What is VSWR and Why It Matters
Push a child on a swing at the wrong moment and energy bounces right back. VSWR tells you how much of your radio signal bounces back instead of being transmitted.
When RF power travels down a transmission line and encounters a load whose impedance differs from the line's characteristic impedance (Z₀, typically 50 Ω or 75 Ω), some of that power is reflected back toward the source. The reflected wave travelling backwards combines with the forward (incident) wave to create a standing wave pattern — a stationary interference pattern of alternating voltage maxima and minima along the line.
VSWR is the ratio of the maximum voltage to the minimum voltage in this standing wave: VSWR = V_max / V_min. A perfect match has VSWR = 1:1 (no reflection, no pattern). An open or short circuit has infinite VSWR (total reflection). Real-world systems fall somewhere in between.
VSWR is directly related to the reflection coefficient Γ (gamma): VSWR = (1 + |Γ|) / (1 − |Γ|). Return loss (RL = −20 log|Γ|) is the same information expressed logarithmically — higher return loss is better. A VSWR of 2:1 equals a return loss of 9.5 dB, meaning 11% of incident power is reflected.
VSWR
2.0 : 1
Γ (reflection coeff.)
0.333
Return loss
9.5 dB
Power reflected
11.1%
The modern standard instrument is a vector network analyser (VNA). A VNA sweeps frequency, measures the complex reflection coefficient S₁₁ at the port, and calculates VSWR, return loss, and impedance continuously. Affordable SDR-based VNAs (NanoVNA, LibreVNA) have made this measurement accessible to hobbyists for under $50.
Older transmit-side measurement uses a directional coupler (SWR bridge) inserted in the coax path. It samples forward and reflected power separately; an SWR meter then displays the ratio. Accuracy degrades when the coupler is not well-matched, and the measurement is only valid at the coupler location — cable losses between coupler and antenna hide the true antenna VSWR.
Best practice is to measure VSWR with a calibrated VNA at the antenna feed point, eliminating cable loss from the measurement. If cable is long, an electrical delay can be applied in the VNA to "de-embed" the cable and reference the measurement to the antenna terminals. Time-domain reflectometry (TDR) mode further pinpoints the exact physical location of impedance discontinuities.
| VSWR | |Γ| | Return Loss | % Reflected | Mismatch Loss | Rating |
|---|---|---|---|---|---|
| 1.0 : 1 | 0.000 | ∞ dB | 0% | 0.00 dB | perfect |
| 1.2 : 1 | 0.091 | 20.8 dB | 0.8% | 0.04 dB | excellent |
| 1.5 : 1 | 0.200 | 14.0 dB | 4.0% | 0.18 dB | good |
| 2.0 : 1 | 0.333 | 9.5 dB | 11.1% | 0.51 dB | acceptable |
| 3.0 : 1 | 0.500 | 6.0 dB | 25.0% | 1.25 dB | poor |
| 5.0 : 1 | 0.667 | 3.5 dB | 44.4% | 2.55 dB | bad |
| ∞ : 1 | 1.000 | 0 dB | 100% | ∞ dB | open/short |