A first transmission line is formed on a first surface of a substrate. A first end of the first transmission line is connected to a transmission line of an unbalanced line. A second end is connected to a first transmission line of a balanced line. A second transmission line extends from a first end of the unbalanced line, alongside the first transmission line and spaced therefrom, and is connected to a second of the transmission lines of the balanced line. A ground plane is formed on a second surface of the substrate, extends from a first end of the balanced line, and is formed along the second transmission line to which the ground plane is connected through one or more vias. A width of a portion of the ground plane adjacent to the unbalanced line is greater than a portion thereof adjacent to the balanced line.