What Do You Mean by Transponder
The combination of transmitter and receiver in satellite is called as Transponder. Transponders are the main and the basic communication electronics unit on the satellite.
Transponder Consists Of :
A transponder consists of receiving antenna, transmitting antenna, low noise amplifier, mixer, power amplifier, local oscillator.
How Transponder Works :
Low noise amplifier amplifies the received signal from earth station. A frequency converter formed by a mixer and local oscillator. The uplink frequencies that are 6 GHz are converted to downlink frequencies that are 4 GHz by frequency conversion. This is necessary because the transponder cannot transmit and receive the same frequency. Otherwise the strong signal of transmitter would overload the receiver and block out the very small uplink signal, thereby stopping any communication. The interference can be avoided by using widely spaced transmit and receive frequencies.
After frequency conversion, the power level of the signal is increased by using power amplifier. Transmitting antenna transmits the downlink signal to the receiving station on earth. Transponders are the basic communication electronics unit on the satellite. A typical communication satellite has 12, 24 or more transponders.
First Use Of Transponder :
First use of transponder was done in Second World War. It was used in aircraft for identification as a part of Friend or Foe system.