A small, loop-type ground radiation antenna for 2.4-GHz WLAN applications in the complex ground with two vertically attached printed circuit boards (PCBs) was designed. In the presence of a vertical PCB (sub-PCB), the antennas in two different positions, far from and near the sub-PCB, were designed. The current distribution of the complex PCB was stronger near the sub-PCB. Therefore, the coupling between the antenna and the complex PCB was stronger when the antenna was located near the sub-PCB. The antenna close to the sub-PCB obtained more than twice the 3:1 VSWR bandwidth and the efficiency bandwidth than the distant antenna.