A harmonic-suppressed miniaturized-element frequency selective surface with a second-order bandpass response

In this paper, we introduce a new method for designing miniaturized-element frequency selective surfaces (MEFSSs) with higher-order bandpass responses, which are free of spurious transmission windows over extremely large bandwidths. The proposed harmonic-free MEFSS is composed of multiple metallic layers in the form of sub-wavelength periodic structures consisting of non-resonant unit cells separated from one another by thin dielectric substrates. Harmonic-free operation in these structures is achieved by using multiple closely-spaced capacitive layers with overlapping unit cells to synthesize a single effective capacitive layer, which leads to a larger capacitance value for a given unit cells size. This allows for shifting the natural resonant frequencies of its constituting elements to considerably higher frequencies and achieving a harmonic-free response over extremely wide bandwidths. Using the proposed method an MEFSS prototype having a second order bandpass response with a fractional bandwidth of 20% at 3 GHz, is designed, fabricated, and its harmonic-free operation in wide field of view (±60°) for both TE and TM polarizations is experimentally verified.