An Ultrathin and Ultrawideband Metamaterial Absorber and an Equivalent-Circuit Parameter Retrieval Method

In this article, we propose an ultrathin and ultrawideband metamaterial (MTM) absorber based on periodically arranged metallic square spirals. The design, characterization, and measurement of the proposal are presented as its equivalent circuit. The lumped elements of the equivalent circuit are extracted using a proposed algorithm based on the least-square method, which presents a straightforward and promising approach and shows a good matching with the electromagnetic simulation. The unit cell of the proposed structure has the square spiral mounted on an FR-4 substrate in front of a conductive plate. The simulated results show an absorptivity of more than 90% from 11.4 to 20.0 GHz, covering the $Ku$ -band for transverse magnetic (TM) and transverse electric (TE) polarizations, and this broadband feature is confirmed by the experimental measurement. Furthermore, the proposed MTM absorber is $\lambda /16.4$ in thickness at the lowest frequency of absorption. The proposed MTM has proper response under oblique incidence from 0° to 50° and shows great performance with regard to the absorbers previously presented in the literature.