A method for determining the complex impedance between a first stage and a second stage in a microwave system includes detecting an incident signal emitted by the first stage and detecting a reflected signal reflected from the second stage. The magnitudes of the incident signal and the reflected signal are measured. The detected incident signal is phase shifted by a first angle to yield a first incident signal and the detected reflected signal is phase shifted by the first angle to yield a first reflected signal. The detected incident signal and the first incident signal are mixed with the detected reflected signal and the first reflected signal. The angle of the reflection coefficient is determined based on the mixing and the magnitudes of the incident signal and the reflected signals.