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This paper presents a superconducting filter with a bandwidth of 0.5 MHz at 40 MHz on a LaAlO3 substrate. It has a compact size of 40 mm × 19 mm, which amounts to only 0.02 λg0 × 0.01 λg0, where λg0 is the guided wavelength of the 50-Ω line on the substrate at the midband frequency. The effect of the resonator structure on the unwound length and, hence, the size of the resonator is analyzed. A spiral resonator with positive mutual inductances between adjacent microstrip turns can greatly reduce the resonator's unwound length and is used to miniaturize the filter size. A feedline structure spiraled inside the spiral resonator is proposed to realize a strong external coupling. Furthermore, a cross-coupling line is added to the six-pole filter to improve the band-edge steepness. The filter is successfully designed and fabricated. The measured results demonstrate a 0.3-dB insertion loss, a -19.2 dB return loss, an 80-dB out-of-band rejection, and steep band edges, which are in good agreement with electromagnetic simulations.