Compact Ultra-Wide-Stopband Microwave Bandpass Filter with Multiple Transmission Zeros by Integrating an L-Shaped Bandpass Filter and a Stepped Impedance Lowpass Filter

A compact ultra-wide-stopband microwave bandpass filter is introduced in this work using an L-shaped quarter-wavelength resonator and stepped impedance resonators. The bandpass filter (BPF-I) is designed at 2.1 GHz (f) using an L-shaped quarter-wavelength line. The lowpass filter is developed at 4 GHz using stepped impedance resonators. The introduced ultra-wide-stopband filter is created through integrating the BPF-I and the lowpass filter. Filter size reduction is achieved through the L-shaped quarter-wavelength line, while the ultra-wide-stopband is attained through the stepped impedance resonators. The mathematical analysis of the introduced ultra-wide-stopband bandpass filter is conducted through the extraction of ABCD and S-parameters. The lumped elements equivalent circuits are realized for the introduced filter, and their S-parameters are plotted. The introduced filter is fabricated on an FR4 substrate, and its S-parameters are measured. The comparison of mathematical analysis, EM simulated, circuit simulated, and measured results, showing good agreement. Measurements show a 140.4% fractional bandwidth (FBW), better than 20 dB return loss, higher than 28 dB rejection up to 30 GHz (14.3f) in the out-of-band with many transmission zeros, and 0.8 dB insertion loss. 0.23g x 0.37g is the overall dimension of the introduced filter.