RFID antenna design with hfss——problem about the S11

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fangyuanhust

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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

Anything Wrong with my calculation?
 

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