A compact planar microstripline balun with suppressed harmonic response

In this paper, shunt open-stubs are used to design reduced size balun with suppressed harmonics. The bandstop response of shunt stubs has been utilized to suppress the harmonics. Design guidelines are presented with design equations and graphs to ease the design process. Transmission line model analysis is carried out to investigate in-band and out-of-band performances of the balun. Experimental results of the designed balun agree well with the theoretical predictions. A prototype balun, with up to sixth harmonic of the operating frequency suppressed along with 75% size reduction, compared to the conventional balun, is implemented in microstrip form.