Duplexers Explained

A duplexer is a specialized filter that allows a repeater to transmit and receive on the same antenna at the same time. Without one, the repeater's transmitter - putting out 25-50 watts - would overload the receiver, which is trying to hear signals measured in millionths of a watt. The duplexer is often the most critical (and most expensive) component in a repeater system.

The problem duplexers solve

A GMRS repeater must transmit on one frequency while simultaneously listening on another frequency 5 MHz away. If both the transmitter and receiver are connected to the same antenna without filtering, the transmitter's powerful signal floods the receiver and makes it deaf to incoming signals. The receiver simply can't hear a faint distant signal when 50 watts of RF is blasting right next to it.

How duplexers work

A duplexer uses tuned cavities - precision-machined metal cylinders that act as extremely sharp bandpass and band-reject filters. A typical GMRS duplexer has six or more cavities arranged in two groups:

• Transmit side: passes the transmit frequency while rejecting the receive frequency. This prevents the transmitter's energy from reaching the receiver through the antenna connection
• Receive side: passes the receive frequency while rejecting the transmit frequency. This protects the receiver from the transmitter's output

The combined rejection - called isolation - needs to be at least 80-90 dB to keep the transmitter from desensitizing the receiver. Each cavity provides 15-25 dB of rejection, which is why multiple cavities are needed in series.

Cavity vs. notch duplexers

Two filter topologies are common in GMRS duplexers:

• Bandpass-notch (BPN): each cavity passes one frequency range while also rejecting (notching) a specific frequency. BPN cavities are more complex and expensive but provide better isolation per cavity, making them the preferred choice for close frequency splits like GMRS's 5 MHz
• Bandpass only: simpler cavities that pass the desired frequency without a built-in notch. More cavities are required to achieve equivalent isolation. Less common for GMRS due to the tight split

For GMRS with its 5 MHz split, bandpass-notch duplexers are strongly preferred. A 6-cavity BPN duplexer will outperform an 8-cavity bandpass-only unit at these frequencies.

Isolation requirements

The minimum useful isolation for a GMRS repeater is typically 80 dB, though 90+ dB is recommended for robust performance. Insufficient isolation results in the receiver being desensitized ("desensed") by the transmitter - the repeater keys up but can barely hear incoming signals. Symptoms include the repeater triggering on strong nearby signals but having poor coverage or failing to hear weak users. More isolation is always better; there's no practical upper limit.

Specifying a duplexer for your frequency pair

When ordering a duplexer, you must provide the manufacturer with your exact frequency pair:

• Your repeater's output (transmit) frequency, e.g., 462.5750 MHz
• Your repeater's input (receive) frequency, e.g., 467.5750 MHz
• Your transmitter power level (to verify power handling)
• Whether you want it pre-tuned to your frequencies (strongly recommended)

A duplexer specified for one frequency pair cannot simply be retuned by the user to a new pair - the physical cavity dimensions are optimized for a particular frequency range, and fine-tuning adjusts within that range. Professional tuning is almost always the right choice.

Tuning requirements

A duplexer must be tuned to your specific frequency pair using specialized test equipment (a spectrum analyzer or network analyzer). Most duplexers ship untuned or roughly tuned and must be precisely adjusted before use. Getting this wrong means poor performance or a completely non-functional repeater.

• Many duplexer manufacturers offer tuning to your frequencies for an additional fee - this is usually worth it
• Re-tuning may be needed if you change frequencies or after physical damage (duplexers are sensitive to being dropped or bumped)
• Temperature changes can cause slight detuning, though quality duplexers are designed to be thermally stable

Common duplexer problems

• Physical damage: dropping a duplexer can shift cavity dimensions and detune it. Handle them carefully. If your repeater's performance suddenly degrades, check if the duplexer was bumped or dropped
• Connector corrosion: outdoor or humid installations can cause PL-259 or N-connector corrosion, adding loss and noise. Inspect connectors annually and apply weatherproofing compound
• Improper installation: reversing the TX and RX ports is a common first-time mistake. Label everything before installation
• Using a VHF duplexer at UHF: a duplexer physically sized for 2-meter frequencies cannot be properly tuned for 70cm/GMRS. Always verify frequency range before purchasing used equipment

Alternatives to a duplexer

If the cost or complexity of a duplexer is prohibitive, there are alternatives:

• Separate TX and RX antennas: mount two antennas with significant physical separation (vertically is best - one high, one low, with 20+ feet between them). The physical distance provides isolation instead of filters. This eliminates the duplexer but requires two antennas, more coax, and a taller structure
• Cross-band isolation: not applicable to GMRS (same band for TX and RX), but in some radio services, using different bands eliminates the need for a duplexer

Choosing a duplexer

• Make sure it's rated for GMRS frequencies (462/467 MHz UHF band) and the 5 MHz split
• Check the power handling - it must handle your transmitter's output with margin
• Look for at least 80 dB of isolation on each side
• Insertion loss (the signal lost passing through the duplexer) should be under 1.5 dB per side
• Buy from a reputable manufacturer who can tune it to your frequencies before shipping

For the full picture on building a repeater, see Setting Up a Repeater.