Measurement technique for symmetrical reciprocal three-port devices using two-port vector network analyzer

A measurement technique is proposed to conduct S-parameter measurements on symmetrical reciprocal three-port devices using a two-port vector network analyzer. The technique takes the effects of reflection from an unmeasured port terminated with a non-ideal 50-Ω load into consideration. These reflected signals cause errors in S-parameter measurements, especially at higher frequencies where the reflection is too large to be ignored. The errors have been overlooked in the traditional three-port measurement using two-port network analyzers. In this paper, analytic equations are presented for some common symmetrical reciprocal three-port devices to improve the accuracy of their S-parameter measurements. The improvement is justified by comparing the data before/after applying the proposed technique with the measured data from four-port vector network analyzers. In addition, error analysis is conducted to ensure that the errors introduced by vector network analyzers would not be deteriorated by the proposed equations.