Simple technique to extend the bandwidth of parallel-coupled microstrip bandpass filter

This paper presents the concept of bandwidth extension in the parallel-coupled microstrip bandpass filter (BPF). By applying a lambda_g/4 shunt-shorted stub at the middle of parallel-line section in the BPF, the wider bandwidth can be obtained. The prototype BPF is designed and implemented on an FR-4 substrate at 1 GHz center frequency in the microstrip structure. The measured insertion loss is lower than -1.9 dB whereas the return loss is kept below -11.8 dB over the frequency range 0.59 to 1.4 GHz. Both simulated and measured results have been confirmed the feasibility of the proposed structure for easy wideband response and rejection performance.