About the VSWR Calculator
This VSWR calculator converts between the different ways of describing how well an antenna or load is matched to its transmission line: voltage standing wave ratio (VSWR), return loss in decibels, and the magnitude of the reflection coefficient |Γ|. You can also enter the load impedance as resistance plus reactance along with the line impedance (50 Ω or 75 Ω) and get all of them directly.
Radio amateurs tuning antennas, engineers reading a vector network analyser, installers checking cellular and Wi-Fi feed lines, and students learning transmission-line theory can all use it. Enter the forward power to see how many watts are reflected back toward the transmitter and how many reach the load.
Results assume a lossless line between the measurement point and the load. Real coax attenuates the reflected wave twice, so a long, lossy cable makes VSWR measured at the transmitter look better than it is at the antenna.
With the default inputs, the vswr is 1.5 : 1. Change any value above to recalculate instantly.
How to use the vswr calculator
- 1Choose which quantity you already know: VSWR, return loss, |Γ| or load impedance.
- 2Enter the value (for impedance, enter R, X and the line impedance).
- 3Optionally enter the forward power from the transmitter.
- 4Read VSWR, return loss, reflection coefficient and mismatch loss.
- 5Compare with the reference table; aim for VSWR below about 1.5 : 1.
Formula and method
When a load impedance differs from the characteristic impedance of the line, part of the incident wave is reflected. The reflection coefficient Γ = (Z_L − Z0)/(Z_L + Z0) is a complex number; its magnitude |Γ| is the ratio of reflected to incident voltage. The forward and reflected waves combine into a standing wave whose maximum-to-minimum voltage ratio is VSWR = (1 + |Γ|)/(1 − |Γ|).
Return loss expresses |Γ| in decibels, −20·log₁₀|Γ|, so a bigger number means a better match. The fraction of power reflected is |Γ|², and mismatch loss −10·log₁₀(1 − |Γ|²) is the resulting reduction in power delivered to the load. Conversions from VSWR or return loss first recover |Γ| and then compute everything else from it.
- Γ
- Reflection coefficient (complex); |Γ| its magnitude
- Z_L
- Load impedance R + jX (Ω)
- Z0
- Characteristic impedance of the line (Ω)
- VSWR
- Voltage standing wave ratio
- RL
- Return loss (dB)
- ML
- Mismatch loss (dB)
Worked examples
VSWR 1.5 : 1 with 100 W forward
|Γ| = (1.5 − 1)/(1.5 + 1) = 0.2, so return loss is −20·log₁₀(0.2) ≈ 13.98 dB. Reflected power is 0.2² = 4% — 4 W of 100 W — and the mismatch loss is only about 0.18 dB.
20 dB return loss
A 20 dB return loss means |Γ| = 10^(−20/20) = 0.1, so VSWR = 1.1/0.9 ≈ 1.22 : 1 and just 1% of the power is reflected.
25 + j25 Ω load on a 50 Ω line
Γ = (−25 + j25)/(75 + j25), so |Γ| = 35.36/79.06 ≈ 0.447 and VSWR = 1.447/0.553 ≈ 2.62 : 1. One fifth of the power (20 W of 100 W) is reflected and the return loss is about 7 dB.
Frequently asked questions
What is a good VSWR?+
A VSWR of 1 : 1 is a perfect match. Below 1.5 : 1 is considered good for most antennas (4% reflected), and up to 2 : 1 (11% reflected) is usually acceptable. Above 2 : 1 many transmitters reduce power to protect themselves.
How do I convert VSWR to return loss?+
First find |Γ| = (VSWR − 1)/(VSWR + 1), then return loss = −20·log₁₀|Γ|. For example VSWR 2 gives |Γ| = 1/3 and a return loss of about 9.54 dB.
Is SWR the same as VSWR?+
In everyday radio use, yes. SWR usually refers to the voltage standing wave ratio; the current standing wave ratio has the same value, so the distinction rarely matters.
Does high VSWR waste power?+
Reflected power is not all lost — with a low-loss line and a tuner or matching network at the transmitter, it is re-reflected back toward the load. The real costs are mismatch loss, extra cable loss and voltage stress on the transmitter output stage.
What VSWR does a 75 Ω load give on 50 Ω coax?+
A purely resistive 75 Ω load on a 50 Ω line gives |Γ| = 25/125 = 0.2 and a VSWR of exactly 1.5 : 1, with 4% of the power reflected.