NanoVNA Antenna Testing

A NanoVNA (Vector Network Analyser) allows you to measure SWR, return loss, and impedance of antennas and coax cables. For LoRa mesh work, the primary use is verifying antennas are resonant at 915 MHz and identifying cable or connector problems. NanoVNAs are available for $50–$80 and are a worthwhile investment for anyone deploying more than a few nodes.

What SWR Means

SWR (Standing Wave Ratio) measures how well the antenna's impedance is matched to the 50-ohm transmission line. A perfect match is SWR 1.0:1. Higher SWR means more power is reflected back from the antenna into the radio.

SWR	Return Loss	Reflected Power	Assessment
1.0:1	∞ dB	0%	Perfect (theoretical)
1.5:1	14 dB	4%	Excellent — acceptable for any use
2.0:1	9.5 dB	11%	Good — acceptable for most use
3.0:1	6 dB	25%	Marginal — investigate root cause
5.0:1	3.5 dB	44%	Poor — replace antenna or cable

SWR Thresholds for LoRa

<1.5: Excellent. No action needed.
1.5–2.0: Good. Acceptable for field use.
2.0–3.0: Marginal. Investigate the antenna and connectors. May cause PA stress over time.
>3.0: Poor. Replace the antenna, cable, or connectors. High SWR can damage the LoRa module PA in some devices.

Calibrating the NanoVNA

Calibration is essential for accurate measurements. The NanoVNA includes an SMA calibration kit (Open, Short, Load, Through). Calibrate at the end of the cable you will use — not at the NanoVNA port itself if you are using a cable extension.

Set frequency range to 800–1000 MHz to bracket 915 MHz.
Connect the Open standard; run calibration step.
Connect the Short standard; run calibration step.
Connect the 50-ohm Load standard; run calibration step.
Save the calibration. Recalibrate any time you change cables.

Measuring an Antenna

Calibrate the NanoVNA as above with the measurement cable attached.
Attach the antenna to the end of the calibrated cable.
On the NanoVNA display, look for the SWR minimum. It should appear near 915 MHz for a properly tuned LoRa antenna.
Note the SWR value at exactly 915 MHz.
If the minimum is significantly off 915 MHz, the antenna is mis-tuned (common with cheap antennas) and may not perform well on your network.

Diagnosing Cable Problems

Coax cable faults (water intrusion, crushed cable, bad connectors) show as unexpected SWR readings. To isolate the problem:

Measure SWR with a known-good antenna connected directly to the NanoVNA (no cable). Note the reading.
Insert the cable and re-measure. Any significant SWR increase points to cable or connector problems.
Flex and bend the cable while watching the NanoVNA. A changing SWR during flexing indicates a broken conductor or intermittent connector.

Common NanoVNA Findings

Finding	Likely Cause	Action
SWR minimum well below 915 MHz	Antenna too long or mis-tuned	Trim antenna if adjustable; replace if fixed
SWR minimum well above 915 MHz	Antenna too short or mis-tuned	Extend antenna if adjustable; replace if fixed
SWR >3 across entire range	Open or short in cable/connector	Inspect and replace cable and connectors
SWR varies with cable movement	Intermittent connector or broken conductor	Re-crimp or replace connector; replace cable
SWR changes when antenna is held	Body capacitance detuning	Normal for handheld antennas; test in free air for comparison

Field Verification Without NanoVNA

If you do not have a NanoVNA, you can still verify basic antenna function by comparing RSSI/SNR values reported by the MeshCore or Meshtastic app when communicating with a known reference node. Compare readings with and without the antenna, and between different antennas, keeping distance and orientation constant. This is less precise than SWR measurement but sufficient for comparative testing.