Low insertion loss, wideband bandpass filters with sharp rejection characteristics

A general circuit configuration using two transmission-line sections is presented to design a sharp-rejection bandpass filter. Sharp rejection characteristic is obtained by creating two transmission zeros across the filter passband by interfering signals propagating through the transmission lines. A simple lossless transmission-line model is used for filter analysis. If an impedance variation of 20-130 Omega is considered, the present method is most suitable for 10-50% 3-dB fractional bandwidth variation. Two prototype bandpass filters having approximately 29% fractional bandwidth at the midband frequency 2.9 GHz are fabricated in microstrip line. The filter structures are simple and easy to fabricate. Theoretical, simulated and measured results are in good agreement.