I've simulated an UHF RFID antenna using HFSS. And I got the S11 calculated by HFSS.
I tried to calculate S11 using the formula Γ=(zc-za*)/(zc+za)
zc for chip impedance (the conjugation of the port impedance setted in hfss)
and za for antenna impedance derived form HFSS
then, S11=10lg|Γ|
but i got this
the red line for result simulated by HFSS, the blue one for calculated