You have to set up the boundary condition for the unit cell. In other words, you have to set up the TM and TE polarization.

Boundary Conditions

In determining the solution of electromagnetic fields for a general antenna or scattering problem, it is necessary to satisfy boundary conditions between different media for the tangential and normal field components. The boundary conditions depend on the type of media and also whether any charges or currents are present at the interface between the media. To derive the boundary conditions from Maxwell’s equations, two vector identities are needed, namely the divergence theorem.

Unit Cell boundary conditions (uc): a single cell with unit cell boundary conditions (uc) on four sides (X and Y boundaries) was used. Electric (Et=0) used in Z min, while Z max should be chosen as open (add space) for floquet mode. The important thing that should take into consideration is the number of floquet mode should be equal to 2, due to electric and magnetic fields.

Unit Cell boundary conditions (uc): a single cell with unit cell boundary conditions (uc) on four sides (X and Y boundaries) was used. Electric (Et=0) used in Z min, while Z max should be chosen as open (add space) for floquet mode. The important thing that should take into consideration is the number of floquet mode should be equal to 2, due to electric and magnetic fields