What Is VSWR?
VSWR (Voltage Standing Wave Ratio) is the ratio of the maximum to minimum voltage amplitude on a transmission line carrying both forward and reflected waves. When a transmission line is terminated in an impedance different from its characteristic impedance Z₀, a portion of the incident power is reflected back, creating a standing wave pattern. VSWR quantifies the severity of this mismatch.
VSWR = V_max / V_min = (1 + |Γ|) / (1 − |Γ|) |Γ| = (VSWR − 1) / (VSWR + 1) Return Loss (dB) = −20 · log₁₀|Γ| Power reflected (%) = |Γ|² × 100 Power transmitted (%) = (1 − |Γ|²) × 100
VSWR Complete Conversion Table
| VSWR | |Γ| | Return Loss (dB) | S11 (dB) | Power Reflected | Mismatch Loss |
|---|---|---|---|---|---|
| 1.00 : 1 | 0.000 | ∞ dB | −∞ | 0 % | 0.00 dB |
| 1.05 : 1 | 0.024 | 32.3 dB | −32.3 | 0.06 % | 0.003 dB |
| 1.10 : 1 | 0.048 | 26.4 dB | −26.4 | 0.23 % | 0.01 dB |
| 1.25 : 1 | 0.111 | 19.1 dB | −19.1 | 1.23 % | 0.05 dB |
| 1.50 : 1 | 0.200 | 14.0 dB | −14.0 | 4.0 % | 0.18 dB |
| 2.00 : 1 | 0.333 | 9.5 dB | −9.5 | 11.1 % | 0.51 dB |
| 2.50 : 1 | 0.429 | 7.4 dB | −7.4 | 18.4 % | 0.88 dB |
| 3.00 : 1 | 0.500 | 6.0 dB | −6.0 | 25.0 % | 1.25 dB |
| 5.00 : 1 | 0.667 | 3.5 dB | −3.5 | 44.4 % | 2.55 dB |
| 10.0 : 1 | 0.818 | 1.7 dB | −1.7 | 66.9 % | 4.81 dB |
| ∞ | 1.000 | 0 dB | 0 | 100 % | ∞ dB |
VSWR Specifications by Application
| Application | VSWR Requirement | Return Loss |
|---|---|---|
| Mobile phone antenna | < 2:1 across band | > 9.5 dB |
| Base station antenna | < 1.5:1 | > 14 dB |
| RF filter port match | < 1.5:1 | > 14 dB |
| Laboratory cable | < 1.1:1 (to rated freq) | > 26 dB |
| Power amplifier load | < 2:1 | > 9.5 dB |
| LNA input (noise matched) | Often 2:1–3:1 | 6–9.5 dB |
VSWR in the Time Domain
In a standing wave, nodes (voltage minima) occur at every half-wavelength from the termination, and antinodes (maxima) occur λ/4 from each node. At the node: Z_node = Z₀/VSWR (resistive). At the antinode: Z_antinode = Z₀ × VSWR. This is the basis of single-stub matching — positioning a stub at the node simplifies the design.
Quick check: VSWR = 2:1 means 11% of power is reflected and 0.51 dB extra insertion loss — acceptable for many systems. VSWR = 3:1 means 25% reflected and 1.25 dB extra loss — significant degradation.
RF View: RF View displays VSWR directly from any SNP file alongside S11 dB and Smith chart — switch views with one tap. BW Marker mode automatically reads VSWR = 2:1 bandwidth from your trace.