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In this brief, a compact four-pole high-temperature superconducting (HTS) narrowband bandpass filter (BPF) based on spiral dual-composite right/left-handed (spiral D-CRLH) resonators is proposed. By incorporating the spiral form into the D-CRLH structure, a greatly miniaturized resonator size and weakened coupling strength between adjacent resonators are analytically illustrated and applied to narrowband circuit design at the VHF band. By virtue of the coding coupling technique, a compact four-pole HTS narrowband BPF prototype was theoretically designed and fabricated on a 0.5mm-thick MgO substrate with double-sided YBa2Cu3O7 films. The BPF centering at 218.5 MHz has a fractional bandwidth of 0.5% with four transmission zeros located outside the passband. The dimensional size of the BPF is 22mm $\times 16$ mm $\times 0.5$ mm. The measured results demonstrate the maximum in-band insertion loss of 0.3 dB and the return loss of better than 15.2 dB, which are in good accordance with the simulated results.

Adopting multimode hairpin ring resonator (HRR) and multiarm defected ground slot (DGS), a new class of dual-band common-mode filter with flexible frequency ratio and bandwidth is proposed in this article. First, the working theory of multimode short-ended HRR is introduced and its multidegree of design freedom is analyzed by changing the physical dimensions. It is noted that the frequency ratio of the second resonant mode to the fundamental resonant mode can be adjusted flexibly, i.e., less than 1.5 or greater than 3. Second, a novel multiarm DGS structure is designed and applied to feed differentially multimode HRR for the desired common-mode noise suppression performance. Third, a pair of microstrip stepped-impedance feeding lines loaded with the multiarm DGS are presented to meet the flexible bandwidth of common-mode filter design. Following these principles, three dual-band bandpass common-mode filters are implemented and fabricated with the flexible frequency ratios of 1.36/2.34/3.39, respectively. The common-mode suppression, which is below 30 dB, is from 1 to 9 GHz for electromagnetic interference rejection. Finally, measured results and simulated results are given to verify the flexibility of the proposed structure and design approach.