Compact sharp-rejection broadband microstrip bandpass filter with wide stopband

A compact sharp-rejection broadband microstrip bandpass filter with a wide stopband is presented. The major building block of the proposed filter is a half-wavelength resonator loaded by a meandering coupled line. The loaded resonator could provide three transmission poles in the passband and two transmission zeros beside the passband. By adjusting the dimensions of the meandering coupled line and using multi-path cancellation technique, one can control multiple transmission zeros for a wide stopband. Analysis and experimental results are included in this paper. From 2.3 to 4.1 GHz, the measured return loss is greater than 15 dB, and the measured insertion loss is less than 0.8 dB. The maximum group delay variation is 0.8 ns, and the core circuit size is 0.05λ_g² at the center frequency. For the upper stopband performance, the rejection level is greater than 20 dB from 4.3 GHz to 10 GHz.