A Broadband Planar Balun Using Aperture-Coupled Microstrip-to-SIW Transition

Based on low-temperature cofired ceramic (LTCC) technology, a broadband 60-GHz planar balun using longitudinal aperture-coupled microstrip-to-substrate-integrated-waveguide (SIW) transition is proposed. The proposed balun consists of a 3-dB SIW power divider and three longitudinal aperture-coupled microstrip-to-SIW transitions. To broaden the bandwidth of the microstrip-to-SIW transition, a matching post and a fork-type structure is employed in this work. With the proposed microstrip-to-SIW transition, it is very convenient to achieve a frequency-independent out-of-phase or in-phase property, and it does not require a tight coupling factor like the marchand balun. A prototype balun operating at 60 GHz is designed, fabricated and measured. The results show that the proposed balun provides a simulated and measured 10-dB return loss fractional bandwidth of 42.3% and >31.8% (limited by the measurement equipment), respectively. Across the operating frequencies, the measured amplitude imbalance and phase imbalance are within ±0.4 dB and ±5°, respectively.