Compact HTS Narrowband Bandpass Filter Based on Spiral D-CRLH Resonator

Abstract

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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 16$ </tex-math></inline-formula> mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 0.5$ </tex-math></inline-formula> 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.