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A high-capacitive frequency selective surface (FSS) with a new structure of folded spiral conductors is proposed as the small-array periodicity and low-frequency resonance FSS for ultra-wide bandwidth absorbers in a multilayer structure. Due to the folded structure with long effective segments and a small gap, a large value of capacitance for the lowest resonating frequency is obtained. Through a combination of the high-capacitive spiral FSS with other conventional FSSs (square loop, square patch) with a medium- and high-frequency resonance, an ultra-wide absorption bandwidth (4.7–50.0 GHz for −10 dB reflection loss) is designed with a small total thickness of 7.0 mm, which is close to the theoretical limit (6.7 mm). Admittance analysis is conducted for better insight into the optimization procedure. The free space measurement with a test sample prepared by the screen printing method also demonstrates a wide-bandwidth absorption result (5.2–44.0 GHz for −10 dB reflection loss, total thickness = 6.5 mm), which is in good agreement with the simulation result. In addition, the angular stability of the proposed wide-bandwidth absorber is discussed for both TE and TM polarizations in association with unit cell periodicity and grating lobes.

Ultrawide Bandwidth Electromagnetic Wave Absorbers Using a High-capacitive Folded Spiral Frequency Selective Surface in a Multilayer Structure