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Designs of a compact lowpass filter using a slotted-ground-plane resonator are presented. Unlike conventional design of cascading bandstop slotted-ground-plane resonators in the literature, the introduced filter is presented, which consists of a quasi-π slot in the ground plane, and a microstrip line with several stepped-impedance (SI) open stubs. The quasi-π slot provides a very sharp cutoff frequency response with low insertion loss. Furthermore, to increase rejection bandwidth, additional transmission zeros are added into frequency characteristics by connecting the SI open stubs at the microstrip line. An equivalent circuit of the proposed lowpass filter is simply represented by resistor, inductor and capacitor (RLC)-lumped elements. According to measured results, compared with a conventional H-slot resonator, the resonance frequency of the resonator is reduced from 3.5 to 2.0 GHz and the ratio of frequency reduction is about 43%. The measured insertion loss at a passband is below 0.3 dB, and the rejection band over 25 dB is from 1.95 to 9.0 GHz.