A Microstrip Lowpass Filter With Miniaturized Size and Broad Stopband

This paper presents a miniaturized wide stopband planar lowpass filter (LPF) using novel stepped impedance hairpin resonators (SIHRs). By embedding a radial stub inside the stepped impedance hairpin resonator, the resulting resonator can achieve a miniaturized size and broad stopband simultaneously. To validate the designed approach, a microstrip LPF is implemented and tested. The filter features a dimension of 0.158 $\lambda$ g $\times$ 0.126 $\lambda$g, where $\lambda$g denotes the waveguide length at the -3-dB cutoff frequency of 1.9 GHz. The measurements demonstrate that the filter offers a 20-dB rejection stopband from 2.45 GHz to 16.3 GHz.