In this paper, a half-wavelength folded-slot antenna, which can be applied to RF module for 3G mobile communications by which security lights are monitored and controlled, is proposed. The proposed antenna can be regarded as modified folded-slot structure which has the size reduced to a half of conventional ${lambda}g$ folded-slot antenna and can be placed at the ground plane edge. In spite of that, the proposed antenna still maintain the advantage of conventional folded-slot antenna that input impedance is close to $50{Omega}$. The antenna is designed and fabricated within the upper space of $40.5{ imes}10mm^2$ on $40.5{ imes}62mm^2$ substrate for 3G mobile communication frequency band. The measured impedance bandwidth and antenna gain are 390 MHz and 2 dBi respectively.