In telecommunications, particularly in radio frequency engineering, signal strength refers to the transmitter power output as received by a reference antenna at a distance from the transmitting antenna. High-powered transmissions, such as those used in broadcasting, are expressed in dB-millivolts per metre (dBmV/m). For very low-power systems, such as mobile phones, signal strength is usually expressed in dB-microvolts per metre (dBμV/m) or in decibels above a reference level of one milliwatt (dBm). In broadcasting terminology, 1 mV/m is 1000 μV/m or 60 dBμ (often written dBu). Examples 100 dBμ or 100 mV/m: blanketing interference may occur on some receivers 60 dBμ or 1.0 mV/m: frequently considered the edge of a radio station's protected area in North America 40 dBμ or 0.1 mV/m: the minimum strength at which a station can be received with acceptable quality on most receivers The electric field strength at a specific point can be determined from the power delivered to the transmitting antenna, its geometry and radiation resistance. Consider the case of a center-fed half-wave dipole antenna in free space, where the total length L is equal to one half wavelength (λ/2). If constructed from thin conductors, the current distribution is essentially sinusoidal and the radiating electric field is given by where is the angle between the antenna axis and the vector to the observation point, is the peak current at the feed-point, is the permittivity of free-space, is the speed of light in vacuum, and is the distance to the antenna in meters. When the antenna is viewed broadside () the electric field is maximum and given by Solving this formula for the peak current yields The average power to the antenna is where is the center-fed half-wave antenna's radiation resistance. Substituting the formula for into the one for and solving for the maximum electric field yields Therefore, if the average power to a half-wave dipole antenna is 1 mW, then the maximum electric field at 313 m (1027 ft) is 1 mV/m (60 dBμ).