Novel Dual-Mode Balun Bandpass Filters Using Single Cross-Slotted Patch Resonator

In this letter, a novel dual-mode bandpass balun filter is presented using a miniaturized cross-slotted patch resonator. This balun filter consists of a single slit-loaded patch etched by a pair of cross slots and a stepped-impedance open stub. An unbalanced input port is orthogonally arranged in-between two balanced output ports along the patch, and each feed line is connected to an additional stub for enhanced coupling. Due to the dual-mode characteristic of the patch resonator, two transmission poles can be easily constructed at both balanced passbands. Asymmetrical width and length of the cross-loaded slots perturb the field of the patch and excite two degenerate modes simultaneously, while the attached open-circuited stub provides an additional degree of freedom for performance tuning. Finally, two prototype balun filters are designed and fabricated at 3.48 and 1.80 GHz, respectively. Measured results achieve good filtering and balun performance and agree well with those from simulations.