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

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