Microstrip bandpass filter with extended stopband based on lowpass filter section

A microstrip bandpass filter with extended stopband is presented in this paper. The extended stopband is obtained through using lowpass filter section in the filter design. The lowpass filter section is composed of SIR resonators with appropriate positions. Equivalent lumped circuit of the lowpass section is given to better understand performance of the proposed filter. This lowpass Alter section suppressed signals in a wide band, thus made the bandpass filter contained them have a wide stopband. Measured results accord well with the simulated results, showing good performance of the Alter.