How to calculate RFID antenna gain using CST MWS?

Hi,

I want to design RFID antenna using CST MWS at 866MHz. This antenna has complex impedance. I am adding an external port in schemetic CST MWS. Then i am adding AC,Combine results and s-parameters tasks from new task tab. S parameters calculate is correct for complex external port. But i can't antenna gain calculation. How to calculate RFID antenna gain using CST MWS?

Please help me.

Thanks.

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