Dual-Mode Microstrip Patch Bandpass Filters With Generalized Frequency Responses

A basic two-pole bandpass filter based on the dual-mode square patch resonator is presented and analyzed to realize generalized frequency responses in terms of two finite transmission zeros (FTZs). The feeding lines of the dual-mode filter are deviated from the center position of the patch resonator to realize different strength of main coupling paths. And then, a novel four-pole patch bandpass filter based on a multilayer dual-mode structure is proposed and analyzed in detail. The input and output ports are designed on the upper substrate layer. The lower dual-mode patch resonator is coupled to the upper one using four coupling apertures in the middle metal layer. The multilayer dual-mode patch filter can be modeled by a dual-extended doublet scheme with source-load coupling. Generalized frequency responses in terms of four FTZs can be achieved. Several design examples using the proposed multilayer dual-mode patch filter are presented to illustrate the high degree of flexibility of FTZs. To verify the proposed structure, two examples were fabricated and measured. The measured results agree well with simulated ones.