Coaxial combline filters using the stepped-impedance resonators

This work presents a series of investigations on the design of a coaxial combline filter using stepped-impedance coaxial resonator (SIRs). First, a full-wave EM simulation code based the finite-element method (FEM) is developed to analyze the resonances a coaxial stepped-impedance resonator (SIR) with a tuning screw. The couplings of the fundamental and high-order resonant modes between a pair of SIRs are also investigated by the FEM simulation. A modified coupling scheme that can effectively suppress the coupling of the high-order modes while provide the required coupling of the fundamental mode simultaneously is then proposed to construct the filter with good spurious performance. Based on the above investigations, a five-pole Chebyshev band-pass filter is designed using the sequential design procedure. The designed filter is also fabricated and measured. Good agreements between the measured and simulated results are observed, which validate the presented investigations.