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As you know Zpv is computed from power and voltage, Zpi power and current, Zvi voltage and current. HFSS computes the voltage by electric field integration across a line from GND to signal. Current is from magnetic field and the power from the poynting vector. The closest value is the computed from p and v. Why? The power is easily computed from E and H*. Voltage integration of E. That means you have defined the impedance line properly and that the current is not being accuratly computed. You must check in the manual how is the current computed and maybe it is no possible to compute the correct value for CPWs structures.
I hope it was helpfull!!
The truth of the matter is that the only valid impedance at microwave is the wave impedance. The rests are just simplifications. As far as the CPW is concerned and if your field is predominantly concentrated into the subtrate it is clear you must use the PV definition. At least the E-field does not vary too much across the metal lines and you can safely identify the contour. On the other hand it requires a good deal of skills to properly feed up the port. Sometimes one may excite some other modes too and then the result is compromised.
For instance, when I designed a slotline mixer, MWS gave me completely bogus results (ver. 3.0) and hfss was fine with PV definition that was within 1% of Cohn's design curves. It is why I guess MWS now does display a line impedance although I have some disgareement with their definition as well.
To recap - for structures, not very dispersive (like CPW) and with a good definition of integration contour you can safely assume PV. For others PI works best.
Well, with coupled lines you have even and odd -modes. It is not clear which one you do mean. Next, HFSS is a bit "careless" when it comes to the port 2D eigenvalue field accuracy. I would push the port triangulation to the max, with some manual seeding. If the aspect ratio is bad you can manualy mesh.