Filtering Balun Based on Multi-Mode Rectangular Cavity Integrated With PCB Feeding Structure

A filtering balun based on multi-mode rectangular cavity is proposed in this paper for the first time. The printed circuit board (PCB) microstrip lines cooperated with coupling slots are utilized as feeding structures. The TE
₁₀₁
mode is employed to construct the passband while the TE
₀₁₁
and TM
₁₁₀
modes are used to generate two out-of-band zeros. Besides, by analyzing the electromagnetic field distributions, the balun characteristic at the two outputs are achieved due to the antisymmetrical arrangement of the output feeding lines. For demonstration, a second order cavity filtering balun is designed and measured. Both good filtering response and balun characteristic are observed in the measurement, which validate the proposed method.