I. Introduction
At microwave frequencies, the use of a calibrated vector network analyzer (VNA) is mandatory to characterize the linear behavior of a device under test (DUT). A number of VNA calibration techniques have been proposed in the literature. Among them, the most commonly used calibration techniques are the thru-reflect-line (TRL) [1], thru-reflect-match (TRM) [2] and thru-reflect-reflect-match (TRRM) [3].