Gain and bandwidth calculation in HFSS...?

I have simulated a rectengular patch antenna(linefed) at the centre frequency of 2.4 GHz. The results (S11 and VSWR) are so confusing.. the whole plot of s11 is below -8dB and VSWR below 1.80.. Is it correct..? How can we calculate gain and bandwidth in that plot? What is the formula to calculate Bandwidth?

Can anybody help me?