A Compact Wide Stopband HTS Filter With Ground Surrounded Quasi-Interdigital Structures

A ground surrounded quasi-interdigital structure (GSQIS) is proposed in this letter. The GSQIS, consisted of several fingers stretching from one end of the microstrip line and the ground, can be conveniently loaded on a quarter-wavelength microstrip resonator. The resonator loaded with the GSQIS can obtain a large ratio of the first spurious frequency fs to the fundamental resonant frequency f0, which is suitable for the design of filters with wide stopbands and high out-of-band rejections. Moreover, because the GSQISs are buried inside the ground, couplings between resonators with or without GSQISs are similar. A compact four-pole high temperature superconducting (HTS) filter at 143 MHz is designed and fabricated using the proposed resonators. The measured 80 dB rejection stop-band of the filter is up to 1040 MHz, which is 7.27 times of the fundamental frequency. The size of the filter layout is 27 mm × 14 mm or 0.033 λg ×0.017 λg, where λg is the guided wavelength at 143 MHz.