This study developed a generalized solution based on modal expansion for scattering by large cavities with known wave functions placed in an infinite perfect electric plane. Under the assumption of a large cavity, to reduce simulation time and simplify expressions, the half‐space above cavity with a strong singular Green's function is substituted by an arbitrary semi‐waveguide. The fields inside the cavity are expanded by the semi‐waveguide eigenfunctions. The corresponding modes are matched to create a system of linear equations for unknown expansion coefficients. To demonstrate the validity and ability of this method, it is applied to several grooves with different shapes (triangular, circular, and elliptical grooves) and then their scattering signatures are compared with each other. The results are also verified by the results obtained by the method of moment. The measured simulation time for both methods shows that this scheme can be an appropriate candidate for analysing the scattered fields by large cavities.
