[…]If you ever find yourself wishing your life was filled with a little more mystery, a little more excitement, save up thirty dollars and go buy a shortwave radio and start scanning the airwaves. With a little luck and a lot of patience, you might come across a band carrying the monotone voice of a man reciting military call letters (“Echo — Kilo — Charlie”), the robotic voice of a woman counting in Russian, or one broadcasting a continual string of beeps, chirps or hums. If you manage to tune in to one of these stations, you could very well be listening in on a coded message intended for a spy.
Once thought to be useless for communications purposes because of their high frequencies and short wavelengths, shortwave radio bands were, it was discovered in the 1920’s, ideal for sending messages over extremely long distances. Shortwave signals are broadcast into the sky, where they are reflected or refracted off of the electrically-charged ionosphere layer of the upper atmoshpere and sent back down to Earth as far as a continent away. By the start of the Cold War a few decades later, countless shortwave radio stations were found to be broadcasting strange, seemingly coded, messages using this technique. At the time, these so-called “number stations” (or “numbers stations”) were widely believed to be in use by various government intelligence agencies to broadcast secret messages to spies over great distances, but none of the codes — if, indeed, that’s what the signals were — were ever cracked and no governments ever officially acknowledged their use.
Number stations remained a mystery and were all but forgotten by the general public until the discovery in the ’70s of a station that was broadcasting a powerful signal comprised of the synthesized voice of a British woman speaking a sequence of five numbers. The station came to be known as “The Lincolnshire Poacher” because two bars from the English folk song of that name served as interval signals. The signal from the station was traced back to the Royal Air Force base at Akrotiri, on the island of Cyprus, providing number station enthusiasts with the first “proof” that government organizations like the British Secret Intelligence Service were behind some, if not most, of the number stations.[…]