The operating range given refers to an open area, i.e. ideal conditions, without obstacles. If there are any obstacles between the receiver and the transmitter it is necessary to foresee decrease of the operating range: for wood and plaster from 5 to 20%, brick from 20 to 40%, reinforced concrete from 40 to 80%. Use of the radio systems near the metal obstacles is not recommended due to considerable reduction of the operating range. In addition, overhead and underground high-voltage power lines as well as cell towers located near devices have a negative impact on the operating range.