Wide bandwidth pyramidal absorbers of granular ferrite and carbonyl iron powders

Pyramidal absorbers with wide bandwidth in the GHz region are designed with magnetic materials of broad loss spectrum for application to small or semi-anechoic chambers. Microwave absorbing properties of pyramidal cone absorbers are analyzed by the transmission line approximation which provides the reflection loss as a function of material parameters and absorber height. It has been found that the magnetic loss spectrum is the most critical parameter to realize wide bandwidth. For the attainment of a broad loss spectrum, two kinds of magnetic fillers are used in appropriate proportions in silicon rubber binder. One is Ni-Zn ferrite with magnetic resonance at low frequency and the other is carbonyl iron with high-frequency relaxation loss. With a controlled geometry of pyramidal cone in those composites, a very wide bandwidth (1-18 GHz with respect to -20 dB reflection loss) has been predicted.