how to set a port when modeling a CPS?

hi, guys, I want to design a balun with a transition from microstrip to coplanar stripline. however, I am coming across some trouble on setting the port for CPS. I placed a pec box next to the end of CPS and set the port on the adjacent face which is large enough and extending beyond the substrate. however, I got different port impedance when I change the size of the port. please give me a hand. Any help will appreciated.

in HFSS, how can the port of the CPS be set?
I post a simple example below. it is a cps with finite length. Both ends are connected with pec boxes and a airbox on which radiation boundary condition is set up.

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