In this paper, a lowpass filter with -3 dB cut-off frequency of 1.69 GHz employing modified hairpin resonator with long straight slots is designed. In the first step, to design the primary resonator, the open stubs of a conventional hairpin resonator are folded inside its free area, which results in a smaller occupied area and an improved frequency response. Next, to control the scattering parameters, asymmetric coupled lines with slots are utilized instead of symmetric open-stubs of the primary resonance cell. In each step, the impact of the employed microstrip transmission lines on the scattering parameters of the designed resonator is determined by extracting the equations of the insertion loss (S21) and return loss (S11) on the basis of their equivalent LC circuit. Finally, by using two modified resonators with slots which are placed symmetrically around (Y) axis, a sharp transition band (264 dB/GHz) and wide stopband from 1.78GHz to 10GHz with a suppressing level of -20 dB are obtained. The overall circuit size is 0.140λg×0.076 λg, which indicates a small circuit size. The proposed lowpass filter has a high figure of merit equal to 72032.
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