A novel compact triple-mode resonator for microstrip bandpass filter design

A novel compact triple-mode resonator is presented in this paper. The triple-mode resonator consists of a square open-loop half-wavelength resonators and a stepped impedance resonator structured in the middle which is connected to the mid-point of the half-wavelength line where a grounding via is located. The grounded via effectively splits the half-wavelength line into two quarter-wavelength resonators. The dimension of the resonator is 15 mm × 16.15 mm. The three resonant modes are analyzed and explained using odd and even modes analysis. To demonstrate the design principle, a threepole Chebyshev bandpass filter is designed and measured. The measured filter has center frequency of 1.0 GHz and passband insertion loss of 1.0 dB. The new resonator has a good stopband response and its first spurious response occurs at approximately three times the passband frequency. The simulated and measured results show good agreement.