Standing-wave ratio (SWR)

The voltage standing-wave ratio (VSWR, often just SWR) measures the mismatch between a transmission line and its load. When a reflected wave combines with the incident wave along the line, the superposition creates a standing-wave pattern — stationary maxima (where the two add in phase) and minima (where they subtract). VSWR is the ratio of the maximum voltage amplitude to the minimum voltage amplitude along the line: VSWR = V_max/V_min = (1+|Γ|)/(1−|Γ|), where Γ = (Z_L−Z₀)/(Z_L+Z₀) is the reflection coefficient.

Extreme cases: Γ = 0 (perfect match, Z_L = Z₀) gives VSWR = 1 (no standing wave, line looks flat — all incident power absorbed). Γ = ±1 (short or open circuit) gives VSWR = ∞ (full reflection, standing wave with nodes at zero voltage). Intermediate mismatches give intermediate VSWR. A VSWR of 1.5:1 means only about 4% of incident power is reflected — excellent for most RF work. VSWR above 3:1 means 25% reflection, usually unacceptable — amplifiers may shut down, connectors may arc, and transmitted signal quality degrades.

In the field, VSWR is measured with a directional coupler and a detector, either on a bench-top network analyser or a portable hand-held SWR meter. Ham-radio operators tune antennas to minimise SWR before transmitting; FM radio engineers maintain broadcast-transmitter VSWR below 1.1:1 to prevent tower damage; microwave system integrators budget per-element VSWR to stay within link-budget limits. Any mismatched component — an antenna off-resonance, a connector with a loose centre-pin, a bent waveguide — shows up in the VSWR measurement before it shows up in the functional performance. It is the single most useful scalar diagnostic in RF troubleshooting.