A wideband high‐temperature superconducting (HTS) duplexer is designed and is characteristic of more than 50% fraction bandwidth in its two channels. The duplexer is composed of a linear half‐wavelength resonator and two channels. Each channel is composed of five dual‐mode resonators with the attenuated second harmonic. To improve the isolation of the duplexer, we loaded a quarter‐wavelength resonator at the output port of Channel 2; besides, a 5‐pole dual‐hairpin lowpass filter is cascaded at the output port of Channel 1 to improve its out‐of‐band rejection. The duplexer was fabricated on a MgO substrate coated with YBCO thin film on its double sides, and the area of the duplexer is 44.42 × 8.00 mm. The measured results showed that the 1 dB bandwidth of the channels is 2.679–4.645 and 5.083–9.014 GHz, respectively; the corresponding fractional bandwidths are 55.7% and 58.1%. The rectangularity of 60/3 dB is 1.51 and 1.11, respectively. The isolation between two channels is larger than 70 dB. The out‐of‐band rejection in the frequency range lower than 15 GHz is larger than 36 dB. The return loss is 10.6 and 6.0 dB and will be improved by subsequent tuning.
Li et al. (Thu,) studied this question.