Easy Design of Ultra-Wideband Balun Using Double-Sided Parallel-Strip Lines with Inverted Outputs

A simple ultra-wideband balun operating at 2.1-9.25GHz frequency band is proposed based on double-sided parallel-strip lines (DSPSLs). The proposed DSPSL balun shares the similar synthetical design procedure with conventional power dividers, which makes the design of the ultra-wide impedance band easy. Furthermore, by inverting the output ports of the DSPSLs, the inherent quasi-opposite phase and quasi-balanced amplitude responses are realized for balun applications. Therefore, an ultra-wideband balun with excellent amplitude and phase balance is proposed. Measurements show that the proposed balun has an ultra-wide impedance bandwidth (
$\mathrm{S}_{11} < - 10\ \text{dB}$
) from 2.1 to 9.25 GHz while the amplitude imbalance and phase imbalance are within 0.5 dB and 6.8° over the operating frequency band, respectively.