Interdigital Capacitor Excitation with HFSS

Hello there,

I'm a pretty newbie to HFSS and I want to simulate a interdigital capacitor to calculate the capacitance with different dielectrica.
So I build the interdigital capacitor and tried to excite it.
Here lies my problem. I excited it with two port connected to ground, but the calculated capacitance from the smith chart with the formula C=-1/(2*pi*f*Im(Z12)) doesn't fit my assumed and measured capacitance at all.

I would be really gratefull for some help.