Ultra-thin wideband absorbers comprised of Frequency Selective Surfaces with concentric square loop elements

An ultra-thin wideband Frequency Selective Surface (FSS) type absorber consisting of a double square loop with a center patch is proposed. The admittance of the FSS is engineered so that it exhibits a capacitive response within the frequency range of interest. The latter is below the substrate's λ/4 resonance, where the substrate is characterized by an inductive admittance. The two admittances cancel each other resulting in an almost zero input reactance. A -10dB return loss is achieved for normal incidence from 12-27 GHz.