Investigations on improving the spurious performance of a coaxial combline filter

A series of investigations on improving the spurious performance of a coaxial combline filter are presented. An equivalent circuit is developed to model the transverse electromagnetic (TEM) resonant frequencies, both the fundamental and higher-order modes, of a stepped-impedance coaxial resonator (SIR) with a tuning screw. Further, a series of EM simulations based on the finite-element method are conducted to study the higher-order resonances involving both TEM and TE₁₁ modes propagating in the high- and low-impedance sections, respectively, in the SIR. These investigations complete the design of the individual SIR with good spurious performance. The couplings of the fundamental and higher-order resonant modes between a pair of SIRs are also studied to obtain a coupling scheme with good suppression on the coupling of the higher-order resonant modes. Two five-pole Chebyshev filters are then designed and compared based on the above investigations. One of the filters is fabricated and measured to verify these investigations. The measured and simulated results share good agreement.