VSWR Calculator

Voltage Standing Wave Ratio (VSWR)

VSWR describes how well a transmission line or antenna system is matched to its load. A perfect match has VSWR of 1:1 with no standing waves. Higher VSWR means more reflected energy and larger voltage swings along the line.

Formulas

From reflection coefficient magnitude:

$$\mathrm{VSWR} = \frac{1 + |\Gamma|}{1 - |\Gamma|}$$

From measured standing wave voltages:

$$\mathrm{VSWR} = \frac{V_{\max}}{V_{\min}}$$

Here $|\Gamma|$ is the magnitude of the voltage reflection coefficient (0 to 1). Return loss in decibels is $RL = -20 \log_{10}(|\Gamma|)$.

Practical Notes

VSWR below 2:1 is often acceptable for many RF systems. Very high VSWR can increase feeder loss, stress transmitters, and cause measurement errors. Always keep $|\Gamma| < 1$ so the denominator stays positive.

Related tools: Cable Impedance Calculator and Modulation Calculator.

Frequently Asked Questions

What is a good VSWR value?

VSWR of 1:1 is ideal. Many applications accept up to 2:1. Higher values indicate a poorer match and more reflected power.

What is the reflection coefficient?

The reflection coefficient Gamma measures how much voltage is reflected at an impedance discontinuity. Its magnitude |Gamma| ranges from 0 (no reflection) toward 1 (nearly total reflection).

How do I measure VSWR with Vmax and Vmin?

Use a slotted line, directional coupler, or VSWR meter to find peak and null voltages along the line. Divide Vmax by Vmin to get VSWR.

What is return loss?

Return loss is the decibel measure of reflected power relative to incident power. Higher return loss (in dB) means less reflection and better matching.

Can VSWR be less than 1?

No. VSWR is always greater than or equal to 1 for passive matched systems. Values below 1 would imply |Gamma| greater than 1, which is non-physical for passive loads.

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Voltage Standing Wave Ratio (VSWR)

Formulas

Practical Notes

Frequently Asked Questions

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