Compact Ultra-Narrowband Superconducting Bandpass Filter Using Symmetrical Spiral-in-Spiral-Out Resonators with Secondary Coupling Structure

This article introduces a new symmetrical spiral-in-spiral-out (SISO) resonator applied to the design of compact ultra-narrowband (UNB) high-temperature superconducting (HTS) bandpass filter (BPF). Compared with the traditional SISO resonator, the proposed symmetrical structure features a weaker coupling strength, which contributes to the miniaturization of the filter. Additionally, the integration of an h-type secondary coupling structure can effectively decrease the coupling strength, thereby reducing the spacing between coupled resonators. Finally, a fourth-order HTS UNB BPF for radio astronomy applications is designed, featuring a center frequency of 1952 MHz and the equal-ripple bandwidth of 0.25%.