Port impedance issue when simulating patch antenna with probe fed

i am simulating patch antenna with probe fed .

i calculated the dimension of feeding coax for 50 ohm impedance using built in macro in CST MWS and then use those dimensions in modeling.
however after simulation when i check the the port impedance it doesnt show 50 ohm but 40 ohm

can anybody know whats wrong with this disagreement

thanks

CST is very ok for coaxial feed. however in your case its mesh problem. Increase the mesh and you will get the right result.

thanks for your helpful comments .
i will try to follow your advice
regards

MWS requires three mesh between port and device e.g. antenna etc.
For coaxial feed if there are three mesh lines in the shied i ever got around 50ohm impedance.