Why reciprocal procedure works?

Accurate characterization of the calibration standards used for the short-open-load-thru (SOLT) calibration of S-parameter measurements of RF devices is crucial if correct measurements are to be obtained. For most reciprocal devices (i.e. with S12=S21) if the calibration coefficients are inaccurate, the 'calibrated' S21A and S12A values appear not to be identical over all frequencies. We find that we can use this apparent violation of reciprocity to develop a novel procedure for standard characterization. This procedure ("reciprocal procedure") searches for the optimum set of calibration coefficients for which the reciprocity of the device is obeyed at all frequencies