A transponder is a wireless device that transmits and receives electrical signals. The word “transponder” is derived from two words: “transponder” and “transmitter”. In telecommunications, the term is abbreviated as XPDR, XPNDR, TPDR, or TP.

A transponder is an automatic device, which acts as a signal receiver, amplifier, and re-transmitter. It is also defined as an automatic device that transmits an encoded message in resoponse to another encoded received signal. A transponder is also considered as a receiver-transmitter generating a signal in response to electronic interrogation.

Transponders were initially created to locate objects, and they are still used for the same purpose at present. World War II saw the initial use of transponders aboard aircrafts to identify whether the craft was “friend” or “foe”. Sending the correct encoded messages in response to predetermined interrogation frequencies would help radar operators to identify pilots.

A transponder operates by receiving an interrogator signal (which “asks” for information) and then transmits a radio wave at an encoded frequency. A built-in frequency converter broadcasts a signal on a frequency other than the one previously transmitted on. The transponder and interrogator signals are identified simultaneously because of the different receiving and transmitting frequencies.

Transponders can also measure distance by formulating the time elapsed from sending the interrogator signal and receiving of the transponder signal.

Transponder technology is now widespread. Highways utilizing electronic toll systems using this technology can automatically compute the toll to be paid. Some cars are equipped with these devices, which help locate the vehicle in cases of emergency.

Mobile or cellular phones have a more compact version of the transponder chip on board. Televisions also employ the use of this device. Ground-based satellite transmitters send compressed digital audio/video to a transponder in their orbiting satellites, and the local stations receive and broadcast the programs to a landlocked dish.