Effective-medium approach to the microwave properties of the high-T_c superconductor-insulator composites

We study microwave transmission through composites consisting of small YBaCuO (BiSrCaCuO) grains embedded in a polymer host. For these composites we evaluate: (a) the effective dielectric constant ζ, (b) the effective magnetic permeability μ, and (c) the losses. We demonstrate that ζ almost does not change upon transition to the superconducting state while μ changes considerably. The dependence of μ on the volume fraction of superconductor is well described by the Clausius-Mossotti approximation.