Low-Profile Higher-Order Narrowband Bandpass Miniaturized-Element Frequency-Selective Surface

We present a low-profile miniaturized-element frequency-selective surface (MEFSS) with narrowband, second-order bandpass response. The miniaturized-element unit cell comprises a capacitively loaded parallel LC unit cell resulting in a narrowband bandpass response. A higher-order filtering response is obtained by implementing five metallic layers FSS formed by two layers of parallel LC resonators with an inductive impedance inverter layer in between them. The proposed design offers a very thin profile and an insensitive frequency response to the polarization of the incident electromagnetic wave. A design procedure is developed for the proposed FSS structure based on an equivalent circuit model analysis. Using this model, a prototype of MEFSS with the second-order bandpass response at 3.34 GHz and a fractional bandwidth of 3.7% is designed, realized, and characterized experimentally. The measurement results of the FSS verify the effectiveness of the FSS design procedure and the stability of its frequency response up to 60° oblique incidence angles.