Not long after getting into the hobby, he ran across one of these transmissions. That was kind of odd. The story also quotes Hugh Stegman, an editor of the shortwave-radio publication Monitoring Times , who speculates that the numbers stations first popped up during the Cuban missile crisis. Shortwave radio, he notes, remains the best way to communicate secret information over great distances, as the devices are portable and can be used in remote sections of the globe. These days, one can hear numbers broadcasts day or night.
Most numbers stations keep very precise schedules.
Certain stations will be on at Wednesdays at 7 P.M., and every few months they'll change the frequency or change the time or something like that. But they keep very fixed schedules. This is considered a strength, strategy-wise. Sudden, unscheduled bursts of transmission would be a dead giveaway to any snooping listener that something is up back in the country of origin. They see that you are sending more often, and then they know something is up.