Astronomers at the Mizusawa Very Long Baseline Interferometry Observatory of the National Astronomical Observatory of Japan are promoting the development of broadband receivers in the ultra-high frequency (UHF) band. The presence of extensive radio frequency interferences (RFI) in the UHF band hinders continuous broadband radio observations in this band. A promising technical solution to this problem is to use a superconducting multi-band bandpass filter as it enables the bundling of several RFI-free bands. We have developed a high-temperature superconducting compact quad-band bandpass filter (QB-BPF) at 0.7, 1.4, 1.6, and 2.1 GHz with low loss and steep cutoff characteristics for broadband receivers. That consists of two six-pole dual-band bandpass filters. We have devised an interdigital feeding structure with a high degree of freedom that enables the external quality factor to be designed individually for each band. A compact QB-BPF was fabricated using double-sided YBa2Cu3Oy thin films on a 36.4 mm × 42.0 mm (0.22λg × 0.25λg) Al2O3 substrate. The measurement results agreed well with the simulated ones, indicating that the QB-BPF overcomes the problem of extensive RFI in the UHF band.

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