A Simple and General Method for Cross-Coupled Bandpass Filters Based on Circular Patch Resonator

In this article, a simple and general method for the design of cross-coupled bandpass filters with compact size is proposed by virtue of the circular patch resonator. Owing to the unique symmetry of the circle, such a circular patch resonator can be freely divided into several circular sector patch resonators along the radii. With this transformation, any two of the divided resonators have a possible coupling region. Specifically, the neighboring resonators have adjacent sides, while the vertices of all resonators are gathered in the center. Given this, the cross couplings can be easily constructed. Moreover, as the circular sector patch resonators with different boundary conditions along two sides have associated resonant properties, both positive and negative couplings can be provided. Consequently, the circular patch resonator should be a good candidate for the design of cross-coupled filters, and the concrete layout of the resultant filter can be determined by the design parameters as extracted from the design specifications. For demonstration, four design examples, including two single-band, one dual-band, and one tri-band bandpass filters, are constructed, fabricated, and measured. All measured results are in good accordance with the synthesized and simulated counterparts, thereby satisfactorily validating the feasibility and universality of the proposed design method.