Short Answer: They're the Same
VSWR (Voltage Standing Wave Ratio) and SWR (Standing Wave Ratio) are used interchangeably in modern RF engineering. Both describe the ratio of maximum to minimum wave amplitude on a transmission line due to impedance mismatch. Technically, SWR is more general (could refer to current, power, or voltage waves), but VSWR became the dominant term because voltage was historically easier to measure.
Definition
VSWR = SWR = V_max / V_min = (1 + |Γ|) / (1 − |Γ|) where |Γ| = reflection coefficient magnitude (0 to 1) Perfect match: VSWR = 1:1 Open circuit: VSWR = ∞ Short circuit: VSWR = ∞
Historical Note on VSWR vs SWR
In the early days of RF engineering (1930s–1950s), standing waves were measured on slotted transmission lines by physically probing the voltage along the line with a sliding probe. Because voltage was measured, the term "Voltage Standing Wave Ratio" (VSWR) stuck. Current standing waves are the inverse pattern (minima where voltage is maximum) but give the same numerical ratio. Today "VSWR" and "SWR" are both acceptable in technical writing.
VSWR in Modern RF Systems
| VSWR | Return Loss | Common Specification |
|---|---|---|
| 1.0:1 | ∞ dB | Ideal (unachievable) |
| 1.5:1 | 14.0 dB | Base station antenna / RF filter port |
| 1.92:1 | 10.0 dB | Mobile phone antenna minimum |
| 2.0:1 | 9.5 dB | Many system specifications |
| 3.0:1 | 6.0 dB | Marginal, needs improvement |