Literal Solutions for Optimal Design of Microwave Absorbing Composites

This paper proposes a new approach for the optimal design of microwave absorbing composites backed by a PEC layer and characterized by frequency dependent permeability and permittivity. The reflection coefficient is expressed as a function of the difference between the absorber hyperbolic input impedance and the free space wave impedance, considering an impinging plane wave with either normal or oblique incidence angle. The impedance matching condition for the selected frequency and incidence angle is solved in terms of literal expression of the optimal thickness, which is compared with the one quarter wavelength thickness given by a new accurate expression. The optimal thickness is used for the design of microwave absorbers made by dielectric-magnetic and dielectric composite materials. The frequency spectra of the absorber input impedances and reflection coefficients are computed in the frequency range from 2 GHz up to 18 GHz and for an incidence angle ranging between 0° and 30°. The obtained results prove the validity of the literal expression of the composite optimal thickness which explicitly shows the crucial parameters for the absorbing performances, obscured in common numerical procedures.