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VSWR, return loss & reflection

Convert a reflection measurement into return loss, VSWR and reflected power.

Convert a reflection measurement

Enter a ratio of 1 or greater, such as 2 for 2:1.

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Return loss

9.54 dB

S11 magnitude: -9.54 dB

VSWR
2 : 1
Reflected incident power
11.11 %

More measurements

Power accounting & reflection magnitude

Reflection magnitude |Γ|
0.33
Accepted incident power
88.89 %
Mismatch loss
0.51 dB

Mismatch loss describes the reduction from incident to accepted power. Accepted power is not antenna radiation efficiency; it may include dissipated power.

Complex reflection & load impedance

A scalar reflection value does not determine a unique load. Add known Γ phase to resolve it. Phase is unnecessary for an exact match. Real Z₀ sets the impedance scale. Use values at the same reference plane.

Leave blank if unknown. Phase wraps over 360°.

Known loads

Complex Γ and load impedance need a known reflection phase.

Model & assumptions

A load impedance ZLZ_L reflects part of an incident wave when it differs from the real reference impedance Z0Z_0. With both impedances in ohms, the complex voltage-wave reflection coefficient is:

Γ=ZL−Z0ZL+Z0\Gamma=\frac{Z_L-Z_0}{Z_L+Z_0}

Its magnitude ∣Γ∣|\Gamma| lies between 0 and 1 for the passive loads covered here. Zero means a match; one means total reflection. A magnitude alone does not identify an impedance because the reflection phase is also needed.

VSWR expresses the same reflection magnitude as the ratio of standing-wave voltage maxima to minima:

VSWR=1+∣Γ∣1−∣Γ∣\mathrm{VSWR}=\frac{1+|\Gamma|}{1-|\Gamma|}

VSWR has no units. It is 1 at a perfect match and grows without bound at total reflection.

Return loss expresses reflection on a dB scale. A smaller reflected wave produces a larger return loss:

RL=−20log⁡10∣Γ∣\mathrm{RL}=-20\log_{10}|\Gamma|

Passive S11 magnitude in dB is the negative of return loss. Return loss itself is not a link-budget penalty: mismatch loss measures the reduction from incident to accepted power. This model covers passive one-port reflection with a positive real reference impedance. Power fractions refer to the stated reference plane; source mismatch and repeated reflections are excluded. Match alone does not establish antenna efficiency, bandwidth or radiated performance.

For example, a 100 Ω load on a 50 Ω reference gives Γ = 1/3: VSWR 2, approximately 9.54 dB return loss and 11.11% reflected power. Open and short circuits both reflect all power, but with different phases.

SourcesKeysight, Understanding the Fundamental Principles of Vector Network Analysis (opens in a new tab)Keysight, S-Parameter Design Techniques (opens in a new tab)

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