Abstract: The broadband transmission-line transformer with 4∶1 impedance transformation ratio is studied, its equivalent model with mutual coupling between transmission-lines is given, and its impedance transformation characteristics are analyzed. Then the permeability change with frequency neglected in conventional equivalent model is taken into account, and the property of the NiZn broadband transmission-line transformer is analyzed as an example. Result comparison between experiment and equivalent model analyses shows that the permeability change with frequency does have effect on analysis results, thus should be considered．