An interesting paper "Measuring Capacitor Parameters Using Vector Network Analyzers" by Stepins, Asmanis, and Asmanis might be of interest to members of this group. The three common methods of measuring components using S11 refection, S21 shunt and S21 series are discussed. It can be downloaded here....
They conclude the following:
"The most problematic parameter to measure accurately with VNA is capacitor ESR. Only shunt-through technique should be used for accurate ESR measurements. However there are some limitations. ESR can accurately be measured only at f(res) and in vicinity of it (ESR measurement error below 3% can be
achieved). For frequencies which are much higher or lower than f(res) the measurement accuracy can be very poor, even using shunt-through technique, because when capacitor reactance is much higher than its ESR, then the measurement error can increase enormously. Accurate measurements of ESR of capacitors with C lower than several tens of nF are impossible. ESR measurements of small-capacitance capacitors using VNA is completely useless."
Roger
Roger Need