This paper introduces a new design technique for a capacitive gap-coupled bandpass filter (BPF) using non-uniform arbitrary image impedances. Based on the proposed BPF equivalent circuit model, the filter's design equations are derived, and they are validated from comparisons of the calculated and simulated results. For this theoretical verification, the BPF using non-uniform arbitrary image impedances is designed using the specifications of: center frequency (fc)=5.8 GHz, fractional bandwidth (FBW)=3.5%, and filter stage (N)=3. The calculated and simulated results of the designed filter show good agreement. The BPF using the proposed design method could provide an advantage that one can arbitrarily determine two different image impedances, which ultimately affects the BPF's coupling gaps and line widths. This could result in suitable filter dimensions, i.e., gaps and line width, for a conventional low resolution photolithography fabrication although a low or high dielectric constant substrate is used for the design.
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