Comparison of the unified perturbation method with the two-scale expansion

The authors consider whether the unified perturbation method (UPM), which converges faster over a wider domain of surface roughness than other perturbations and intrinsically possesses characteristics similar to the two-scale expansion without requiring a free parameter, can be improved by applying the two-scale concept. To do so, they expand the unknown source current in a two-scale approximation. It is shown that the UPM performs best without two-scale expansions. The optimum spectral split of the various two-scale theories considered is presented