An ultra-narrow band eight-pole dual-mode HTS patch filter

An ultra-narrow band eight-pole dual-mode HTS patch filter was designed and fabricated. The filter is constructed by four corner-cut square patches, each of which has two resonating modes. The influence of the change of the corner-cut position on the filter response was investigated by simulations. Four small rectangle patches were added in the topology of the filter as tuning tools. By employing this unique tuning structure, the simulated filter responses and current density distributions on the patches have been greatly improved. The filter was fabricated using double sided YBCO films on a 3 in. LaAlO3 substrate. The measured results showed satisfactory performance with center frequency 2.494 GHz, fractional bandwidth 0.2%, return loss in passband better than 16 dB, and out-of-band rejection of the filter better than 90 dB at 15 MHz away from its center frequency.