Quarter-Mode Dielectric-Filled Resonators Based on Virtual Magnetic Walls

A compact quarter-mode (QM) dielectric-filled resonator (DR) is proposed, along with a design methodology employing virtual magnetic walls (VMWs). The operating principle of the QM DR is discussed, focusing on the realization of VMW through an open aperture (OA), which eliminates the aspect ratio limitation. The implementation of VMWs is analyzed using transmission line theory. Compared to conventional full-mode (FM) resonators, the proposed QM DR achieves a 67% reduction in size and weight. This compact design makes the QM DR well-suited for the development of compact bandpass filters (BPFs), offering enhanced out-of-band selectivity. To validate the proposed approach, a fourth-order QM DR BPF was designed, fabricated, and experimentally tested. The experimental results confirm the improved out-of-band selectivity, demonstrating the effectiveness of the proposed design.