Non-Uniform Pitch Helical Resonators for Dual-Passband Filter Design

摘要

This paper presents a study of a non-uniform pitch helical resonator(NPHR) structure and the coupling mechanisms to design dual-passband filters. Theprevious research analyzes NPHR as a type of step impedance resonator (SIR),however, it does not give analytical equations or prediction for the dual-resonance characteristics of the NPHRstructure discussed in this paper. Consequently, a circuit model is proposed toanalyze the dual-resonance characteristics of NPHRs. Analytical equations arederived, showing that the frequency ratio of a dual-band NPHR can be determinedby the ratio of turns. EM simulation and experimental results have shown goodagreement with the circuit analysis. The derived analytical equations fromcircuit model can be used for fast design of NPHRs. A step-width aperture isproposed to independently control the coupling coefficients at the each band ofNPHRs. A third order dual-passband filter has been designed and fabricated. Thefilter has 3.3 and 3 fractional bandwidths at 789 MHz and 2402 MHz,respectively. The designed prototype filter shows that NPHRs can be utilized torealize compact filters with dual-band characteristics. The filter design canbe extended from engineering perspective for application in wireless communicationsystems.