Multiband High-Order Bandstop 3-D Frequency-Selective Structures

A general methodology for the design of multiband high-order three-dimensional (3-D) frequency-selective structures (FSSs) is proposed. The methodology is based on employing multiple resonators inside a unit cell (UC) to obtain a multiband response, and by controlling the coupling between identical resonators, a high-order filtering performance is achieved. This provides more transmission zeros in the operating bands, which lead to improved bandwidth and high selectivity. Detailed analysis for the coupling between identical resonators is provided as well as equivalent circuit models to explain the operating principle. Three structures are designed, fabricated, and measured to verify the proposed methodology. The first structure is a triband secondorder FSS with center frequencies at 9.6, 13.4, and 17.5 GHz and their -3-dB fractional bandwidths of 13.5%, 8.5%, and 12.6%, respectively. The second structure is designed to have a dualband third-order response with center frequencies at 13.2 and 19.6 GHz and their -3-dB fractional bandwidths of 16% and 13.3%, respectively. The last structure is designed to exhibit a dual-band second-order response under both TE and TM polarizations with center frequencies at 11.2 and 13.8 GHz and -3-dB fractional bandwidths of 5.5% and 4%, respectively.