Combline Filters using Conical Resonators

This paper describes a novel configuration for combline filters employing conical rods as central conductors in the resonators. With respect to resonators with uniform rods, this shape allows a reduction of the capacitive loading for a given length of the resonator; as a consequence, a larger gap at the rods end can be employed (it is particularly convenient in high power filters). In this paper a general procedure is presented for analyzing non-uniform resonators under TEM approximation for the fundamental resonant mode; using this procedure, a simple design method for combline filters using non-uniform resonators has been developed. A test filter operating in the frequency band 824-894 MHz has been designed and fabricated for verifying the novel configuration here introduced; the experimental measurements obtained from this prototype have shown to be in good agreement with the expected performances.