VSWR Calculator Formula

Understand the math behind the vswr calculator. Each variable explained with a worked example.

Use the VSWR Calculator

Formulas Used

VSWR

vswr = (1 + gamma) / (1 - gamma)

Reflection Coefficient

reflection_coeff = gamma

Return Loss

return_loss = -20 * log10(gamma)

Mismatch Loss

mismatch_loss = -10 * log10(1 - pow(gamma, 2))

Power Reflected

power_reflected_pct = pow(gamma, 2) * 100

Variables

VariableDescriptionDefault
z_loadLoad Impedance(Ω)75
z0Characteristic Impedance(Ω)50
gammaDerived value= abs((z_load - z0) / (z_load + z0))calculated

How It Works

What is VSWR?

VSWR (Voltage Standing Wave Ratio) measures impedance mismatch between a transmission line and its load. A perfect match is 1:1.

Formulas

Gamma = Z_load - Z_0/Z_load + Z_0

VSWR = (1 + Gamma) / (1 - Gamma)

Return Loss = -20 x log10(Gamma) dB

Interpretation

  • VSWR 1.0: Perfect match (0% reflected)
  • VSWR 1.5: Good match (4% reflected)
  • VSWR 2.0: Acceptable (11% reflected)
  • VSWR 3.0+: Poor match (25%+ reflected)

    • Worked Example

    75 ohm load on a 50 ohm transmission line.

    z_load = 75z0 = 50
    1. 01Gamma = |75 - 50| / |75 + 50| = 25 / 125 = 0.2000
    2. 02VSWR = (1 + 0.2) / (1 - 0.2) = 1.2 / 0.8 = 1.50
    3. 03Return loss: -20 x log10(0.2) = 13.98 dB
    4. 04Power reflected: 0.04 = 4.0%

    Frequently Asked Questions

    What VSWR is acceptable for most applications?

    Below 2:1 is acceptable for most systems. Below 1.5:1 is good. Antenna systems often specify 1.3:1 or better.

    Can VSWR damage equipment?

    High VSWR reflects power back to the transmitter, causing heating. Most transmitters reduce power automatically above VSWR 3:1.

    What is the relationship between VSWR and return loss?

    They are different ways to express the same mismatch. Return loss = 20 x log10((VSWR+1)/(VSWR-1)) dB.

    Ready to run the numbers?

    Open VSWR Calculator