A high-performance ultra-narrow bandpass HTS filter and its application in a wind-profiler radar system

This paper reports a recent development of a narrowband high-temperature superconducting (HTS) bandpass filter with a fractional bandwidth of 0.35% in the L band. The filter exhibits a 10-pole quasi-elliptic function response implemented with a cascaded quadruplet coupling structure. The measured filter shows a midband insertion loss of 0.26 dB and a return loss better than -15 dB over the passband. Steep rejection slopes are obtained at the band edges and the measured skirt slope has exceeded 120 dB MHz^-1, resulting in an excellent out-of-band rejection very close to the band-edge. Combining this filter together with a special designed low-noise amplifier (LNA) as well as a cryo-cooler, an HTS subsystem was then constructed and mounted in the front end of a wind-profiler radar system, substituting the corresponding conventional parts. A field trial has shown that the HTS filter could markedly improve the sensitivity as well as the anti-interference capability of a wind-profiler radar system.