A Stopband-Expanded Low-Pass Filter with Five Transmission Zeros

A novel compact low-pass filter with five transmission zeros is presented. This filter is composed of two symmetrical open-circuited transmission lines and an anti-coupled transmission line loaded with an open-circuited transmission line. By adjusting the parameters of coupled line and open-circuited lines, the five zeros can be excited in stopband. Thus an expanded stopband, a sharp transition and a compact size can be achieved at the same time. A filter with this structure is designed, fabricated, and tested. The measured results of the filter agree well with the simulated results and evidently demonstrate good LPF performance: compact size and a wide stopband of 1.15 to 5.17 GHz with the rejection higher than 20 dB.