A VHF band HTS filter based on modified single-spiral resonators for radio astronomy application

A 10-pole quasi-elliptic bandpass filter (BPF) with a relatively wide passband in the very high frequency (VHF) band was designed and fabricated using YBCO thin films deposited on both sides of a LaAlO3 substrate. Single-spiral resonators were modified to generate strong coupling and reduce the parasitic coupling. We analysed the coupling polarities between the modified resonators. Two pairs of attenuation poles were introduced in the filter for sharp cut-off response. The measurements showed that the filter has a centre frequency of 255.8 MHz, fractional bandwidth of about 12.3%, insertion loss of smaller than 0.265 dB, and the return loss in the passband of better than 17.5 dB, in good agreement with computer simulations.