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A highly-selective, closely-spaced, tri-band bandpass threedimensional frequency selective surface (3D FSS) is proposed based on the topology of the microwave filter in this paper. The unit cell of the proposed 3D FSS is constructed by combining an air-filled square waveguide and a cuboid dielectric resonator. Both top and bottom layers of the dielectric resonator consist of double square loops, while the middle layer contains an embedded square loop. Due to its inner electromagnetic coupling in the unit cell, multiple transmission zeros/poles are introduced for obtaining high frequency selectivity, two second-order passbands, and wide out-of-band rejection. Concentric square loops with very close sizes are designed to realize small band ratios. In order to understand the working principle of the proposed 3D FSS, the surface current distributions at the frequencies of transmission poles are analyzed. Finally, an FSS prototype is fabricated and measured. The measurement results show a stable response under the incident angles up to 60◦ for both TE and TM polarizations is realized. Besides, the proposed 3D FSS has a relatively compact unit cell size.

Highly-selective, closely-spaced, tri-band bandpass three-dimensional frequency selective surface