A Wideband HTS Planar Microwave Balun

This paper presents a high-temperature superconducting (HTS) planar balun. The balun exhibits an excellent amplitude and phase balance performance. The balun was fabricated using double-sided HTS YBCO films on a 0.5-mm-thick 3-in LaAlO₃ substrate. The balun demonstrates a wideband operation and a low insertion loss. The center frequency of the balun is 1.25 GHz. The return loss bandwidth of 18 dB is 1.02 GHz (the fractional bandwidth is 82%). The phase imbalance is less than 7. The amplitude imbalance is less than 0.7 dB, and the insertion loss is less than 0.31 dB over 0.69-1.77 GHz.