z in of antenna in cst

Hi,
You can check by calculating port modes only to check the impedance. You can also check the return loss and if the value is at least below -10dB at your desired frequency, then the antenna is matched.

s11 is return loss?

i think, you don't know all parameter of microstrip antenna. you can read in7-ANTENNA FUNDAMENTALS .pdf
follow me, relation of all parameter as: S11, Z_anten, Z_feed. VSWR is parameter you need.