Dual-Mode Filtering Baluns Based on Hybrid Cavity-Microstrip Structures

Filtering baluns are proposed based on the hybrid cavity-microstrip structures in this article. The cavity acts as the resonator to obtain the filtering response. In the meanwhile, the microstrip feeding networks are used to excite multiple modes and achieve the balun characteristics simultaneously. The coupling mechanism between the slots and the cavities is utilized to analyze the single- and dual-band filtering baluns. Therefore, by combining the benefits of the cavities and the microstrip lines, multifunctional filtering baluns are realized. Besides, to reduce the volume and insertion loss further, a filtering balun based on the dielectric cavity-microstrip structure is presented and designed. It possesses high selectivity passband response, magnitude imbalance of 0.075 dB, and phase difference of 180° ± 0.2°. It is much smaller than that fed by the waveguide. On the other hand, it is easy to integrate with other components and robust in structure compared to that fed by the probe. Thus, the proposed hybrid structure not only maintains the high performance of the cavity but also has the flexibility of the planar circuit.