Abstract:A novel coupledline balun structure is proposed and investigated in this paper. The balun has a simple structure which only consists of a pair of coupled line and a single transmission line. In addition, it can be realized on a single layer PCB without any via. The analytic design formulas of the proposed balun are given and the impedance range of the ideal circuit under different port impedance ratios is analyzed. The design parameters are very flexible, and the operating bandwidth of the balun can be adjusted by changing the impedance value of the transmission line. Moreover, the design equations indicate that the proposed balun structure is very suitable for designing the balun with large port impedance ratio. A prototype balun that operates at 1.5 GHz is simulated, fabricated and measured. The test results are in good agreement with the ideal circuit and the threedimensional simulation curve.