How VSWR, return loss and mismatch loss relate
When a load doesn't match the impedance of its line, part of the wave reflects back toward the source. VSWR, return loss and the reflection coefficient are three ways of describing the same reflection; mismatch loss is the power that never reaches the load because of it.
- Reflection coefficient:
Γ = (Z_L − Z₀) / (Z_L + Z₀) - VSWR:
(1 + |Γ|) / (1 − |Γ|) - Return loss:
RL = −20·log₁₀|Γ| - Mismatch loss:
ML = −10·log₁₀(1 − |Γ|²)
Worked example: an antenna at 1.5:1
A VSWR of 1.5:1 means |Γ| = 0.2 and a return loss of 14.0 dB. The load reflects 4% of the power, a mismatch loss of only 0.18 dB.
Good to know
- 2:1 reflects 11% of the power (0.51 dB) and is the usual limit in antenna and RF port specs. Mismatch loss stays small until VSWR passes about 3:1.
- Cable loss hides a bad antenna: each dB of cable between the meter and the antenna improves the measured return loss by 2 dB. Measure at the feed point when you can.
- Return loss is quoted as a positive number; S11 is the same quantity with a negative sign.
- Pro adds the VSWR your radio actually sees through the cable and the mismatch uncertainty against the source.