039.2
K 5154.3, 7039.3, 8495.3
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.

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”, 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.

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 (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

FSK beacons

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 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:

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):

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.

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