This paper presents a study of a non-uniform pitch helical resonator
(NPHR) structure and the coupling mechanisms to design dual-passband filters. The
previous 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 NPHR
structure discussed in this paper. Consequently, a circuit model is proposed to
analyze the dual-resonance characteristics of NPHRs. Analytical equations are
derived, showing that the frequency ratio of a dual-band NPHR can be determined
by the ratio of turns. EM simulation and experimental results have shown good
agreement with the circuit analysis. The derived analytical equations from
circuit model can be used for fast design of NPHRs. A step-width aperture is
proposed to independently control the coupling coefficients at the each band of
NPHRs. A third order dual-passband filter has been designed and fabricated. The
filter has 3.3% and 3% fractional bandwidths at 789 MHz and 2402 MHz,
respectively. The designed prototype filter shows that NPHRs can be utilized to
realize compact filters with dual-band characteristics. The filter design can
be extended from engineering perspective for application in wireless communication
systems.
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