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Understanding Antenna Duplexers

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What is a Duplexer?

A duplexer is a device that allows bi-directional communication over a single channel. In 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 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.
  • 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.
  • Supply enough isolation to prevent receiver desensitization.

Diplexer vs Duplexer. What is the difference?

A diplexer is a passive device that combines two inputs into a common output. The signals on inputs 1 and 2 occupy different frequency bands. Consequently, the signals on inputs 1 and 2 can coexist on the output without interfering with each other. It is also known as a cross band combiner. A duplexer is a passive device that allows bi-directional (duplex) communication of transmit and receive frequencies within the same band over a single path.



Types of Duplexers

There are two basic types of duplexers: Band Pass and Band Reject.

Bandpass Duplexers:

  • Generally will have higher branch loss than pass-reject type, 1.5 dB per branch or higher being expected
  • Far superior for dense site use. The multiple cavity strings provide added selectivity for the receiver and a high order of spurious and harmonic rejection for the transmitter
  • Requires larger, higher “Q” cavities, and more of them, resulting in higher cost and need for greater site space occupancy
  • Through use of correct branch cable lengths and careful loop coupling adjustments, this duplexer type can be tuned for a broad “nose” response to accommodate multi-frequency transmitters and receivers
  • Impractical for closely spaced TX-RX pairs, compared to pass/notch types. Higher costs than pass notch types due to requiring larger cavities

Band Reject Duplexers (a.k.a. Pass/Notch):

  • Lower insertion loss than band pass types for same TX-RX spacings
  • Since pass band is broad, little help is provided in receiver front end selectivity except for the transmit carrier notch; this can be a real problem when placed at high density sites
  • Can use smaller volume cavities for a given TX-RX spacing, saving space
  • Lower cost to manufacture; savings in materials and labor

High and Low Band Duplexers

To choose the right duplexer for your application, please browse the full Tessco duplexer offering.

  • Duplexers, 30-88 MHz
  • Duplexers, 118-174 MHz
  • Duplexers, 300-512 MHz
  • Duplexers, 650-960 MHz

For further information, download the EMR Duplexer Manual.




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