SIW planar balun with enlarged bandwidth

An X-band substrate integrated waveguide (SIW) balun, based on a Y-type divider with enlarged bandwidth, is presented in this study. In most SIW Y-type dividers, the undesired TE30 mode either leads to the deterioration of the in-band reflection or constrains the bandwidth by a transmission zero. The cause of the transmission zero is analysed and the modes coupling, rather than the step impedance matching, is utilised to reduce its impact. In the traditional Y-type divider, the frequency band is split by this transmission zero, while in the proposed wideband configuration, the co-existences of the TE10 and TE30 modes are employed to combine the two non-contiguous bands. A broadband balun is designed and fabricated, with the improved Y-type wideband divider serving for the power-dividing part. The dividing ports are connected to the reversely placed transitions, achieving inherent out-of-phase performance and good amplitude property. The measurement of the balun suggests a 43.94% bandwidth from 8.23 to 12.89 GHz, with 15 dB return loss. The measured amplitude and phase imbalance between two output ports are below 0.7 dB and ±3°, respectively. The performance of the fabricated balun is in agreement with the simulation, and it is promising for wideband applications.