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The design of absorbers for use in electromagnetic anechoic chambers has been a problem of interest for many years. The surface of such absorbers is created from periodically arrayed lossy materials, generally backed by an anisotropic ferrite material that absorbs low frequencies. To study the problem numerically and produce computer aided absorber designs, a computational scheme is required. In this work, the finite element-boundary integral (FF-BI) variant of the FEM is used because the method of moments is inefficient when applied to inhomogeneous structures, and because periodic boundary conditions are difficult to handle in FDTD. On the other hand, the FE-BI method has been already been efficiently and flexibly applied to the study periodic structures involving isotropic and inhomogeneous materials.