To prevent heatstroke among firefighters, an activity support system that visualizes the biometric data of firefighters has been proposed. The antenna must be capable of transmitting and receiving irrespective of the situation on the back of the antenna because the repeater may be used near a bulkhead or an obstacle in the building during system operation. In this study, a prototype antenna with an artificial magnetic conductor (AMC) was fabricated and investigated to reduce the influence of the back of the antenna. The results demonstrated that the antenna with AMC had a good maximal realized gain and impedance matching in 1.9 GHz band. In addition, it was possible to prevent the degradation of the maximal realized gain, even when the antenna was placed on a metal plate.