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VSWR, Return Loss and Reflected Power: What the Numbers Mean

A 1.5:1 VSWR means 4% of the incident power is reflected at the mismatch. It also corresponds to 13.98 dB return loss, but only 0.177 dB of mismatch loss. Those numbers describe different things; treating return loss as lost forward power gives the wrong answer. ^{[1, 2]}

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Start with the reflection coefficient

On a line with characteristic impedance Z₀ terminated by load Z_L, the complex voltage reflection coefficient is Γ = (Z_L − Z₀)/(Z_L + Z₀). Its magnitude ρ = |Γ| is between zero for a match and one for total reflection in a passive load. The phase of Γ matters when several mismatches interact, but VSWR and return loss use only ρ. ^{[1, 2]}

VSWR = (1 + ρ)/(1 − ρ) · ρ = (VSWR − 1)/(VSWR + 1)

Return loss = −20 log₁₀ ρ · Reflected power = 100ρ² %

For VSWR = 1.5, ρ = 0.2. The reflected voltage is 20% of the incident voltage, but the reflected power is 0.2² = 4%. Return loss is −20 log₁₀(0.2) = 13.98 dB. Higher positive return loss means a better match; an instrument may instead plot S11 as −13.98 dB. ^[1]

Why mismatch loss is much smaller than return loss

For a single passive mismatch with the source matched to Z₀, the fraction of incident power accepted by the load is 1 − ρ². Its mismatch loss is −10 log₁₀(1 − ρ²), or 0.177 dB for the same 1.5:1 VSWR. Return loss describes the reflection relative to the incident power; mismatch loss describes the reduction of delivered power. Neither figure includes cable, conductor or dielectric attenuation. ^{[1, 3]}

Quantity	At 1.5:1 VSWR	Meaning
\|Γ\|	0.2	Reflected voltage-wave ratio
Return loss	13.98 dB	Incident-to-reflected power ratio
Reflected power	4%	Power travelling back from the mismatch
Mismatch loss	0.177 dB	Accepted-power reduction with a matched source

Convert VSWR, return loss and reflected power →

What to check on a real board

A load can have a good match at one frequency and a poor one elsewhere. Read S11 or S22 over the full operating band, at the same reference impedance as the system. A connector, via or cable adds frequency-dependent loss as well as reflections; S21 describes forward transmission through the measured network. Two separated mismatches can reinforce or cancel depending on their phase, so adding their VSWR figures is not a channel calculation. ^{[1, 3]}

Inspect S11 and S21 across frequency in a Touchstone file →

Tools used in this guide

VSWR, Return Loss & Mismatch Loss Calculator – Convert between VSWR, |Γ|, return loss, mismatch loss and reflected power, find Γ of a complex load, and bound the mismatch uncertainty and cascaded VSWR of two ports.
S-Parameter Viewer (.s2p, .s4p) – Open .s1p to .s16p Touchstone files in the browser: insertion and return loss, phase, group delay, mixed-mode SDD21/SCD21 for 4-ports, TDR impedance and interface loss budgets.
Attenuator Pads – Matched Pi, T and bridged-T pads with the nearest standard values, their actual loss and return loss, and resistor power.

Sources

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