In this paper, a microstrip lowpass filter using rectangular resonators is designed. To weaken the undesired harmonics from 10 to 20 GHz, a stair-shaped suppressing cell and also high impedance elements at the edges are utilized. It should be noted that for analyzing the proposed filter, an LC model is used and the design is done in such a way that the coupling effects of the close-up lines are reduced as much as possible, so that they are ignored in the circuit analysis. The designed filter has a cut-off frequency of 1.01 GHz and the transition band is only 0.1 GHz. In addition to compression of dimensions, the stopband bandwidth is 18.7 times the cut-off frequency (18.7 × fc). The attenuation rate in the passband reaches 0.081 dB which indicates that the passband is flat. In the stopband, disturbing harmonics have been dampened to −23.5 dB and a good bandwidth is achieved. The figure of merit of the proposed filter is 180 387 and it is a good benchmark for the overall comparison of the proposed filter with the previous works.
