Compact and very sharp roll-off low-pass filter using folded flag resonator

In this paper, a microstrip low-pass filter (LPF) using folded-flag-shaped resonators is designed and presented. Its base resonator length is 12.4 mm, and the proposed low-pass filter has a sharpness of 0.08 GHz. The filter cut-off frequency is 1.44 GHz, and the return loss is reduced to a level of -17 dB, indicating a flat pass-band. Also, the suppressing cell is used to weaken the disturbing harmonics by a stepped impedance structure and two high-impedance stubs with an L-shaped structure, which results in a bandwidth of 17.78 GHz. The circuit area of the LPF is 20.9 x 7.5 mm(2), which it has created a suitable normalized circuit size (NCS) rate due to the -3 dB cut-off frequency. The figure of merit (FOM) of the proposed filter is 114450, and it indicates that the microstrip LPF responses are optimal as compared to previous works.