why the bandwidth is determined at VSWR=2.5？

In the HFSS antenna simulation result,why should the bandwidth determined at 2.5:1 VSWR or 7.3dB return loss?
who can give me some brief explanation,thanks~！

I suggest you look at the microwave or principle, in-depth understanding of Caigou！

what is differenr of return loss and vswr?

RETURN LOSS= 20 * LOG((vswr+1)/(vswr-1))

if the vswr you're tunning in handy is about 2.5,it means that the antenna maybe acceptable and you can turn to the TRP/TIS test
However,it depends on the system ,the result for waht you're chasing is in or out of your imagination. so you maight as well make good preparation in your heart！