MathBench

VSWR Calculator

Enter whichever number you have (VSWR, return loss, reflection coefficient, or mismatch loss) and get the other three, plus how much power reaches the load.

Result

—VSWR
—return loss (dB)
—reflection coefficient |Γ|
—mismatch loss (dB)
—power reflected
—power delivered

How this calculator works

All four quantities on this page describe the same physical thing: how well a load (usually an antenna) is matched to its transmission line, so knowing any one pins down the rest. Pick which number you have, type it, and the calculator converts through the reflection coefficient to fill in the others, along with the percentage of power reflected and delivered.

Switching the "I know the…" selector re-expresses the current answer in the new parameter: handy when a datasheet quotes return loss but your antenna analyzer reads VSWR.

The formula

|Γ|  = (VSWR − 1) ÷ (VSWR + 1)
VSWR = (1 + |Γ|) ÷ (1 − |Γ|)
RL   = −20 × log₁₀(|Γ|)        dB
ML   = −10 × log₁₀(1 − |Γ|²)   dB
Reflected power fraction = |Γ|²

The reflection coefficient magnitude |Γ| is the hub every conversion passes through: the ratio of reflected to incident voltage at the load. These are the standard transmission-line relations found in any RF text (e.g., Pozar, Microwave Engineering). Return loss uses the positive-dB convention.

Worked example

Say your antenna analyzer reads VSWR 2.0:1 at the band edge:

  1. Reflection coefficient: (2 − 1) ÷ (2 + 1) = 0.333
  2. Return loss: −20 log₁₀(0.333) = 9.54 dB
  3. Reflected power: 0.333² = 11.1%
  4. Mismatch loss: −10 log₁₀(1 − 0.111) = 0.51 dB

So the dreaded 2:1 band edge costs about half a decibel, barely a tenth of an S-unit. The larger costs are usually elsewhere: a protective transmitter folding back its output, or feedline loss climbing on the reflected round trip.

Assumptions & tips

  • Measure at the antenna, judge at the radio. Coax loss attenuates the reflected wave too, so VSWR measured at the transmitter end always looks better than at the feedpoint. A long, lossy cable can hide a badly matched antenna entirely.
  • Mind return-loss sign conventions. Properly it's a positive dB quantity. Many VNAs display S11 as a negative number. Enter it here as positive, 20 dB return loss, not −20.
  • VSWR is frequency-dependent. A single number means little without its frequency. Sweep the band and note where the minimum sits: off-center minimums usually mean the antenna needs lengthening or shortening.
  • Don't chase decimals below 1.5:1. The mismatch loss difference between 1.5:1 and 1.1:1 is under 0.15 dB, invisible in practice. Spend that effort on a better feedline or antenna height instead.
  • High power raises the stakes. At kilowatt levels, a 3:1 mismatch produces serious voltage peaks along the line. Check your tuner's and connectors' ratings before keying up into a known mismatch.

Frequently asked questions

What is an acceptable VSWR?

For most amateur and commercial installations, 1.5:1 or better is considered good and 2:1 is generally acceptable: at 2:1, only 11 percent of the power reflects and the mismatch loss is about half a decibel. Many solid-state transmitters begin folding back output power above roughly 2:1 to protect their finals, which is the practical reason to care.

Will high VSWR damage my radio?

Modern solid-state radios protect themselves by reducing power as VSWR rises, so damage is rare. You lose performance instead. Older transmitters and some amplifiers without protection can overheat their output stages into a bad mismatch. The reflected power itself does not "burn up the antenna". The stress lands on the transmitter and, with high power into a severe mismatch, on the feedline via voltage peaks.

Why is mismatch loss so small even at VSWR 2:1?

Because power reflects in proportion to the square of the reflection coefficient. At 2:1, the reflection coefficient is 0.33, so the reflected fraction is 0.33² ≈ 11 percent, a mere 0.5 dB. The lesson: chasing a 1.2:1 match down to 1.1:1 buys you hundredths of a decibel, effort usually better spent on cable loss.

Sources

  1. Microwave Engineering, 4th edition. David M. Pozar, Wiley, 2011. wiley.comChapter 2 develops the transmission-line relations this calculator inverts: |Γ| = (VSWR − 1)/(VSWR + 1), the reflected power fraction |Γ|², and the definition of return loss.
  2. Understanding the Fundamental Principles of Vector Network Analysis, application note 5965-7707. Keysight Technologies. keysight.comGives VSWR as (1 + ρ)/(1 − ρ) alongside the return-loss and reflected-power columns an analyzer displays: the source of the sign convention warning in the tips, since instruments often show S11 as a negative number.
  3. Recommendation ITU-R V.574-5: Use of the decibel and the neper in telecommunications. International Telecommunication Union, 2015. itu.intThe convention for expressing losses and gains in decibels, under which return loss and mismatch loss are quoted here as positive quantities.
  4. IEC 60050-726, International Electrotechnical Vocabulary — Part 726: Transmission lines and waveguides. International Electrotechnical Commission, Edition 2.0, 2025. The standardised terminology for transmission-line reflection and standing-wave quantities, including the standing-wave ratio this page reports.