Editing Duplexer (section)

==Types==

===Transmit-receive switch===
[[Image:Bsp_Duplex-eng.svg|thumb|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 [[Sutton tube#Soft Sutton tube|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===
{{Main|Circulator}}

===Hybrid===
A [[hybrid coil|hybrid]], such as a [[magic T]], may be used as a duplexer by terminating the fourth port in a [[electrical termination|matched load]].<ref>{{cite book
 |last1=Sisodia
 |last2=Gupta
 |date=2007
 |title=Microwaves: Introduction To Circuits,Devices And Antennas
 |url= https://books.google.com/books?id=iEvgmwH1esgC&pg=SA4-PA40
 |location=New Delhi
 |publisher=New Age International
 |page=4.40
 |isbn=9788122413380
}}</ref>
This arrangement suffers from the disadvantage that half of the transmitter power is lost in the matched load, while [[thermal noise]] in the load is delivered to the receiver.

===Orthomode transducer===
{{Main|Orthomode transducer}}

===Frequency domain===
[[Image:Repeater duplexer.png|right|190px]]

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. These are effectively a higher-performance version of a [[diplexer]], typically with a narrow split between the two frequencies in question (typically around 2%-5% for a commercial two-way radio system).

With a duplexer the high- and low-frequency signals are traveling in opposite directions at the shared [[Port (circuit theory)|port]] of the duplexer.

Modern duplexers often use nearby frequency bands, so the frequency separation between the two ports is also much less. For example, the transition between the uplink and downlink bands in the [[GSM frequency bands]] may be about one percent (915&nbsp;MHz to 925&nbsp;MHz). Significant attenuation (isolation) is needed to prevent the transmitter's output from overloading the receiver's input, so such duplexers employ multi-pole filters. Duplexers are commonly made for use on the 30-50 MHz ("low band"), 136-174 MHz ("high band"), 380-520 MHz ("UHF"), plus the  790–862 MHz ("800"), 896-960 MHz ("900") and 1215-1300 MHz ("1200") bands.

There are two predominant types of duplexer in use - "notch duplexers", which exhibit sharp notches at the "unwanted" frequencies and only pass through a narrow band of wanted frequencies and "bandpass duplexers", which have wide-pass frequency ranges and high out-of-band attenuation.

On shared-antenna sites, the bandpass duplexer variety is greatly preferred because this virtually eliminates interference between transmitters and receivers by removing out-of-band transmit emissions and considerably improving the selectivity of receivers. Most professionally engineered sites ban the use of notch duplexers and insist on bandpass duplexers for this reason.

[[File:Kathrein-weiche.jpg|thumb|350px|Commercial 19" rack mount antenna filter<br>Isolation typical >75 dB <br> Insertion Loss typical < 1.0 dB]]

''Note 1:''  A duplexer must be designed for operation in the [[frequency]] [[band (radio)|band]] used by the receiver and transmitter, and must be capable of handling the output [[Power (physics)|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]]