Effective-medium model dependence of the radar reflectivity of conducting particle films

We present a numerical study of the frequency dependent, complex effective permittivity e for a composite material which consists of lossy graphite-type microsphere inclusions randomly imbedded in a dielectric matrix, with a view towards assessing the suitability of such a composite for its use as a radar absorbing material (RAM). This suitability is determined by the material possessing a large effective absorptivity while at the same time not giving rise to an overly large reflectivity. In this vein we here evaluate, as a function of frequency up to 20 GHz, the effective magnitudes of Re e and Im e for particulate composites and their dependence on the volume fraction φ of the particles, while independently varying conductivities. Our calculations are carried out using both the effective-medium theory (mixture theory, valid for small φ), and the multiple-scattering theory of Tsang and Kong valid for general φ but for small particle sizes. Multiple-scattering effects lead to increased effective absorptiv...