A Wide Bandpass Elliptical Filtering Response Frequency-Selective Slab Based on Slotline- Backed Split-Ring Resonators

This article presents a wide bandpass elliptical filtering response 3-D frequency-selective slab (3D-FSSlab) based on slotline-backed split-ring resonators (SRRs). Field analysis is performed to determine the resonant frequency of slotline-backed SRRs at first. Physical dimensions are then mapped into an equivalent circuit model (ECM). Subsequently, the elimination of the transmission pole on the right side of the passband and the enhancement of transmission characteristics within the passband are achieved through the ECM analysis. Finally, to verify the proposed design method, single- and dual-polarization prototypes are modeled, simulated, fabricated, and measured. The measured results reveal that the proposed 3D-FSSlab achieves a fractional bandwidth (FBW) of 39.7% at 3.88 GHz, surpassing existing designs by a minimum of threefold with comparable angular stability and out-of-band suppression. It also provides excellent out-of-band cutoff characteristics with a rectangular coefficient of 1.17 and a suppression level of 10 dB. Furthermore, the design exhibits excellent angular stability, with an angular instability factor of 1.02 and a maximum insertion loss (IL) of 1.1 dB in the passband at an oblique incidence angle of 45°. The proposed 3D-FSSlab can be used in multiband antenna systems to enable space-division multiplexing.