A communications satellite receives a signal from Earth, amplifies it, and retransmits it in the form of a television signal, a telephone signal or any other communications signal. Most other satellites also receive and transmit signals, however. The instrument necessary for this is a transponder.
A transponder receives a signal from a station on Earth at one frequency, 6 GHz, for example. The transponder then amplifies the signal and shifts it to a new frequency, say 4 GHz. A transmitter within the transponder amplifies the shifted signal using a travelling wave tube. A travelling wave tube is just a hollow tube that is designed to fit a wave of a particular frequency. Therefore, the width of the tube depends on the frequency of the signal. This will create a travelling wave.
The Canadian communications satellites, the Anik series, all use transponders. There are actually 12 five-watt travelling wave tubes aboard Anik A that boost the signals so that they can be easily received on Earth. Each of these tubes could carry one television program or 960 voice channels. After the signal is amplified, it is then transmitted back to Earth at its new frequency.