Duplexer

A duplexer is an electronic device that allows bi-directional (duplex) communication over a single path. In radar and radio communications systems, it isolates the receiver from the transmitter while permitting them to share a common antenna. Most radio repeater systems include a duplexer. Duplexers can be based on frequency (often a waveguide filter), polarization (such as an orthomode transducer), or timing (as is typical in radar).^[1]

Types

Transmit-receive switch

TR duplex switching

In radar, a transmit/receive (TR) switch alternately connects the transmitter and receiver to a shared antenna. In the simplest arrangement, the switch consists of a gas-discharge tube across the input terminals of the receiver. 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.

Circulator

Orthomode transducer

Frequency domain

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. See diplexer.

Commercial 19" rack mount antenna filter
Isolation typical >75 dB
Insertion Loss typical < 1.0 dB

Note 1: A duplexer must be designed for operation in the frequency band used by the receiver and transmitter, and must be capable of handling the output power of the transmitter.

Note 2: A duplexer must provide adequate rejection of transmitter noise occurring at the receive frequency, and must be designed to operate at, or less than, the frequency separation between the transmitter and receiver.

Note 3: A duplexer must provide sufficient isolation to prevent receiver desensitization.

Source: from Federal Standard 1037C

References

1. Rinehart, Ronald E. (1991). Radar for Meteorologists. University of North Dakota. radar engineers have added the automatic switch (also called a duplexer) in the ... As soon as the transmitter stops sending a signal, the duplexer switches so that the receiver is now connected to the antenna.