Development of UWB HTS Bandpass Filters With Microstrip Stubs-Loaded Three-Mode Resonator

We developed ultra-wideband high-temperature superconducting bandpass filters for Japan's low-band spectrum. First, we designed a low-band filter with a microstrip three-mode resonator loaded with two open stubs. The three-mode resonator is characterized by the ease of controlling the frequency response compared with other resonators. In addition, we loaded a low-pass filter consisting of a microstrip coupled-line hairpin unit to suppress all unwanted high-frequency harmonics. The simulated filtering characteristics of the low-band filter showed a wide passband within 3.44-4.67 GHz with a minimum insertion loss of 0.06 dB. After that, we fabricated a low-band filter with YBCO thin films deposited on sapphire substrates. The frequency response of the fabricated filter was clearly observed at 70 K, and the measured response without any tuning matched well with the simulated one. The measured filtering characteristics showed a wide passband within 3.46-4.80 GHz with a minimum insertion loss of 0.01 dB. The box resonances of the copper housing were suppressed by introducing radar absorbent material under the top metal.