For decades, SWLs have been hearing stations that do nothing but read blocks of numbers, usually using a woman's voice, in a variety of languages and on innumerable different frequencies. All available evidence indicates that some of these transmissions may be somehow connected to espionage activities. These are the numbers stations, the most enduring mystery on the shortwave bands. Even though they do not operate on any fixed schedule or frequencies known to the public, numbers stations are really very easy to hear. Just tune outside the established shortwave broadcasting or ham radio bands and you'll hear several with patient tuning. While numbers stations can be heard any time on any frequencies, most seem to be heard in North America during the evening and night hours on frequencies from 3 to 12 MHz. Most numbers stations heard in North America transmit in Spanish. Other languages often heard include English, German, and scattered other languages such as Chinese and Russian. Almost all will use a woman's voice, although on rare occasions a man's voice might be used. You will quickly notice that the numbers you hear sound much like the "intercept" messages used by telephone companies to give out new numbers when you dial a discontinued number, and a similar technology is obviously in use to generate numbers messages. There are two main types of English and Spanish numbers transmissions you can hear. The first, the five-digit stations, transmits messages in blocks of five digits. The second, the 3/2-digit stations, are so named because there is a distinct pause between the third and fourth digit of each group. For years, stations were heard in Spanish and English transmitting numbers in four-digit blocks. These now seem to have been replaced by the 3/2-digit stations, although some four-digit German numbers stations are still heard in Europe. Five-digit Spanish stations are the most frequently heard variety. They begin transmissions with something like "atención 341 67" repeated for several minutes. The three-digit group is believed to the identifier of the recipient of the message, while the second number is the number of five-digit groups in the message. Sometimes as many as three different messages may be sent in the same transmission. Transmissions usually conclude with the words "fínal, fínal." Almost all of these stations use AM. The next most commonly heard type is the 3/2-digit Spanish station. These open with a three-digit group sent three times (again believed to be the intended recipient) followed by "1234567890." This sequence is repeated for several minutes. After ten tones, something like "grupo 154, grupo 154" is sent. The number following "grupo" is the number of groups that will be transmitted. Numbers stations using English, German, and other languages generally follow similar formats, although often with some significant variations. Some numbers stations using Morse code have been reported , although the Morse is usually sent using audio tones over AM instead of as CW. Morse code numbers stations were once common, but their number has steeply declined in recent years. One interesting aspect of numbers broadcasts is that a message may be repeated hours or days later on the same or different frequency . The purpose of this is not known, although one logical reason would be to allow the intended recipient another opportunity to receive the message in the event they were unable to listen at the earlier time. Another possible explanation is that some messages could be for training and practice rather than to actually convey information. The formats used by the five-digit and 3/2-digit stations follow two well known cryptological techniques. The first is the one-time pad system, which fits with the format of the five-digit stations. In this system, both the sender and recipient have copies of a code pad. The code pad will have several columns of numbers, seemingly at random. The received numbers are added or subtracted from the numbers on the sheet of the one-time pad being used, and the results are compared to a master code key list. Each number block will usually represent a complete word or phrase rather than a sequence of characters. After each page in a one-time pad is used, it is torn from the pad and destroyed. (The CIA has reportedly developed one-time pads whose pages turn into chewing gum upon contact with saliva!) One-time pads as small as postage stamps are known to exist. While the one-time method sounds crude, messages sent using it are unbreakable so long as a copy of the pad used doesn't fall into the hands of the "opposition." The 3/2-digit stations seem to be using some variation of the dictionary code system. In this system, a book available to the sender and recipient is used as the basis of message encoding and decoding. The first three digits represent the page number, while the last two digits are the position of a certain word on the page, usually counting from the upper left corner of the page. This method has the advantage of eliminating the risk of delivering one-time pads to agents and having those pads discovered. The big disadvantage has traditionally been a lengthy, slow encoding process, although computers can now handle this easily and quickly. It may be no accident that 3/2-digit first appeared in the early 1980s, when PCs began to greatly increase in power! Many 3/2-digit transmissions are made using reduced carrier AM. This modulation method reduces the carrier power and puts additional power in the sidebands. This method allows improved intelligibility and reception over normal AM, but is compatible with shortwave radios that only receive AM signals. While some dispute this conclusion, there is evidence that most numbers stations may be connected with intelligence and espionage operations . Books such as The Spy Who Got Away by David Wise, Widows by William R. Corson, Susan B. Trento, and Joseph J. Trento, and KGB Today by John Baron, have all detailed espionage operations, both American and Soviet, involving the use of messages sent via radio as number groups. Other spies, like England's Geoffrey Prime, have been arrested with one-time pads and a shortwave radio in their possession. However, not all numbers stations may be connected to espionage. Some transmissions, especially those using a man's voice or sent "live" rather than taped, may be connected to drug smuggling or other illegal activities. The five-digit Spanish transmissions appear to be originating within Cuba. These often have hum and other technical difficulties, and sometimes audio from Radio Havana Cuba has been heard mixed in with the numbers messages! This indicates the five-digit Spanish numbers and Radio Havana Cuba probably use the same transmitting facilities. Another possibility is that some numbers stations operate from embassies and consulates in the United States and elsewhere. Under international law, embassies and consulates have the right to maintain radio facilities to communicate with their home countries. Since embassies and consulates are "extraterritorial" to the nation they are located in, stations there could be used for numbers transmissions. There are still other mysteries surrounding numbers stations. SWLs equipped with spectrum analyzers have found data bursts buried within numbers transmissions . Subaudible tones have been detected on other numbers signals. And listeners like Brian Webb of California and Zel Eaton of Missouri have heard numbers transmissions, in the AM mode, on frequencies above 30 MHz during sporadic-E openings. This is noteworthy because frequencies above 30 MHz are normally restricted to local reception. There is another category of mystery station closely related to numbers stations, known as "phonetic" stations . These transmit messages using groups of five letters from the international phonetic alphabet (alpha, bravo, charlie, delta, etc.). These stations are often heard repeating a phrase like "charlie india oscar two" for hours before any message is actually sent. It is widely believed that phonetic stations are operated by the Mossad, Israel's intelligence service . In addition to Israel itself, it is believed that these stations also operate from Israeli embassies. (Some of these stations have been heard with local quality signals in the Washington area, for example.) While most SWLs couldn't care less about numbers stations, others are obsessed with them. And it's understandable. . . . how often do you get to play James Bond and match wits with the CIA, the Mossad, and other intelligence services of the world?