How to use it
- Choose what you start from: VSWR, Return loss (dB), |Γ| or Load.
- Enter the value. For a load, type its resistance and the characteristic impedance of the line (usually 50 Ω or 75 Ω).
- Read the VSWR, return loss, reflection coefficient, the percentage of power reflected and delivered, and the mismatch loss.
Formulas
- |Γ| = (VSWR − 1) / (VSWR + 1), and VSWR = (1 + |Γ|) / (1 − |Γ|)
- Return loss RL = −20·log₁₀|Γ| dB, so |Γ| = 10^(−RL/20)
- Reflected power = |Γ|², delivered power = 1 − |Γ|²
- Mismatch loss ML = −10·log₁₀(1 − |Γ|²) dB
- Resistive load: |Γ| = |R − Z₀| / (R + Z₀)
Worked example
An antenna measures VSWR 2. Then |Γ| = (2 − 1)/(2 + 1) = 0.3333. The return loss is −20·log₁₀ 0.3333 = 9.54 dB, the reflected power 0.3333² = 11.11 %, so 88.89 % reaches the antenna, and the mismatch loss is −10·log₁₀ 0.8889 = 0.5115 dB.
A return loss of 20 dB means |Γ| = 0.1, VSWR = 1.1/0.9 = 1.2222 and 1 % of the power reflected.
Tips
- Measure at the antenna if you can. Cable loss makes a bad antenna look better at the radio end: the reflected wave is attenuated twice.
- Return loss is positive. Some analysers show S11 as a negative number (−14 dB); its magnitude is the return loss.
- A perfect match (VSWR 1, |Γ| = 0) has an infinite return loss, so that line is left out of the results.