RCWA implementation in calculating difraction from radio-absorbing material

The RCWA method in solution of the scattering problem for a radio-absorbing material is presented. The goal of this study is to check applicability of the method for calculation of reflection indices of materials used in modern unechoic chambers. The RCWA method is considered with some enhancements, ETMA and partial solution approach in particular. Calculations are conducted using characteristics of a commercial wedge-shaped radio-absorbing material. The method of layered approximation is also considered. RCWA and approximate method results are compared. Good agreement between them is shown including the most important case of incident angles close to $90^\circ$.