Understanding Your 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.

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.

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.

A duplexer must provide sufficient isolation to prevent receiver desensitization.

Also, the terms duplexer and diplexer have been used interchangeably for many years. Duplexer and diplexer have the same literal meaning but a duplexer has been used with regard to wireless (land mobile) systems and diplexer has been used in microwave system application.

Duplexer Types

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

Band Pass:

• 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 (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

More…

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 (PDF)