Hi Dr. Torben,
Thank you for your kind reply. I ran the model, however, the problem that I am seeking is an eignevalue problem and the geometry is not the FSS you have used. The geometry is a grounded substrate with a via and conducting patch on top. Then the natural modes are usually found from an eigen mode solver after adding an air layer and PML. There is a lengthy procedure to follow by varying the phase among three periodic axes called Brillouin zone (from solid state physics). The final results are represented by a dispersion diagram frequency versus phase for a number (usually three) for TM and TE modes. I have searched the literature on FEKO and dispersion diagram and did not find results reported using FEKO. Currently, Lumerical, CST (application note attached), and HFSS have separate eignemode solver to perform such calculations.
Therefore, I wonder if you can solve this problem using FEKO?
Thank you and best regards