Also known as: standing wave ratio, SWR, VSWR
Standing wave ratio (SWR, often VSWR for voltage standing wave ratio) measures how well an antenna is impedance-matched to its feedline and radio at a given frequency.1 A perfect match is 1:1; larger numbers mean a worse match and more reflected energy.
How it works
When the antenna’s feedpoint impedance does not equal the feedline’s characteristic impedance (usually 50 Ω), part of the wave travelling down the line is reflected back at the junction. The forward and reflected waves overlap and, because they travel in opposite directions, add and cancel at fixed points along the cable to form a stationary interference pattern — a standing wave. SWR is defined as the ratio of the maximum voltage on that pattern to the minimum:
- SWR = (1 + |Γ|) / (1 − |Γ|), where Γ is the reflection coefficient — the fraction of the wave’s amplitude that bounces back.2
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A matched load reflects nothing, Γ = 0, so SWR = 1:1. - A total mismatch (open, short, or pure reactance) reflects everything, |Γ| = 1, and SWR runs to infinity.
SWR is thus a single dial that summarizes the same mismatch that return loss expresses in decibels and Γ expresses as a complex number. A 2:1 SWR corresponds to roughly 11% of the power reflected, about 9.5 dB return loss — noticeable but rarely fatal for receive.
In practice
- Frequency dependence — SWR is only meaningful at a stated frequency. A resonant antenna is well-matched over a band and rises sharply outside it; an SWR sweep across the band shows the usable bandwidth.
- Measurement — an SWR meter, directional wattmeter, or a NanoVNA reads it directly. On receive-only setups a VNA is the practical tool.
- Transmit vs receive stakes — for a transmitter, high SWR sends reflected power back into the amplifier and can damage it, so a good match is safety-critical. For a receive-only SDR, the reflected energy is small and the real cost is a modest loss of signal reaching the radio.
Relevance to SDR
For a scanner, SWR matters less than it does for a transmitter, but a reasonably matched, resonant antenna still delivers more signal to the SDR than a badly mismatched one, helping SNR on weak control channels. A wildly high SWR usually points to a fault worth fixing — a wrong-band antenna, a broken element, or water in the coax — rather than a subtle tuning issue. GopherTrunk sees only the signal that survives the feedline; it has no way to correct a poor match, so keeping SWR sensible is an antenna-and-feedline task done before the ADC.
Sources
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Standing wave ratio — Wikipedia, on impedance matching, reflected power, and VSWR. ↩
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Reflection coefficient — Wikipedia, for Γ and the SWR = (1+ Γ )/(1− Γ ) relationship.