The thickness dependence of resonance frequency in anisotropic composites with long conductive fibers

The resonant frequency of long conductive fibers embedded in an anisotropic composite as a function of layer thickness is studied both numerically and experimentally and is discussed in terms of the equivalent permittivity of the layer. A simple empirical exponential law is suggested to fit the thickness dependence of the equivalent permittivity. The law involves a critical thickness value, below which the layer can no longer be treated as a bulk material for a fiber of given length and thickness. The critical thickness is retrieved from numerical data for different permittivity values of isotropic and anisotropic layers and for different fiber length.