Quarter wavelength coaxial line cavities are commonly used structures in constructing diplexer and band pass filters at the Very and Ultra High Frequencies (V-UHF). Despite its wide use of this structure, design rules of selecting geometrical details concerning the input/output feeding points are determined in each case experimentally, which is a tiresome process. In order to achieve an accurate and reliable design and to avoid cut and paste to achieve fine tuning of resonance at the desired frequency and matching at the input/output lines a field analysis is implemented to develop accurate design rules. In this context the effect of feeding structure geometrical details as well as inner-outer conductor gaps are examined. Furthermore, in order to achieve an additional control of the filter bandwidth the use of a tapered coaxial line is also examined and computed. The performance of various structures is examined and design curves are provided to ease the work of designer.
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