Duplexers, Combiners and Couplers Explained (and Duplexer vs Diplexer)

Sites with more than one transmitter rarely have room, or budget, for a separate antenna per radio. Combiners, duplexers and couplers solve that problem in different ways, and the three terms are often used loosely. Here's what each one actually does, how a duplexer differs from a diplexer, and where Renair's HCMR150-4-DU and AC-0305 UHF Coupler Series fit.

What a duplexer does

A duplexer lets a single antenna transmit and receive at the same time on different frequencies, without the transmitter's output overwhelming the receiver sitting a few megahertz away. It's the component that makes simultaneous TX/RX possible on one feedline, and it's essential on repeaters and on any base station where transmit and receive frequencies are close together.

Duplexer vs diplexer: what's the difference?

The two names sound almost identical, and both let two signals share one antenna, but they solve different problems:

• A duplexer separates transmit and receive within the same band, where the two frequencies may be only a few megahertz apart. That requires sharp, high-rejection filtering, typically using cavity filters.
• A diplexer combines or separates two different bands that are far apart, such as VHF and UHF, so two radios on different bands can share one dual-band antenna and one feeder. Because the bands are widely spaced, the filtering is simpler and the unit is usually much smaller.

A simple rule: if the two signals are in the same band, you need a duplexer; if they are in different bands, you need a diplexer.

What a combiner does

A combiner lets multiple transmitters share one antenna port, each on its own frequency, without one channel's power leaking into another and desensitising it. The HCMR150-4-DU combines both functions in one hybrid unit: four VHF TX channels (144–160 MHz or 160–175 MHz) onto a single antenna feed, with an integrated duplex filter so the same antenna handles simultaneous receive. It's built for 19-inch rack or wall mounting, rated to 50 W per channel, with insertion loss of just 0.3 dB and inter-channel isolation of 25 dB: low enough loss and high enough isolation that four radios genuinely behave as four independent channels rather than degrading each other. For smaller sites, the HCMR150-2-DU is a hybrid 2-channel VHF TX/RX combiner and duplex filter.

What a directional coupler does

A coupler is a different job entirely: it samples a small, known proportion of the RF power passing through a feedline (for test equipment, monitoring, or feeding a secondary receiver) without disrupting the main signal path. The AC-0305 UHF Coupler Series covers 300–500 MHz at up to 200 W, with coupling values from 6 dB through to 30 dB across the AC-06 to AC-30 variants, so the sampled signal can be scaled to whatever the monitoring equipment needs.

Choosing between them

• Need one antenna to transmit and receive at once on a single radio? You need a duplexer.
• Need two radios on different bands, such as VHF and UHF, to share one antenna? You need a diplexer.
• Need several transmitters sharing one antenna? You need a combiner, or, as with the HCMR150-4-DU, a combiner with a duplexer built in.
• Need to sample power for test gear or a monitoring receiver without touching the main signal? You need a directional coupler such as the AC-0305 series.

See full specifications for the HCMR150-4-DU and AC-0305 coupler series, browse the wider range at renair.co.uk/products, or ask our team which unit suits your site.

For the base station antennas these typically feed, see our base station antenna buying guide. For public safety repeater sites specifically, our TETRA antenna guide covers the wider system design.

Frequently asked questions

What is the difference between a duplexer and a diplexer?

A duplexer separates transmit and receive frequencies within the same band so one antenna can do both at once. A diplexer combines or separates two different bands, such as VHF and UHF, so they can share one antenna.

Can the HCMR150-4-DU be used outside the 144–175 MHz band?

No, it's purpose-built for VHF in the 144–160 MHz and 160–175 MHz ranges. For UHF combining requirements, speak to our sales team about equivalent UHF options.

What does coupling value (dB) mean on the AC-0305 series?

It's the ratio between the power on the main line and the power delivered to the coupled port. A 30 dB coupler (AC-30-0305) samples a much smaller fraction of the signal than a 6 dB coupler (AC-06-0305). Choose based on what your monitoring or test equipment needs at its input.

Does adding a combiner reduce my transmit power?

Yes, slightly: the HCMR150-4-DU has an insertion loss of around 0.3 dB, a small, predictable reduction that should be accounted for in the site's link budget rather than a reason to avoid combining.

Related products

• HCMR150-4-DU
• HCMR150-2-DU
• AC-0305 UHF Coupler Series
• BASE001 A/B/C/D/E/X

Related reading

• Base Station Antenna Buying Guide
• RF Connectors and Mast Adapters for Installers
• UHF Antenna Types: Folded Dipoles to Yagis