designing capacitor hfss

Hi all,
I am try to make microstrip antenna in HFSS. I used matching capacitor. In my design, I used cylindrical PEC and I defined it as a RLC element in the design. However its effect changes with its legth, I wonder it normal or is there another way to define constant capacitance in HFSS? Please help me.
Thanks
not: I added my design here.