A 4-term reflectorneter error model for systems with variable coupling

Traditional error models for the reflectometers in vector network analyzers (VNAs) use 3 terms and assume the coupling is independent of device loading. !n some situations, this assumption may be suspect. These cases include remote VSWR monitoring systems (in RF power systems for example), some mm-wave measurement tools, low-cost W measurement devices and experimental wafer-probe-head measurement tools. A rudimentary 4-term error model and a newer calibration procedure has been developed that significantly improves results; in some cases reducing errors by a factor of 5. Perhaps more importantly, this model allows a better assessment of uncertainty in such measurement systems when a traditional calibration procedure is used for practical reasons.