Compact LPF using T-shaped resonator with sharp roll-off and ultra wide stopband

In this paper, a microstrip lowpass filter with an ultra-wide stopband with sharp roll-off and compact size is designed and fabricated. To provide a very small dimension, T-shaped and stepped impedance have been used. The cut of frequency at -3 dB is 3.1 GHz. In the proposed structure, insertion loss is less than 0.12 and return loss is more than 15 dB. To achieve a -20dB attenuation level in the stopband, modified L-shaped structure is utilized. This filter has an ultra-wide stopband that is expanded from 3.37 GHz to 37.5 GHz also the suppression level is greater than -20dB. The size of the fabricated LPF is about 16.6 * 13.5 mm2 which corresponds to an electrical of 0.28ƛg * 0.22ƛg, where ƛg is the guided wavelength at 3.1 GHz, which is very small. The proposed filter is designed, simulated, optimized, fabricated and measured. There is good agreement between the simulated and measured results.