Ultra thin absorbers are studied in detail to provide a comprehensive model for their absorption mechanism. It is shown that the transmission line (TML) approach is not able to model the absorber frequency response correctly. It results in large errors when the thickness to wavelength (d/λ) ratio is below a certain level. It is explained that large amplitudes of high order Floquet modes and excitation of non-transverse component of the scattered field at the absorption frequency are the reasons for the TML model inaccuracy. It is illustrated that at small d/λ ratios, the structure becomes a localized lossy resonator which exhibits absorption even in a finite size array configuration. For a resistive squared patch periodic array, the resonant frequency can be estimated fairly accurately by the cavity model of a single perfect conductor patch antenna.
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Kazemzadeh et al. (2010) studied this question.
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