Return Loss, VSWR & Reflected Power
Four connected ways to describe impedance mismatch in an RF path—and why reference planes and measurement conditions matter.
Reflection coefficient
The complex reflection coefficient, usually written as Gamma (Γ), is the reflected voltage wave divided by the incident voltage wave at a defined reference plane. Its magnitude ranges from zero for an ideal match to one for total reflection in a passive, single-port example.
The magnitude alone does not show the phase of the reflection; a vector network analyzer can measure both magnitude and phase.
Converting between return loss and VSWR
Return loss is minus 20 times the base-10 logarithm of the reflection coefficient’s magnitude. VSWR is (1 + |Γ|) divided by (1 – |Γ|). Reflected power as a fraction of incident power is |Γ| squared, under the usual matched-reference assumptions.
| VSWR | Return loss (approx.) | Reflected power (approx.) |
|---|---|---|
| 1.2:1 | 20.8 dB | 0.83% |
| 1.5:1 | 14.0 dB | 4% |
| 2.0:1 | 9.5 dB | 11.1% |
A higher return-loss value generally means a better impedance match; a VSWR closer to 1:1 also means a better match. These values describe a particular measurement reference plane and frequency.
Why measurement location matters
Connectors, adapters, cable loss and fixtures influence what the instrument sees. A lossy cable may reduce the magnitude of a reflection measured at the instrument while the actual load mismatch remains unchanged. Calibrating a VNA at the intended device connection moves the measurement reference plane and makes comparisons more meaningful.
What to include in a component requirement
- Operating frequency band and reference impedance (commonly 50 ohms in coaxial RF systems)
- Maximum permitted VSWR or minimum return loss across the band
- Connector or waveguide interface and test configuration
- Average and peak power, including expected reflected-power conditions
Discuss an RF component requirement
Share the frequency, performance, interfaces and quantity you need. Impulse can help evaluate relevant product options; confirm specific ratings against the manufacturer datasheet.
