Novel Class of Microstrip Bandpass Filters with Improved Upper Rejection Band

In this paper, we present a novel class of microstrip bandpass filters with improved upper rejection band. The proposed filters are composed of two half-wavelength resonators and two shortended microstrip feed lines for input and output. Using voltagewave analysis, we examine the resonance and coupling properties at harmonic frequencies. It is found that different combinations of the feed line and the resonator with proper selection of the coupling regions can suppress spurious responses. Benefiting from this approach, two single-band and one dual-band bandpass filters are designed, fabricated and measured. Simulated and measured results indicate that the upper rejection bands of the proposed filters are increased up to near thirdand fifth-order harmonics, respectively. And the rejection level during the stop-bands is kept below 20 dB.