A new method to rapidly design 2D metamaterials for rectangular waveguides by rebuilding their dispersion properties is proposed. The Modal Expansion Theory (MET) is revisited for theoretical surfaces with fixed surface impedances ZS. Then, it is pursued for real dispersive anisotropic surfaces, which have surface impedances that are dependent on the frequency and the incidence angle. An algorithm which calculates the correct incidence angle of the guided electromagnetic mode at each frequency is presented. By including this algorithm in the MET and by combining it with a code based on the Finite Element Method (FEM) to calculate the surfaces impedances, dispersion diagrams of waveguides with real 2D anisotropic walls are correctly rebuilt. This is validated by comparing the results for two different metamaterials, corrugation- and T-structure, corrugations and metamaterials, with those obtained using a commercial software.
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