Simulating antenna scattering parameter S11

Hello,

I am trying to get the scattering parameter S11 of a patch antenna. In order to avoid reflections, I use PML which is thickness of 1 wavelength.

However, I have marked that S11 results change accordingly to mesh size. I am using physics-defined mesh from normal to extremely fine, and the results are always different, which is confusing.

My question is - how is it possible to tell whether mesh is accurate enough to give trustworthy results?

please find the model attached.

Thanks and regards,
Arturs