Expressions are derived for calculating estimates of the systematic errors in dual six-port or four-port measurements of reflection coefficient and scattering parameters due to imperfections in the transmission-line standard used to calibrate the system. A new mathematical model for a four-port reflectometer makes it easier to visualize and analyze these errors. In this new model, two of the three parameters needed to characterize a four-port can be determined without standards. All imperfections in the standard perturb only the third parameter which acts as an impedance transformer.
Cletus A. Hoer, "On-line accuracy assessment for the dual six-port ANA: Treatment of systematic errors," IEEE Transactions on Instrumentation and Measurement, vol. 36, no. 2, pp. 514-519, June 1987.
The complete article is available for download from the publications page.