Abstract: A microwave balun using microstrip broadside coupled lines is disclosed for achieving a broadband double balanced mixer or modulator performance. The use of a double layered structure for broadside coupled microstrip lines, called overlapped microstrip lines, makes a balun design equivalent to well known double coaxial Marchand design. It is feasible by means of thin film and MMICs technology. The most advantageous applications of the balun include its simple and inverted options useful for mixer or modulator design considering both the balun and semiconductor junctions manufactured in one compact volume of the lower substrate. A balun circuit layout is one-sided fixed to a contiguous ground plane without using slots or holes in it and othersided open from a top side. The application is dedicated to an approximate frequency range of at least 1 to 18 GHz.