This letter proposes a novel analysis and design method of a continuously adjustable bandpass combline filter. It investigates the feedline design specifications and introduces an external quality factor (Qext) tuning structure to achieve a constant fractional bandwidth over 60% tuning bandwidth. The design approach allows to determine the optimum feedline structure for the filter andis verified by full-wave simulation and measurement. The results show a constant fractional bandwidth of 4.5% over the entire operating frequency range between 225-400 MHz.
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