Design of a Quad-Band Branch Line Balun Using Extended Pi-Shaped Coupled Lines

In this letter, a quad-band balun employing Pi-shaped and series connected coupled lines is proposed. The closed form design equations are derived analytically. Based on these equations, a quad-band balun operating at 1.8/3.6/5.8/7.8 GHz is designed, fabricated and tested. The measurement result shows that, the return loss and isolation are better than 18 dB and the bandwidth obtained is more than 150 MHz at each operating frequency band, which is feasible for most applications. Good agreement is found between the theoretical predictions and experimental results.