Effective medium theory for large particulate size composites

The present study extends our previous work that concerned the effective permittivities and absorptivities of particulate composites consisting of graphite-type microsphere inclusions in a dielectric matrix, and their suitability as radar-absorbing materials. Whereas in that work spheres of one size (radius a=50 μm) had been considered only, we here study the dependence of the results on the sphere radius of up to one or several millimeters, and find sizable effects on permittivities for a⩾1 mm, and below that for the absorptivities, indicating the increasing importance of multiple scattering effects. While the multiple-scattering formulas utilized here are based on a low-frequency approximation, we devise a modification of these formulas in order to extend their validity to higher frequencies (or larger particle sizes).