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

We present a numerical study of the frequency dependent, complex effective permittivity epsilon for a composite material which consists of lossy graph ite-type microsphere inclusions randomly imbedded in a dielectric matrix, w ith a view towards assessing the suitability of such a composite for its us e as a radar absorbing material (RAM). This suitability is determined by th e material possessing a large effective absorptivity while at the same time not giving rise to an overly large reflectivity. In this vein we here eval uate, as a function of frequency up to 20 GHz, the effective magnitudes of Re epsilon and Im epsilon for particulate composites and their dependence o n the volume fraction phi of the particles, while independently varying con ductivities. Our calculations are carried out using both the effective-medi um theory (mixture theory, valid for small phi), and the multiple-scatterin g theory of Tsang and Kong valid for general phi but for small particle siz es. Multiple-scattering effects lead to increased effective absorptivities by adding scattering losses to the intrinsic losses of the media. We commen t on the optimal values of the medium parameters and packing fractions for composite RAM materials. Percolation effects (transition from matrix domina nce to particle dominance) are studied, both for the effective-medium theor y and for the multiple-scattering theory, and compared for the two cases. ( C) 1999 American Institute of Physics. [S0021-8979(99)08618-1].