Testing a Duplexer

A duplexer allows a repeater’s transmitter and receiver to share a single antenna. It uses resonant cavities — typically notch or bandpass/reject types — to pass the desired frequency while rejecting the other. Without adequate isolation between TX and RX, the transmitter would desensitize or damage the receiver front end.

The NanoVNA-H can measure all the critical duplexer parameters: insertion loss, rejection, port-to-port isolation, and return loss. This tutorial walks through each measurement for a typical VHF or UHF repeater duplexer.

What You Will Learn

• How to connect the NanoVNA-H to a duplexer for each measurement
• Measuring passband insertion loss and center frequency
• Measuring rejection at the opposite frequency
• Measuring total port-to-port isolation
• Evaluating return loss at each port
• Pass/fail criteria for repeater service

Equipment Needed

• NanoVNA-H (firmware 1.2.40 or later recommended)
• Two SMA-to-N adapters (or SMA-to-UHF/SO-239, depending on your duplexer connectors)
• Quality coax jumpers — short, low-loss cables with connectors matching your adapters
• 50-ohm termination load with N or UHF connector (for isolation and return loss measurements)
• The duplexer under test

Caution

Never apply RF power through the NanoVNA-H. Disconnect the repeater transmitter before connecting the VNA to any duplexer port.

Test Setup

A typical 4-cavity duplexer has three ports: TX input, RX output, and Antenna (common). The TX and RX cavities are tuned to pass their respective frequencies while rejecting the other.

You will make three separate connections during testing:

TX cavity passband (S21):

• Port 1 (CH0) to the TX input port
• Port 2 (CH1) to the Antenna port
• RX port terminated with 50-ohm load

RX cavity passband (S21):

• Port 1 (CH0) to the Antenna port
• Port 2 (CH1) to the RX output port
• TX port terminated with 50-ohm load

Port-to-port isolation (S21):

• Port 1 (CH0) to the TX input port
• Port 2 (CH1) to the RX output port
• Antenna port terminated with 50-ohm load

Calibration

Calibrate with the adapters and jumper cables in place so the calibration reference plane sits at the duplexer connectors. Any loss or mismatch in the adapters and cables will be removed from the measurement.

1. Set the sweep range

   Cover both the TX and RX frequencies with margin on each side.

   For a 2-meter repeater with 600 kHz offset (e.g., TX 146.940 MHz, RX 146.340 MHz):

   • START: 144M
   • STOP: 149M

   For a 70-cm repeater with 5 MHz offset (e.g., TX 444.000 MHz, RX 449.000 MHz):

   • START: 440M
   • STOP: 455M

2. Perform full SOLT calibration

   For best accuracy, perform OPEN, SHORT, LOAD on Port 1 and THRU between ports.

   See Full Calibration for the step-by-step procedure.

   If you only need S21 measurements, an enhanced response calibration (THRU only) is sufficient — see Enhanced Response Calibration.

3. Verify calibration

   Connect the THRU directly between the cable ends (bypassing the duplexer). S21 should read close to 0 dB across the span. If it deviates by more than 0.3 dB, recalibrate.

Tip

Save your calibration to a slot before starting measurements. If you accidentally bump a connector or need to restart, you can recall it instantly.

Measurements

Passband Insertion Loss

1. Connect the TX cavity path

   Port 1 to TX input, Port 2 to Antenna port, 50-ohm load on RX port.

2. Set trace to S21 LOGMAG

   You should see a passband peak near the TX frequency.

3. Place a marker at the peak

   Use MARKER > SEARCH > MAX to snap the marker to the highest point.

4. Read insertion loss

   The S21 value at the marker is the insertion loss. A healthy cavity duplexer shows -0.5 to -2 dB at the passband center.

5. Repeat for the RX cavity

   Reconnect: Port 1 to Antenna port, Port 2 to RX output, 50-ohm load on TX port. Read the peak S21 value the same way.

If insertion loss exceeds 3 dB, suspect dirty or damaged connectors, a corroded cavity, or a cavity that has drifted far off frequency.

Passband Center Frequency

With the marker still at the S21 peak (MAX search), read the frequency directly from the marker readout. For a properly tuned duplexer:

• The TX cavity passband center should sit at the repeater’s transmit frequency
• The RX cavity passband center should sit at the repeater’s receive frequency

A center frequency that is off by more than a few kHz indicates the cavity needs retuning.

Passband Bandwidth

1. Note the peak S21 value

   For example, -1.0 dB at the TX frequency.

2. Calculate the -3 dB target

   The target is 3 dB below the peak: -1.0 - 3.0 = -4.0 dB.

3. Use delta markers

   Place marker 2 on the lower skirt where S21 crosses -4.0 dB, and marker 3 on the upper skirt at the same level. The frequency difference between them is the -3 dB bandwidth.

   See Delta Markers for details on setting these up.

4. Record the bandwidth

   Typical cavity bandwidths range from 100 kHz to 500 kHz depending on coupling adjustment. Tighter coupling (narrower bandwidth) gives better rejection but higher insertion loss.

Rejection at the Opposite Frequency

This is the single most important duplexer measurement. Each cavity set must reject the opposite frequency to prevent desensitization.

1. While measuring the TX cavity passband (S21)

   Place a marker at the RX frequency. Read the S21 value.

   For a well-tuned 4-cavity duplexer, expect -60 to -90 dB of rejection.

2. While measuring the RX cavity passband (S21)

   Place a marker at the TX frequency. Read the S21 value. Same expected range.

3. Verify against repeater requirements

   A repeater running 50 W typically needs at least 70 dB of rejection in each leg to avoid receiver desensitization.

Note

The NanoVNA-H has roughly 70 dB of dynamic range below 300 MHz and somewhat less above. If you need to verify rejection deeper than 70 dB, you may need to reduce the sweep span to improve noise floor, or use a higher-end analyzer.

Port-to-Port Isolation

This measures the total isolation the receiver sees from the transmitter, accounting for both cavity legs plus any leakage paths.

1. Connect for isolation measurement

   Port 1 to TX input, Port 2 to RX output, 50-ohm load on Antenna port.

2. Set trace to S21 LOGMAG

   The entire trace should show deep rejection.

3. Place a marker at the TX frequency

   The S21 value is the total isolation at the transmit frequency.

4. Place a second marker at the RX frequency

   Read the isolation at the receive frequency as well.

Expected values:

• 4-cavity duplexer: greater than 80 dB isolation
• 6-cavity duplexer: greater than 90 dB isolation
• 2-cavity duplexer: 50-70 dB (marginal for high-power repeaters)

Return Loss

Poor return loss at a duplexer port means reflected power, which wastes transmitter energy and can cause intermodulation products.

1. Connect Port 1 to the TX input

   Terminate the Antenna and RX ports with 50-ohm loads.

2. Set trace to S11 LOGMAG

3. Place a marker at the TX frequency

   Read S11. A well-matched duplexer port shows better than -20 dB return loss (equivalent to less than 1.2:1 SWR) in the passband.

4. Repeat for the RX port and Antenna port

   Move Port 1 to each port in turn, terminating the other two.

Interpreting Results

Parameter	Good	Marginal	Fail
Insertion loss	< 1 dB	1 - 3 dB	> 3 dB
Rejection at opposite freq	> 70 dB	50 - 70 dB	< 50 dB
Port-to-port isolation	> 80 dB	60 - 80 dB	< 60 dB
Return loss	> 20 dB	15 - 20 dB	< 15 dB

A duplexer with any parameter in the “Fail” column should not be placed into repeater service until retuned or repaired.

Tuning Tips

If the duplexer needs adjustment, the NanoVNA-H can serve as a real-time tuning aid.

Tip

Enable marker tracking with MARKER > SEARCH > TRACKING MAX to watch the passband center move in real-time as you adjust tuning screws. This makes tuning much faster than the traditional signal-generator-and-power-meter method.

• Turn tuning screws slowly. A quarter turn can shift a VHF cavity by several hundred kHz. Use a non-metallic tuning tool if available.
• Tune for minimum insertion loss first. Bring the passband center to the target frequency, then optimize coupling for the desired bandwidth.
• Expect interaction between cavities. After adjusting one cavity, re-check the others. The shared antenna port means they load each other slightly.
• Tighten lock nuts carefully. It is common for the frequency to shift as the lock nut is tightened. Hold the tuning screw in place while tightening.
• Re-measure isolation last. After all individual cavities are tuned, reconnect for the port-to-port isolation measurement to confirm the system meets spec.

Common Issues

Rejection looks adequate per-cavity but isolation is low

Check for RF leakage around the duplexer — loose covers, corroded gaskets, or poor grounding between cavities. Also verify the 50-ohm load on the antenna port is making good contact.

Insertion loss is high on one cavity

Possible causes: oxidized cavity interior, damaged connector, or a tuning screw that has been adjusted beyond its useful range. Inspect the cavity interior if accessible. A cotton swab with contact cleaner can help with corrosion on the tuning mechanism.

Return loss is poor despite good insertion loss

The cavity coupling may be set for the wrong bandwidth. Adjusting the coupling loop (if accessible) or the coupling screw changes both bandwidth and match. Some duplexers have separate tuning and coupling adjustments.

Measurement floor limits rejection readings

Reduce the sweep span to just a few MHz around the frequencies of interest. A narrower span concentrates the sweep points and can improve dynamic range by 5-10 dB. Lowering the IF bandwidth also helps at the cost of slower sweep speed.

Next Steps

• Characterizing a Filter — General filter measurement techniques
• Full Calibration — Maximize measurement accuracy
• Your First S21 Measurement — Transmission measurement basics
• Using Markers — Marker placement and search functions