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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]


Transmit-receive switch

File:Bsp Duplex-eng.svg
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 between 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.


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


  1. Rinehart, Ronald E. (1991). Radar for Meteorologists. University of North Dakota.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles> 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.

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