S11 parameter (return loss) variables

hello guys,

I'm working on XFDTD and I'm getting S11 parameter for my rectangular microstrip antenna S11 value of -25 dBa (amplitude) at my frequency and S11 of -15 dBi (isotropic)... which one is true and what's the difference between them?
I'm trying to calculate the bandwidth from this graph and I'm not sure which one is the right value for the bandwidth?

thanks,
Sam

Dear roommate
would you please tell me more about your test setup?