I would recommend to use Floquet ports with appropriate boundaries. When setting the Floquet ports, you will have an opportunity to set the incoming wave direction. It would be better, than using WG-port, since that one does not simulate pure plane wave.

You can learn about Floquet theory from here:

P. A. Kuchment, Floquet theory for partial differential equations, Russian Mathematical Surveys, 1982, Volume 37, Issue 4, 1–60

How we can plot graph of Scattered Electric Field (dB (v/m) Vs distance (R (m)) from RIS in CST ?

Hello,

Few papers and links have been given.

https://space.mit.edu/RADIO/CST_online/mergedProjects/3D/special_overview/special_overview_waveguideover.htm

https://www.edaboard.com/threads/simulating-fss-in-cst-mw-studio.134499/

Thanks

Thank you Smrity Dwivedi for sharing

This can be done with Floquet Ports by observing the responses at oblique incidence.