Hybrid Capacitive and Inductive Coupling Paths for Implementing Quasi-Elliptical High-Order Bandpass FSS With High Out-of-Band Suppression
Hao Jiang, Shaowei Liao, Quan Xue
Abstract
This work proposes a novel filter circuit model to design the high-order bandpass frequency selective surfaces (FSSs) with quasi-elliptical filtering response and high out-of-band suppression. First, the improved filter circuit, consisting of three lumped parameter resonators and stepped impedance, is established by integrating capacitive and inductive coupling second-order circuits. Second, the odd- and even-mode analysis method demonstrates that the filter circuit can excite four transmission poles (TPs). Since the hybrid coupling paths can produce other series resonance, the filter circuit can exhibit four transmission zeros (TZs) in the out-of-band approaching the existing TPs, resulting in a quasi-elliptic filtering response and high out-of-band suppression. Third, two dual-polarized FSSs based on the corresponding circuit models have been designed using a compact hexagonal composite structure. The final simulated FSS can provide a broad −1 dB passband within 10.9–12.5 GHz (13.7%), the out-of-band suppressions at the lower and upper sides of −26.9 and −36 dB, respectively, and an excellent roll-off rate in dual sidebands. Finally, one FSS prototype is fabricated and experimentally characterized for further confirmation. Experimental results agree with simulated ones, indicating the device exhibits significant features, for example, the wideband transmission, excellent out-of-band suppression, and quasi-elliptic filtering response that remains stable at incident angles ranging from 0° to 60°.