A 12-Pole K-Band Wideband High-Temperature Superconducting Microstrip Filter

A 12-pole wideband high-temperature superconducting (HTS) microstrip bandpass filter is reported in this paper. The filter has a center frequency of 23 GHz and a bandwidth of 4.2 GHz. The filter was fabricated on a 2-in-diameter 0.25-mm-thick MgO wafer with double-sided YBCO films. Both edge and end coupling structures were employed in this paper to achieve the desired bandwidth. An insert coupling structure was used for input/output coupling to realize the remarkably strong coupling. A microwave-absorbing material was used to eliminate cavity resonances. The measured results show a midband insertion loss of 1.3 dB, a return loss better than 9.5 dB and an out-of-band rejection of over 55 dB on both sides of the passband.