Miniaturized Dual-Band Bandpass Filter With Sharp Roll-Off Using Ring-Loaded Resonator

In this paper, a miniaturized dual-band bandpass filter (BPF) with good selectivity based on ring-loaded resonator (RLR) and 0° feed structure is proposed. First, the RLR is investigated theoretically and numerically, and a miniaturized Filter I is designed based on the RLR and filter synthesis approach. Secondly, based on the coupling structure of the previous BPF and theory of 0° feed structure, the sub Filter II is formed. By appropriately adjusting the location of the I/O ports, the dual-band filter can be achieved simply. The proposed dual-band BPF provides compact size and satisfactory out-of-band rejection between each band with multiple transmission zeros (TZs) introduced by source/load coupling and a special coupling scheme.