VSWR Calculator
When a load impedance does not match the transmission line, part of the incident wave reflects back toward the source, creating a standing wave. The voltage standing wave ratio summarises how bad the mismatch is, and it links directly to the reflection coefficient, the return loss, and the power lost to reflection. This calculator takes a real load impedance and a line (source) impedance, then returns the VSWR, the reflection coefficient magnitude, the return loss in decibels, and the mismatch loss in decibels.
VSWR formula
gamma = |ZL - Z0| / (ZL + Z0)
VSWR = (1 + gamma) / (1 - gamma)
Return loss = -20 * log10(gamma) dB
Mismatch loss = -10 * log10(1 - gamma^2) dB
The reflection coefficient magnitude ranges from 0 (perfect match) to 1 (full reflection). VSWR and the decibel figures all follow from it. Return loss is undefined for a perfect match because no power is reflected.
VSWR facts
- A VSWR of 1 to 1 is a perfect match with no reflected power.
- A VSWR of 2 reflects about 11 percent of the incident power.
- Return loss of 20 dB corresponds to a VSWR of about 1.22.
- Antenna systems often target a VSWR below 1.5 or 2.
- An open or short circuit gives an infinite VSWR and zero return loss.
VSWR: frequently asked questions
What is VSWR?
Voltage standing wave ratio (VSWR) measures how well a load impedance matches the transmission line. It is the ratio of the maximum to the minimum voltage on a mismatched line. A perfect match gives 1 to 1; larger values mean more reflected power. VSWR is computed from the magnitude of the reflection coefficient.
How is the reflection coefficient calculated?
For a real load and line impedance, the reflection coefficient gamma equals (load impedance minus line impedance) divided by (load impedance plus line impedance). Its magnitude ranges from 0 for a perfect match to 1 for a total reflection (open or short circuit).
What is return loss?
Return loss is the reflected power expressed in decibels below the incident power: return loss equals minus 20 times the base-10 log of the reflection coefficient magnitude. A higher return loss in decibels means a better match; 20 dB return loss corresponds to a VSWR of about 1.22.
What is mismatch loss?
Mismatch loss is the fraction of incident power not delivered to the load because it is reflected, expressed in decibels: minus 10 times the log of (1 minus the reflection coefficient squared). At a VSWR of 2, mismatch loss is about 0.51 dB, meaning roughly 11 percent of the power is reflected.
Why does VSWR matter?
High VSWR means reflected power that can heat or damage transmitters, reduce delivered power, and distort signals on the line. Antenna and RF systems are usually specified to keep VSWR below a target such as 1.5 or 2 to protect equipment and maintain efficiency.
Official sources
- NIST: Physical Measurement Laboratory: RF reflection metrology.
- IEEE: IEEE standards for transmission-line measurements.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.