Duplexer is a circuit that is designed to allow the use of a common antenna for both transmits and receive functions. Figure 1 shows the case of a branch-guide duplexer for radar using TR and ATR switches. For simplicity of illustration, the waveguides are shown as two-wire lines. When the radar transmitter pulse is present, it travels directly to the antenna. The TR and ATR switches also fire, and short circuits are produced. These are transformed by the quarter guide wavelength lines to open circuits at the main line. When no transmit pulse is present, the ATR is an open circuit that is transformed to a short circuit at the transmitter port. It is further transformed by the quarter guide wavelength main line to an open circuit at the antenna port. The TR switch acts as a normal waveguide, passing the receive signal from the antenna to the receiver.
The branch-line duplexer is a relatively narrowband device because it relies on the length of the waveguides that connect the switches to the main waveguide. This type of duplexer generally has been replaced by the balanced duplexer in modern radars.