Simulation of a RFID tag antenna with CSt MWS

Hello
I have a problem in the simulation of a RFID tag antenna with CSt MWS, when I use a discrete port with an impedance of 50 ohms, I have Z11, but when I change the impedance of the discrete port, the Z11 change a little , however Z11 must not depends on the impedance of the discrete port because it represents the input impedance of the antenna, someone can tell me how to fix it ?

How much Z11 changes? Z11 is calculated from S-paramters, so they should also change. Do you use adaptive meshing?

One should not change Z11, your problem in matching chip impedance to the input impedance of the Tag antenna.