i draw a rectangular (sheet) in HFSS and define it as lumped port excitation and characteristic impedance of 50 ohm, but after the simulation and plotting port Z0 for Z0(p1) and Z0(p2) in rectangular plot section of results, i saw a characteristic impedance of 34 ohm, not 50 ohm.
i think i define the lumped port sheet wrong.
how can i design the size of this sheet?
If you want to know Z0 of a transmission line, it is determined by the transmission line structure, not the port settings. To measure this Z0, you should use a wave port. A lumped port may give a wrong value for "Port Z0".
I believe the Z0 setting for lumped ports is only used for adjusting S-parameters (they have to be normalized to a certain impedance) after the simulation is finished.