A novel system composed of a circular aperture slot antenna and a Common-Mode (CM) noise rejection filter is presented. This antenna is differentially fed by microstrip coupled transmission lines. In order to eliminate CM noise, a notch filter based on three non-periodical defected ground structures (DGS) was implemented. The whole system achieves a fractional impedance bandwidth of about 127%. Radiation patterns in E and H planes for different frequencies were obtained for the system. Finally, measurements in the transversal plane show attenuation up to 13 dB when the system with filter was compared against one without it. Good agreement between simulated and measured results can be observed.