Telecommunication
The other meaning of the term "channel" in telecommunications is seen in the phrase communications channel, which is a subdivision of a transmission medium so that it can be used to send multiple streams of information simultaneously. For example, one radio station can broadcast radio waves into free space at frequencies in the neighborhood of 94.5 MHz (megahertz) while another radio station can simultaneously broadcast radio waves at frequencies in the neighborhood of 96.1 MHz. Each radio station would transmit radio waves over a frequency bandwidth of about 180 kHz (kilohertz), centered at frequencies such as the above, which are called the "carrier frequencies". Each station in this example is separated from its adjacent stations by 200 kHz, and the difference between 200 kHz and 180 kHz (20 kHz) is an engineering allowance for the imperfections in the communication system.
Soldatensender Calais
"Soldatensender Calais" operated on the mediumwave band on 833 kHz (360 metres), 714 kHz (420 metres), and 612 kHz (490 metres), with an associated shortwave station "Kurzwellensender Atlantik" created to broadcast to U-boat crews. The station used a 500 kilowatt transmitter originally built for American broadcaster WJZ, in Newark, New Jersey. This transmitter had lain unused at the factory after the United States Federal Communications Commission imposed a 50 kW power limit on all U.S. stations, and so RCA was glad to sell it overseas and the British Secret Service bought it for £165,000. Codenamed Aspidistra, it was installed in a huge, underground bunker near Crowborough in Sussex, England, where it was briefly the world's largest medium wave station, perfect for deceptive "black" operation.