What Are Satellite TV Transponders?
By Charles Clay
The satellites which broadcast satellite television signals to homes and businesses are essentially orbital repeating stations. A transponder on a television satellite receives a television signal beamed from large satellite uplink stations on the ground, then processes, encodes, amplifies and rebroadcasts the signal over a large area of the Earth's surface. There, it is picked up by small satellite receivers and transmitted to a digital receiver in the customer's living room, where it is decoded and displayed on a connected television screen.
Transponders
A transponder is a device for receiving and rebroadcasting a signal. Usually, this signal is amplified by the transponder, and can be encoded or modified in other ways. The word "transponder" is derived from the words "transmitter" and "responder." The transponders aboard television satellites are designed to receive signals from uplink stations, amplify them and rebroadcast them, taking advantage of the enormous line of sight provided by the satellite's altitude. The signal is encoded to ensure that only paying subscribers are able to access the signal.
Uplink Stations
Satellite uplink stations are large Earth-based broadcasting units designed to transmit a tight beam of information to orbital satellites. They look like large upward-facing satellite dishes, usually nine or 10 meters across. While they are capable of producing powerful transmissions, any terrestrial broadcast from these stations would be blocked within several hundred miles by the curvature of the Earth. By broadcasting to orbital satellites, they are able to blanket large portions of a continent with a single signal.
Channels
Most television satellites carry 24 to 56 transponders. Each transponder is capable of receiving, amplifying and broadcasting a compressed stream of data carrying multiple channels. Analog satellite systems can broadcast around 30 channels per transponder, while digital systems use compression techniques to carry up to 200 channels on a single transponder. Television transponders are designed to retransmit received signals at a different frequency than the uplink signal to avoid interference.
Television Dish Receivers
The dish receiver that sits on top of the television customer's home is similar in many ways to "rabbit ear" television antennae. Its function is to receive and focus the signals broadcast by the orbital transponders, scrub the signal of interference and noise and route the signal to the satellite receiver attached to the television. In switching channels, the customer instructs the receiver to decode and display various data streams broadcast by different transponders aboard the orbital satellite.