Signal strength in telecommunications
In telecommunications, particularly in radio frequency, 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). Relationship to average  radiated power. 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 formula_2 is the angle between the antenna axis and the vector to the observation point, formula_3 is the peak current at the feed-point, formula_4 is the permittivity  of free-space, formula_5 is the speed of light in a vacuum, and formula_6 is the distance to the antenna in meters. When the antenna is viewed broadside (formula_7) the electric field is maximum and given by Solving this formula for the peak current yields The average power to the antenna is where formula_11 is the center-fed half-wave antenna's radiation resistance. Substituting the formula for formula_12 into the one for formula_13 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). For a short dipole (formula_15) the current distribution is nearly triangular. In this case, the electric field and radiation resistance are Using a procedure similar to that above, the maximum electric field for a center-fed short dipole is RF signals. Although there are cell phone base station tower networks across many nations globally, there are still many areas within those nations that do not have good reception. Some rural areas are unlikely to ever be covered effectively since the cost of erecting a cell tower is too  high for only a few customers. Even in areas with high signal strength, basements and the interiors of large buildings often have poor reception. Weak signal strength can also be caused by destructive interference of the signals from local towers in urban areas, or by the construction materials used in some buildings causing significant attenuation of signal strength. Large buildings such as warehouses, hospitals and factories often have no usable signal further than a few metres from the outside walls. This is particularly true for the networks which operate at higher frequency since these are attenuated more by intervening obstacles , although they are able to use reflection and diffraction to circumvent obstacles. Estimated received signal strength. The estimated received signal strength in an active RFID tag can be estimated as follows: In general, you can take the path loss exponent into account: The effective path loss depends on frequency, topography, and environmental conditions. Actually, one could use any known "signal power" dBm at any distance r as a reference: Estimate the cell radius. When we measure cell distance "r" and received power pairs, we can estimate the mean cell radius as follows: Specialized calculation models exist to plan the location  of a new cell tower, taking into account local conditions and radio equipment parameters, as well as consideration that mobile radio signals have line-of-sight propagation, unless reflection occurs.