A Design of Microstrip Lowpass Filter with Wide Rejection Band and Sharp Transition Band Utilizing Semi-Circle Resonators

Abstract

In this paper, a microstrip lowpass filter with -3 dB cut-off frequency of 2.1 GHz consisting of three cascaded resonators with different semi-circle patches and four suppressors employing radial stubs has been proposed. To indicate the role of each employed microstrip transmission line in the structure of its resonance cell, the equations of the transfer function and transition zero of the resonator have been calculated, separately. The designed filter has been constructed and tested, and a good agreement between the results of simulation and measurement has been achieved. In the whole rejection region, a return loss better than +0.28 dB and a 19.656 GHz stopband bandwidth with high rejection level of 32 dB have been obtained. Moreover, a flat insertion loss close to zero in the passband and sharp cutoff slope (203.57 dB/GHz) can verify desired frequency response. The proposed filter has a high figure of merit equal to 24241.69.