High-Impedance Surfaces (HIS) comprising lossy Frequency Selective Surfaces(FSS) are employed to design thin electromagnetic absorbers. The structure,despite its typical resonant behavior, is able to perform a very widebandabsorption in a reduced thickness. Losses in the frequency selective surfaceare introduced by printing the periodic pattern through resistive inks andhence avoiding the typical soldering of a large number of lumped resistors. Theeffect of the surface resistance of the FSS and dielectric substratecharacteristics on the input impedance of the absorber is discussed by means ofa circuital model. It is shown that the optimum value of surface resistance isaffected both by substrate parameters (thickness and permittivity) and by FSSelement shape. The equivalent circuit model is then used to introduce theworking principles of the narrowband and the wideband absorbing structure andto derive the best-suited element for wideband absorption. Finally, theexperimental validation of the presented structures is presented.
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