Dual-Band Helical Filters Based on Nonuniform Pitch Helical Resonators

A helical resonator with two-sectional different pitches is proposed in this paper. The frequency ratios of the first two resonant modes of the helical resonator can be tuned to desirable values in a certain range by adjusting the pitches. The proposed resonator is applied to design dual-band bandpass helical filters. To achieve control over dual-band filtering characteristics, a feeding structure with step-rectangular plate and a coupling window with three apertures are also proposed to realize the expected external quality factors and coupling coefficients in the two bands. To validate the design, a second order and a fourth order dual-band helical filters are fabricated and measured. The measured results show good agreement with simulation ones.