I have been using the formula
Z0=Zs*((1+S11)^2-S21^2/(1-S11)^2-S21^2)
to extract the characteristic impedance from measured S-parameters of a transmission line (TL). This calculation for Z0 works for some transmission lines but shows discontinuities (resonant features) at specific frequencies for others. It appears the phase of S11 plays a role. For TLs with symmetric S11 phase, I don't see the abrupt change.
Is this the right formula to use? Is there another formula? Also, what is the best way to measure Z0 of a TL using VNA?
Thanks,
Reyan