A duplexer is used:

1. To couple two different antennas to a transmitter without mutual interference
2. To allow the one antenna to be used for reception or transmission without mutual interference
3. To prevent interference between two antennas when they are connected to a receiver
4. To increase the speed of the pulses in pulsed radar (BEL 2006)

A duplexer is used to allow the one antenna to be used for reception or transmission without mutual interference.