Few QSL responses have been received from numbers stations by shortwave listeners who sent reception reports to stations that identified themselves or to entities the listeners believed responsible for the broadcasts, which is the expected behaviour of a non-clandestine station. The Czech Ministry of the Interior and the Swedish Security Service have both acknowledged the use of numbers stations by Czechoslovakia for espionage, with declassified documents proving the same. Many stations from this era continue to broadcast and some long-time stations may have been taken over by different operators.

According to an internal Cold War-era report of the Polish Ministry of the Interior, numbers stations DCF37 (3.370 MHz) and DFD21 (4.010 MHz) transmitted from West Germany beginning in the early 1950s. Numbers stations were most abundant during the Cold War era. It is reported that Archduke Anton of Austria in his youth during World War I used to listen in to their transmissions, writing them down and passing them on to the Austrian military intelligence. Amongst amateur radio enthusiasts there is an interest in monitoring and classifying numbers stations with many being given nicknames to represent their quirks or origins.Īccording to the notes of The Conet Project, which has compiled recordings of these transmissions, numbers stations have been reported since World War I with the numbers transmitted in Morse code. Numbers stations have been reported since at least the start of the first world war and continue to be in-use today. Stations may have set frequencies in the high frequency band. Most stations have set time schedules, or schedule patterns however, some have no discernible pattern and broadcast at unpredictable times. Most identified stations use speech synthesis to vocalize numbers, although digital modes such as phase-shift keying and frequency-shift keying, as well as Morse code transmissions, are not uncommon. A numbers station is a shortwave radio station characterized by broadcasts of formatted numbers, which are believed to be addressed to intelligence officers operating in foreign countries.

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