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"
MX — an ENIGMA 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......