This paper presents a class of ultra-thin metamaterial absorbers, which consists of periodic microstrip lines on top of a planar lossy substrate backed by a conducting metallic plate. A highly efficient full-wave analysis method was developed to solve the electromagnetic response of the absorbers. The in uence of electromagnetic properties of the substrate and physical dimensions of the microstrip lines were analyzed. Genetic algorithm was used to optimize the absorption bandwidth of the absorbers. Effective permeability and permittivity of the absorbers were retrieved to shed a new light on the absorption mechanism of the absorbers and to explain their ultimate bandwidth limit. It was found that the ultimate bandwidth limit of the metamaterial absorbers is the same as that of normal absorbers.
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