Duplexer

In radar and radio communications systems, it isolates the receiver from the transmitter while permitting them to share a common antenna.

Duplexers can be based on frequency (often a waveguide filter), polarization (such as an orthomode transducer), or timing (as is typical in radar).

When the transmitter is active, the resulting high voltage causes the tube to conduct, shorting together the receiver terminals to protect it, while its complementary, the anti-transmit/receive (ATR) switch, is a similar discharge tube which decouples the transmitter from the antenna while not operating, to prevent it from wasting received energy.

In radio communications (as opposed to radar), the transmitted and received signals can occupy different frequency bands, and so may be separated by frequency-selective filters.

Significant attenuation (isolation) is needed to prevent the transmitter's output from overloading the receiver's input, so such duplexers employ multi-pole filters.

[3][4] The first duplexers for radar, sometimes referred to as Transmit/Receive Switches, were invented by Robert Morris Page and Leo C. Young of the United States Naval Research Laboratory in July 1936.