The effect of measurement errors in the S-matrix of a reciprocal 2-port device is recognized in the (usually low) difference between S12 and S21, as the device were nonreciprocal. This ``false non-reciprocity'' is analyzed in the present paper, and it is verified that, for low loss device, the difference acts principally on the phases of S12 and S21. This anomaly can be removed if a numerical correction is applied to the experimental S-matrix. In doing so, it is proved that the residual measurement errors have comparable amplitudes on all scattering parameters.
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