Multiband electromagnetic wave absorber based on reactive impedance ground planes

A novel structure able to perform multi-band absorption is presented. As is well known, the most famous and simplest radar absorbing material is the Salisbury screen. Improved solutions, aimed at a thickness reduction, have been presented in the literature, conceptually based on metamaterials and employing properly designed high-impedance surfaces (HIS) in practice. The absorber presented here combines the working principles of the previously mentioned designs in order to join the two types of absorber into a single structure. The novel design comprises a single resistive sheet mounted at a fixed distance from a reactive surface. The latter is a composite layer of a frequency selective surface printed on a grounded dielectric slab acting as an artificial magnetic conductor at lower frequencies, while responding as a perfect electric conductor at higher frequencies. The distance of the resistive sheet can therefore be dimensioned so as to recover the Salisbury functions in the upper band, while operating as a λ/10 absorber in the low-frequency regime. The structure is explained by means of a simple circuital model and measured data are reported to verify the proposed concepts. It is finally shown that low-frequency resonance can be advantageously adjusted by introducing some active elements in the HIS.