Transmission Technology
Although few numbers stations have been tracked down by location, the technology used to transmit the numbers has historically been clear — stock shortwave transmitters using powers from 10 kW to 100 kW.
Amplitude modulated (AM) transmitters with optionally–variable frequency, using class-C power output stages with plate modulation, are the workhorses of international shortwave broadcasting, including numbers stations.
Application of spectrum analysis to number station signals has revealed the presence of data bursts, RTTY-modulated subcarriers, phase-shifted carriers, and other unusual transmitter modulations like polytones. (RTTY-modulated subcarriers were also present on some U.S. commercial radio transmissions during the Cold War.)
The frequently reported use of high tech modulations like data bursts, in combination or in sequence with spoken numbers, suggests varying transmissions for differing intelligence operations.
For spies in the field, low tech spoken number transmissions continue to have advantages in the 21st century. High tech data receiving equipment is difficult to obtain, and being caught with more than a civilian shortwave news radio could be construed as evidence of spying. Yet governments' embassies, aircraft, and ships at sea are known to possess complex receiving equipment that could make regular use of encrypted data transmissions from the home country. These probably include charts and photos that require more transmitted data than can be sent efficiently using spoken numbers.
Read more about this topic: Numbers Station
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