An investigation of novel capacitor-based coupling scheme and its applications to microstrip bandpass filters

In this paper, we present a novel capacitor-based coupling scheme for realizing both harmonic-suppressed and miniaturized microstrip bandpass filters. The proposed coupling scheme employs lumped-/distributed-capacitors to provide the desired coupling strength between the resonance elements instead of traditional parallel-coupled lines. Certain coupling regions are replaced by coupling points resulting in high freedom degree of the filter topologies. Thus, the filter structure can be effectively miniaturized by meandering microstrip lines. To investigate the properties of the novel coupling scheme, an analytical equation is derived for describing the coupling coefficient variation vs. the positions of coupling points and the values of coupled-capacitors. Theoretical analysis indicates that several zero-voltage points can be chosen to suppress the harmonics, and the coupling coefficient can be easily tuned in a wide range by adjusting the capacitance of the coupled-capacitor. For demonstration, two sample filters are designed and fabricated. Simulation and experimental results agree well, validating the proposed methodologies.