Copied from Wikipedia and modified to suit this site.
A duplexer is a device that allows bi-directional communication over a single channel, and in ham radio is typically used in repeaters
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.
Note 1: A duplexer must be designed for operation in the frequency or radio 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.
Further information about duplexer design can be found here:
- 2m duplexer construction guide
- Commercial duplexer specifications
- Duplexer theory and tune-up
- How to tune a repeater duplexer
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