A wideband high selectivity filtering balun

This letter presents a wideband planar multiple-functional balun which simultaneously has balun and filtering response. By utilizing the geometrical symmetry character of substrate integrated waveguide bandpass filter, a wideband balun is developed with high selectivity. Multiple slots are etched on the bottom and top layers to improve the isolation performance of the proposed balun. As demonstrations, an X-band filtering balun is designed and verified by experiments. Good agreements are obtained between simulated and measured results, which show that the measured filtering balun has an operation bandwidth from 8.7 to 11.7 GHz, that is, around 30.27% fractional bandwidth, a maximum 5.6 degree output phase imbalance, and a small insertion loss of 4.1 dB at center frequency of 10.2 GHz. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1107–1110, 2015