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Docs Radio & RF Design VSWR Measurement

VSWR Measurement

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How to measure VSWR on a 433 MHz antenna without expensive gear, and how to read the results.

VSWR Measurement

Voltage Standing Wave Ratio (VSWR, or SWR) measures how well the antenna matches the 50 ohm system. It is the first number to check on any antenna.

What SWR means

SWR is the ratio of the maximum to minimum voltage along the feed line, caused by reflected power. Perfect match: 1:1. Infinite mismatch (open or short): infinite.

SWRReflected powerJudgment
1.00%Ideal
1.54%Excellent, standard target
2.011%Acceptable
3.025%Poor, fix it
5.044%The radio’s protection should refuse

Measuring without a network analyzer

The practical options at 433 MHz:

  1. NanoVNA (about $30): the honest tool. Calibrate, sweep 400 - 470 MHz, read SWR at 433.92 MHz.
  2. SWR meter designed for 70 cm: works inline between radio and antenna in a low-power test.
  3. Two-beacon comparison: no absolute number, but a good antenna vs a bad one on the same receiver shows a clear RSSI gap (see Two Beacon Bench Test).

Procedure with a NanoVNA

  1. Calibrate open, short, load.
  2. Set sweep 400 - 470 MHz, 101 points.
  3. Connect the antenna in free space, away from your body and metal.
  4. Read the SWR minimum and its frequency. Resonance should sit inside 433 - 434 MHz.

Common results

ReadingMeaning
SWR min well below 433 MHzAntenna too long, trim
SWR min well above 433 MHzAntenna too short, extend
SWR flat and high everywhereBroken coax or connector
SWR changes when you touch itBad ground plane or connection