A Quasi-Elliptic Microstrip Bandpass Filter Using Modified Anti-Parallel Coupled-Line

A novel band-pass filter (BPF) with lower and upper stopband zeros is proposed. The filter mainly consists of two antisymmetric modified anti-parallel coupled-lines and double pairs of transmission lines. Compared with the traditional anti-parallel coupled-line, the new filter has a better performance at both pass band and stop band. On the basis of even-odd mode method and network theory, the filter has been analyzed in detail. In addition, a simple method to create transmission zeros is investigated, which is beneficial for controlling the stopband zeros. The proposed filter is implemented on RT/Duroid 4350 substrate. Measured results show low insertion loss of less than 1.6 dB in the pass band and good suppressing of more than 22 dB in the lower stop band. Measured results show a good agreement with simulated results.