I. Introduction
Microwave filters using high-temperature superconducting (HTS) materials are suitable for applications that require high sensitivity, such as radio astronomy and radar receivers. The low surface resistance of HTS films guarantee the low loss characteristic in multi-pole filters [1]. Many reported HTS filters work at the VHF or UHF bands [2]–[7], whereas only a few HTS filters at the HF band can be found in the literature [8]–[10]. In low frequency applications, filter miniaturization becomes a severe issue, because the HTS film size is limited to two or three inches. Lumped elements are used to implement compact filters [4] at a few hundreds of MHz. However, it is difficult to extend lumped-element HTS filters to lower frequencies, because large capacitors such as parallel plate capacitors are unavailable in planar HTS films [11]. In [8], a three-pole lumped-element 15 MHz filter is demonstrated, using a whole two-inch HTS wafer. Distributed-element resonators in filters are often winded into spirals [2], [3], [5]–[7], [9] or meander lines [10] to achieve compact sizes. In [5] the relative sizes of different resonators are compared. The single spiral resonator provides the most compact size, followed by the spiral-in-spiral-out (SISO) resonator. To further reduce circuit size, chip capacitors can be loaded to the resonators. In [10], a 50 pF chip capacitor is used to bring down the resonant frequency from 26.1 MHz to 18.5 MHz.