A Novel HTS Ultra-Narrowband Bandpass Filter Using Hairpin Meander-Line Resonator With Gradient Line-Width

A six-pole high-temperature superconducting ultra-narrowband bandpass filter (BPF) using microstrip hairpin meander-line resonators is proposed in this article. The hairpin resonators are meandered first to enclose the whole electromagnetic field in the structure, aiming to greatly reduce the coupling between two adjacent resonators. To further decrease the coupling, the line-width of the meander-line resonators are gradually increased from the inner turn to the outside turn. An ultra-narrowband BPF with center frequency of 845.1 MHz and the equal-ripple bandwidth of 0.17% is designed, fabricated, and measured. Measured minimum in-band insertion loss is 0.55 dB and the measured in-band return loss is better than 14.5 dB. The measured results are in good agreement with the simulated ones.