Ultra-thin absorbers comprised by cascaded high-impedance and frequency selective surfaces

The theoretical basis for an ultra-thin broadband absorber is established which is comprised of a mushroom-type high impedance surface (HIS) and a pixelized frequency selective surface (FSS). The latter is engineered to exhibit a prescribed series RLC circuit response and it is placed at an electrically small distance above the HIS. Through a transmission line analysis, it is demonstrated that the admittances of the two structures cancel each other, resulting in an almost zero input reactance and a resistance that fluctuates around that of free space within the frequency range of interest. The resulting structure has a total thickness that does not exceed 2 mm while a - 10dB return loss is achieved for normal incidence from 9-13GHz.