microstrip impedance

Hello,

I have designed a microstrip line. It is just a line z0=50 and two ports, one each side of the line.

I simulate to get the S parameters and then I calculate the Z parameters.
The result of the Z12 parameter is really surprising. In the real part, the resonances are changing between negative and positives.
Does anyone know why it is happening? do you can consider it as a negative resistance?

First re-define what Z12 is in your mind ...You'll find the right answer.

Thank you for replying.

My definition is https://en.wikipedia.org/wiki/Impedance_parameters

If I see the equivalent circuit of the Wikipedia https://en.wikipedia.org/wiki/Impeda...equivalent.svg and the real part of the Z12 is negative, then is the microstrip line similar to a negative resistance?

Is such thing possible with passive elements ?

Question is how you set reference planes in your simulated S-parameter "measurement". If you don't apply port extension to eliminate the transmission line delay, S12 and S21 as well as Z12 and Z21 will rotate around the origin with increasing frequency. Interpreting a (180° + n*360°) rotation as negative impedance is an error of reasoning.