A microwave or millimeter-wave balun is provided. The balun uses three edge-coupled lines along with a plurality of air bridges instead of two edge-coupled lines used in a conventional planar Marchand balun. The first edge-coupled line and the second edge-coupled line are substantially parallel, and the third edge-coupled line is disposed also substantially in parallel between the first edge-coupled line and the second edge-coupled line. The plurality of air bridges are transmission lines between the first edge-coupled line and the second edge-coupled line. The air bridges have total width longer than one half of the total length of the first edge-coupled line or the second edge-coupled line. By combining three edge-coupled-lines and a plurality of air bridges, the Marchand balun has a higher coupling coefficient and increases the operation bandwidth. The microwave monolithic integrated circuit (MMIC) mixer based on the balun can provide compact size compared to conventional ones.