Hybrid structure-based broadband field-rotation balun for millimeter-wave applications

A broadband balun structure is developed through a hybrid combination of substrate integrated waveguide (SIW) and antipodal fin-line topologies. The signal at the balanced port is divided between two unbalanced ports using a broadband SIW power divider which is capable of an equal power division with low phase and amplitude imbalance over a wide bandwidth. The 180° phase difference is made by the rotation of E-field polarization through the antipodal fin-line sections in two unbalanced branches. Due to the independency of this phase shifting with respect to propagation constant and wavelength, a constant 180° phase difference is achieved in a wide range of frequency. Using a printed circuit board (PCB) process, the proposed balun is prototyped and demonstrated for operation at millimeter-wave frequencies from 70 GHz to 90 GHz. The measurement results show an excellent performance with very low insertion loss of less than 1 dB and 180° phase difference between the unbalanced ports in a broad band of frequency.