Design of dual-bandpass filter using zeroth-order resonance and Bragg frequency

This study presents a novel compact dual-bandpass filter using zeroth-order resonance and Bragg frequency of the composite right/left handed transmission line (CRLH-TL) resonator. By taking advantage of the Bragg frequency, two resonance modes are obtained with only one CRLH-TL unit cell. The equivalent circuit for the novel resonator is proposed and analysed. The resonance frequencies are given by using the circuit model and the input impedance is calculated. Dimensions of the resonator can be obtained based on the values of the lumped elements in the equivalent circuit. Compared with the conventional half-wavelength resonator, the size of the presented resonator is only about one-tenth of it. Then, the zeroth-order resonance and Bragg frequency modes are demonstrated using the dispersion curve. A dual-bandpass filter is designed to verify the resonator analysis. The source-load coupling is involved to improve the frequency selectivity of the filter. The fabricated filter is compact with a size of only 0.09 λg × 0.16 λg. The two central frequencies are located at 2.4/3.3 GHz and the filter has a wide upper stopband of over 4 GHz. Moreover, the fabricated filter has five transmission zeros and good frequency selectivity is observed.