Compact Triple-Band High-Temperature Superconducting Filter Using Multimode Stub-Loaded Resonator for ISM, WiMAX, and WLAN Applications

A compact triple-band high-temperature superconducting (HTS) bandpass filter using multimode stub-loaded resonator is proposed in this paper. The resonant characteristics of the multimode resonator are investigated by the even/odd-mode analysis. To sharpen the passband skirts, a source-load coupling configuration with a nonresonant node is arranged to produce several extra transmission zeros for realizing high-selectivity performances. For demonstration purposes, a triple-band filter operating at 2.45 GHz (ISM band), 3.5 GHz (WiMAX band), and 5.2 GHz (WLAN band) is designed, fabricated, and measured. Measurements are in good agreement with the electromagnetic simulations. The measured minimum insertion losses of the triple-band HTS filter are 0.16, 0.55, and 0.22 dB, whereas the return losses are greater than 16.7, 10.2, and 17.5 dB, respectively. Its overall size is only 8.3 mm $\times$ 8.6 mm.