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VSWR calculator

VSWR (voltage standing wave ratio) says how much of the wave an antenna sends back down the feedline. The outgoing and returning waves overlap, so the voltage along the cable is high in some places and low in others. VSWR is the ratio of the highest to the lowest: 1.0 : 1 means nothing comes back, 2.0 : 1 sends 11% of the power home and costs about 0.5 dB.

: 1

As an antenna analyzer reads it. Values below 1 are not physical.

W

Used only to put the reflected fraction into watts.

VSWR
1.50 : 1
ratio of maximum to minimum voltage
Return loss
14.0 dB
higher is a better match
Reflection coefficient
0.200 |Γ|
reflected voltage fraction
Mismatch loss
0.18 dB
power lost to the mismatch alone
Reflected power
4.0 %
= 0.4 W of 10 W comes back
forward power at the loaddelivered 96.0 %reflected 4.0 %voltage along the feedline (relative)VmaxVminλ/2VSWR = Vmax / Vmin = 1.50 : 10λ/2λ3λ/2distance from the load · the pattern repeats every λ/2

The top bar splits the forward power: green reaches the antenna, amber turns around. Below it is the voltage along the feedline — Vmax where the two waves reinforce, Vmin a quarter wavelength further on where they oppose, repeating every λ/2. The crests are broad and rounded while the troughs are narrow and sharp: that asymmetry is real, and it grows with the mismatch.

Assumes a lossless line at the measurement plane. Measured VSWR at the radio end always looks better than at the antenna, because feedline loss attenuates the reflected wave twice. The diagram draws a purely resistive load, which puts a voltage maximum at the antenna; a reactive load slides the whole pattern along the line without changing the ratio.

VSWR reference table

VSWR|Γ|Return loss (dB)Reflected power (%)Mismatch loss (dB)
1.0 : 10.000∞0.00.00
1.2 : 10.09120.80.80.04
1.5 : 10.20014.04.00.18
2.0 : 10.3339.511.10.51
3.0 : 10.5006.025.01.25
5.0 : 10.6673.544.42.55
10.0 : 10.8181.766.94.81

Five ways of stating the same mismatch, computed with the formulas below. A perfect 1.0 : 1 match reflects nothing, so its return loss is infinite; at 2.0 : 1 the antenna still receives 89% of the power.

How it works

  1. 01

    The antenna sends part of the wave back

    The transmitter pushes a wave down the feedline. An antenna whose impedance matches the cable absorbs all of it; any other impedance cannot, and the remainder turns around and travels back toward the radio. The reflection coefficient |Γ| is the fraction of the voltage that comes back: 0 for a perfect match, 1 for an open or a short. At |Γ| = 0.333 the antenna returns a third of the voltage and 11% of the power.

  2. 02

    Two waves on one cable make a standing pattern

    Outgoing and returning waves share the cable and simply add. Where their crests meet, the voltage reaches 1 + |Γ| times the forward wave; a quarter wavelength further on they oppose and it drops to 1 − |Γ|. Those peaks and troughs stay put along the line — a standing wave — and repeat every half wavelength. VSWR is the ratio between them, VSWR = VmaxVmin. With nothing reflected there is no pattern at all: the voltage is the same everywhere, and the ratio is 1 : 1.

  3. 03

    Convert freely

    VSWR, return loss and |Γ| are one number in three costumes, so any of them can be the starting point. VSWR 1.5 : 1 is |Γ| = 0.200 and 14.0 dB return loss; VSWR 2.0 : 1 is |Γ| = 0.333 and 9.5 dB. Network analyzers report return loss or S11, antenna meters report VSWR.

  4. 04

    Read the damage honestly

    A 2.0 : 1 mismatch reflects 11% of the power, which is 0.5 dB — usually less than the coax loses on the way up the mast. The real costs sit elsewhere: solid-state amplifiers fold their output back to protect themselves, the reflected power heats the feedline, and voltage peaks stress connectors.

Formulas

Reflection coefficient and VSWR
|Γ| = VSWR − 1VSWR + 1 ⟺ VSWR = 1 + |Γ|1 − |Γ|
  • |Γ| — magnitude of the reflection coefficient, 0 … 1
  • VSWR — voltage standing wave ratio, ≥ 1 (quoted as VSWR : 1)
Voltage along the line
|V(z)| = |1 + Γ·e−j2βz| = 1 + |Γ|2 + 2|Γ|·cos(4πz/λ)
  • z — distance from the load, m
  • λ — wavelength on the line, m (β = 2π/λ)
  • Vmax = 1 + |Γ|, Vmin = 1 − |Γ|, repeating every λ/2
Return loss
RL (dB) = −20·log10|Γ|
  • RL — return loss, dB, positive and larger for a better match
Reflected power and mismatch loss
PreflPfwd = |Γ|2 ; ML (dB) = −10·log10( 1 − |Γ|2 )
  • Pfwd — power travelling toward the load, W
  • Prefl — power reflected back toward the source, W
  • ML — mismatch loss, dB

Worked example

VSWR 2.0 : 1 with 10 W forward
  1. |Γ| = 2 − 12 + 1 = 0.333
  2. Return loss: −20·log10(0.333) = 9.5 dB
  3. Reflected: 0.3332 = 11.1% → 1.1 W comes back
  4. Mismatch loss: −10·log10(0.889) = 0.51 dB
  5. → the antenna still gets 8.9 W — VSWR 2 : 1 wastes far less than its reputation

FAQ

What VSWR is acceptable?
Below 1.5 : 1 is a sensible design target for a transmitting antenna, and up to 2.0 : 1 is fine for most installations — that is 11% of the power reflected and about 0.5 dB lost. Beyond roughly 2.5 : 1 many solid-state transmitters begin reducing output to protect the final stage, so the practical limit is set by the radio, not by the lost fraction.
Does a high VSWR damage the radio?
Reflected power itself is rarely the problem at low levels; heat and voltage are. A high-power stage driven into a bad mismatch sees standing-wave voltage peaks of 1 + |Γ| times the forward wave and dissipates the returning energy, which is why modern radios sense the reflected wave and fold back power. At a few watts, even an open circuit is usually harmless.
Why does my VSWR look better through long coax?
Feedline loss attenuates the forward wave on the way up and the reflected wave on the way back, so the meter at the radio sees |Γ|radio = |Γ|antenna·10−2L/20 with L the cable loss in dB. Through 3 dB of coax, an antenna sitting at 5.0 : 1 reads 2.0 : 1 at the radio. Measure at the antenna if you want to know what the antenna is doing.
Is a perfect 1.0 : 1 match achievable?
Only at spot frequencies, and only for that one set of conditions — weather, ice, nearby metal and feedline routing all move the match. Bandwidth matters more than the dip: an antenna that stays under 2.0 : 1 across the whole band beats one that reaches 1.0 : 1 at a single frequency.
Why are the voltage minima sharper than the maxima?
Because the two waves add as vectors. Near a crest the sum is already large and a small phase change barely moves it, while near a trough the two amplitudes almost cancel, so the same phase change is a large relative change. Expanding 1 + |Γ|2 + 2|Γ|·cos(4πz/λ) around a minimum shows the dip narrowing as |Γ| approaches 1 — at VSWR 10 : 1 the troughs are visibly needle-shaped, at 1.2 : 1 the curve is almost a sine.

Matched? Now check the path — a perfect antenna into a blocked path is still a dead link. Waveshed draws line-of-sight and coverage over real terrain, free in your browser.

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