In this paper, a novel design method for dual-band baluns operating with a high frequency ratio between the two bands is presented. The circuit is designed based on a dual-band cross-shaped impedance transformation circuit combined with a dual-band Pi-shaped phase shifter. The investigation results show that the proposed balun structure achieves a frequency ratio f2/f1 of up to 4.9, which exceeds the typical limits reported for most previously published dual-band balun circuits. The enhanced frequency ratio between the two bands enables the circuit to be applicable in wideband applications. To evaluate the effectiveness of the proposed design method, a prototype of dual-band balun operating at two bands f1=0.7 GHz and f2=2.3 GHz was designed, fabricated and measured. The measured results demonstrate that the proposed dual-band balun not only achieves a significantly large frequency ratio between the two bands but also ensures good performance in key parameters, particularly exhibiting a substantial improvement in amplitude imbalances. At the operating frequencies, the balun exhibits an insertion loss of less than 0.9 dB, isolation better than 23.4 dB, amplitude imbalance less than 0.19 dB, and phase difference of 180 ± 4.4 degrees
Hung et al. (Wed,) studied this question.