A Compact Microstrip Lowpass Filter with Ultra-Wide Rejection Band and Sharp Transition Band Utilizing Combined Resonators with Triangular Patches

In this paper, a microstrip lowpass filter with -3 dB cut-off frequency of 1.8 GHz consisting of two resonators with different triangular patches and four high-low impedance resonators as suppressing cells has been designed. To design this filter, the influence of each transmission line on the frequency response of the utilized resonators has been clarified by calculating the equations of S21 and S11 based on the lumped circuit of them separately. The designed filter has been constructed and tested, and an good agreement between the simulation and measurement results has been achieved. The stopband covers an ultra-wide frequency range from 1.94 to 34.556 GHz with a suppression level of -21.2 dB. Furthermore, the insertion loss in the passband region is close to zero from DC to 1.68 GHz and an acceptable return loss (17.44 dB) in this band can prove desired in-band character. Moreover, the proposed filter provides a sharp transition band equal to 228 (dB/GHz). The designed lowpass filter has a high figure of merit equal to 26583.4.