So what I have at hand is a cheap network analyser that can present the measured impedance in form of the values of different equivalent circuit topologies, e.g. R-L series or RL parallel.
What I want to do is measure the electrical parameters of a simple PCB antenna structure, which is typically modeled as as capacitor (mainly inter-winding capacitance) parallel to ( R + L).
So I did the "SOL" calibration and measured S11. Concerning R and L, I chose the available Network Analyser Representation of the equivalent circuit called "RL" (I have no C//(R+L) representation available) This measurement shows the values for both, R-L series and R-L parallel equivalent circuits and I get reasonable values so no problem here.
But how can I determine the value of the capacitor? Is it possible at all? I thought of writing down the formula for the real and imaginary parts of the impedance of the topology I want to model (i.e. C//(R+L)) and solve it using the measured values of S11. Besides of being inconvenient I think I would get a wrong result since any physically capacitance present in the structure is already indirectly included in the values i get for R-L (by having a slightly smaller L than without cap), right?
(Of course the inclusion of the capacitor in the presented R-L values is possible only for a specific range of frequencies because above some frequency the capacitor will dominate and "get visible" in the measurements, but that does not help either...)