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

We present a numerical study of the frequency dependent, complex effective permittivity ε 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 ε and Im ε 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 absorptivities by adding scattering losses to the intrinsic losses of the media. We comment on the optimal values of the medium parameters and packing fractions for composite RAM materials. Percolation effects (transition from matrix dominance to particle dominance) are studied, both for the effective-medium theory and for the multiple-scattering theory, and compared for the two cases.