To see the effect of a metasurface on an incident plane wave, I can use E Field& H field monitor for near field in frequency domain solver with unit cell boundaries and floquet port excitation. But to have far field, I can use farfield/RCS monitor if I can exclude sidewalls, based on :
https://space.mit.edu/RADIO/CST_online/mergedProjects/3D/special_postpr/special_postpr_pp_farfield.htm
section:
Farfield Contribution of Unit Cells / Periodic Structures
but I don't know how to exclude sidewalls.