Microstrip Lowpass Filter with Ultrawide Stopband and Sharp Roll-off

Abstract

In this paper, a novel microstrip lowpass filter using curved T-shaped and stepped-impedance resonator with the folded lines is presented. The cutoff frequency of the proposed filter is adjusted to 1.5 GHz. The transition band is 0.21 GHz from 1.5 to 1.71 GHz with corresponding attenuation levels of −3 and −20 dB, respectively. The stopband with minimum attenuation level of 20 dB is obtained from 1.71 to 20 GHz, and the insertion loss is <0.1 dB. The return loss is more than 20 dB, and the overall size of the filter is \({0.017\lambda_{g}^{2}(28 \times13\,{\rm mm}^{2})}\). The proposed filter is fabricated and measured. The simulation and measurement results are in good agreement.