Design of single- and dual-band BPFs using folded 0° feed structures and embedded resonators

In this paper, a novel symmetric primary structure including two coupled stepped-impedance main-resonators is presented. The primary structure is divided into three different bandpass filters (BPFs) of equal size such as two derived single-band BPFs operating at 0.9 and 1.8 GHz frequencies, respectively, and the third one is a dual-band BPF operating at 0.9/1.8 GHz passbands concurrently. To validate the proposed design methodology, three prototype BPFs have been designed, fabricated and measured successfully using folded main-resonators with 0° feed structures to create the 0.9 GHz passband. Then by embedding two pairs of double-spiral open-loop sub-resonators into the main-resonators with the cross-coupled arrangement, the tuneable 1.8 GHz passband is created. Also, by inserting extra pairs of sub-resonators, the 1.8 GHz BPF can be cascaded to achieve the selective higher-order modes. The dual-band BPF has been optimized not only to reduce the band-ripples and remove the spurious frequencies but also to miniaturize in overall size. The advantages of measured insertion losses, less than 0.9 dB in the single-band BPFs, 0.7 dB in the dual-band BPF with a good return loss and improved band selectivity are achieved with a high flexibility for changing the primary structure into some exclusive BPFs.