how to improve phase accuracy in MWS?

it is strange that MWS is giving very inaccurate phase output. I have been trying to improve the phase output by adjusting some parameters, like increasing the number of meshcells. So far, nothing works. The problem is quite simple. I got only free space and periodic boundary conditions on two directions and open boundary conditions on one direction. There is only a plane wave and nothing more. I have increased mesh too much, even I cannot distinguish two mesh points on my screen (12000000 meshcells). Still the phase is very inaccurate. Any ideas how to improve the accuracy

Hi irfan1
Try to use open boundary instad of periodic boundary just to see if it works.
PL

After playing more with the structure, I found that I have to adjust the PML settings. This improves the accuracy of the magnitude of the S-parameters.I hope phase is improved the same ammount