On Sun, Mar 3, 2024 at 11:04 AM, W0LEV wrote:
But given the small difference between the S21 and the Y21 method, I'll
continue with the S21 method.
It's not always small, Dave. The first plot shows VNA R & X for data from a ground probe using a two-port series measurement. The second plot shows the resulting ground permittivity and conductivity. Dim traces are for the S21 method, normal traces are for Y21. The Y21 permittivity and conductivity traces are similar to those derived from a single-port S11 measurement. The traces for the S21 method are in left field.
In this application, common-mode current arises because the unknown (ground) is distributed and easily couples to the VNA cables and the VNA itself. It manifests itself as rather large stray shunt impedances. The Y21 method effectively null them.
Brian
