In this paper, the systematic design for a dual-band balun featuring impedance transformation and high output isolation is presented. A dual-band balun capable of conducting transformation between an unbalanced 50-$\Omega $ line and a balanced one of 300-$\Omega $ is proposed for its operations at 2.4/5.2 GHz with high output isolation. The combination of a dual-band $\lambda $ /4 stepped-impedance-stub line and a dual-band $3\lambda $ /4 T-junction coupled line produces an output phase difference of 180° at the two operating frequencies for the balun. An important feature of the proposed design is the 1:6 impedance transformation from an unbalanced source to a balanced load. High isolation can also be achieved by introducing an isolation circuit between the output ports. The measured isolation is better than -26 dB. The phase imbalance is under 3°, and the magnitude imbalance is less than 0.9 dB within the bandwidths.
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