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This study presents simple, compact designs for a bandstop resonator with a slotted ground plane. By adding two parasitically coupled patches and embedding them with shorting pins, the resonance frequency of the slotted-ground-plane resonator is significantly shifted down and an improvement in harmonic suppression is easily achieved. The two identical patches are symmetrically placed beside a transmission line on the top of the slotted ground plane. The patches have a parasitic capacitance, which increases the equivalent capacitance of the resonator so that the resonance frequency is reduced and the quality factor is increased because of the narrow rejection bandwidth. Furthermore, the technique of loading the patches with two shorting pins significantly reduces the size of the resonator. Moreover, the harmonics at high frequencies are suppressed by varying the locations of the shorting pins. A corresponding equivalent-circuit model for each topology is developed, and the lumped-circuit parameters are extracted using a simple circuit analysis method. Compared with a conventional slotted-ground-plane resonator, with a quality factor of 7.628, the measured results show that the resonator with the shorted patches provides a quality factor greater than 23.