Directivity And VSWR Measurements: Understanding Return Loss Measurements

By: Doug Jorgesen and Christopher Marki

The characterization of microwave networks requires discriminating between forward and backward traveling waves.

Unfortunately no directional device is perfect, leading to potentially dramatic measurement errors. In this note we show that return loss and VSWR measurements are greatly complicated by the finite performance of the directional device used to measure the reflected power.

Explicit expressions are derived for measurement error as a function of directivity, return loss, and reflection phase. The only accurate and convenient way to make return loss measurements is with a well matched high directivity directional coupler or bridge.