In this paper, a semicircle ended stub resonator, cascaded to a modified radial patch to design a compact lowpass filter with sharp roll-off and wide stopband is proposed. This filter has 3 dB cutoff frequency at 1.54 GHz. The transition band is only 0.26 GHz from 1.54 GHz to 1.8 GHz with corresponding attenuation levels of -3 dB and -20 dB respectively. Maximum insertion loss is 0.1 dB in the passband, and the stopband bandwidth with the attenuation level better than -20 dB is extended from 1.8 GHz up to 13.93 GHz. So, a wide stopband is achieved. The proposed filter is designed, fabricated and measured, where there is a good agreement between the simulation and measurement results. The results show that a roll-off rate of 65.4 dB/GHz together with a relative stopband bandwidth of 154% with the suppression level of -20 dB is obtained while achieving a high figure of merit (FOM) of 23509.
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