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SWR, VSWR, and Return Loss Explained

Before deploying an antenna on your mesh node, understanding how to measure and interpret antenna performance can save you from poor coverage or potential hardware damage.

What is SWR?

Standing Wave Ratio (SWR) — more precisely Voltage Standing Wave Ratio (VSWR) — measures how well an antenna is impedance-matched to your transmission line and radio. A perfect match is 1:1. Most radios are designed for 50-ohm impedance.

  • SWR 1.0:1 — Perfect match. 100% of power transferred to antenna.
  • SWR 1.5:1 — Excellent. ~96% power transferred. Imperceptible in practice.
  • SWR 2.0:1 — Good. ~89% power transferred. Acceptable for most deployments.
  • SWR 3.0:1 — Poor. ~75% power transferred. Antenna should be investigated.
  • SWR 5.0:1+ — Bad. Significant reflected power. Can damage some transmitters over time.

At LoRa power levels (typically 10-30 dBm / 10mW-1W), a high SWR is unlikely to damage hardware immediately, but it does reduce effective radiated power and range.

Return Loss

Return loss is another way to express the same measurement, preferred by RF engineers:

Return Loss (dB) = -20 * log10((SWR-1)/(SWR+1))

SWR 1.5:1  =  -14 dB return loss
SWR 2.0:1  =  -9.5 dB return loss
SWR 3.0:1  =  -6 dB return loss

Higher return loss (less negative dB) is better. A return loss of -14 dB or better is considered a good antenna match.

Why Antennas Have Poor SWR

  • Wrong resonant frequency — Antenna cut for wrong frequency. 433 MHz antennas will not match at 915 MHz.
  • Damaged antenna — Broken internal element or damaged connector.
  • Loose or oxidized connector — Resistance at connection point adds to mismatch.
  • Incorrect antenna for your radio's impedance — Most LoRa radios are 50-ohm; some antennas are 75-ohm (designed for cable TV).
  • Near-field interference — Conductive material too close to the antenna element.

Measuring SWR Without a VNA

If you don't have a NanoVNA, you can still estimate antenna performance:

  • Two-node range test — The most practical field test. Compare RSSI/SNR at a known distance with the suspect antenna vs. a known-good stock antenna.
  • RF power meter + dummy load — Measure forward and reflected power at the transmitter. RF power meters for 915 MHz are available for $20-50.
  • RSSI comparison — Place a second node 100m away. Compare received RSSI with the suspect antenna vs. the stock rubber duck. A 3 dB improvement = double the power effective radiated.

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