How to find the Q factor for a resonant antenna?

Hi,

I want to reformulate my previous post by simple question: how to find the Q factor for a resonant antenna or a resonant Transmission Line using S parameters from ADS or HFSS simulation.
I know that Q is equal to fcenter/bandwith. How to calculate the bandwith, can i find this bandwidth from -10dB or VSWR(S11),VSWR(S21).

I find this in edaboard forum but unfortunatly is not clear specially for the loaded Q

Thank you for your help