By definition, ERP is the abbreviation for Effective Radiated Power that is directed in a given direction. It is the power supplied to an antenna multiplied by the antenna gain in a given direction. If the direction is not specified, the direction of maximum gain is assumed and the type of reference antenna should be specified. It is also the product of the power supplied to the antenna and its gain relative to a half-wave dipole in a given direction and it is the effective radiated power of a transmitter (with antenna, transmission line, duplexers, etc.) Also, it is the power that would be necessary at the input terminals of a reference half-wave dipole antenna in order to produce the same maximum field intensity. ERP is usually calculated by multiplying the measured transmitter output power by the specified antenna system gain, relative to a half-wave dipole, in the direction of interest.
As a matter of definition, dB is the accepted abbreviation for decibel(s). One tenth of the common logarithm of the ratio of relative powers, equal to 0.1B (bel). Further, dBW is the abbreviation for dB referenced to one watt. The decibel is the conventional relative power ratio, rather than the bel, for expressing relative powers because the decibel is smaller and therefore more convenient than the bel. The dB is used rather than arithmetic ratios or percentages because when circuits are connected in series, expressions of power level, in dB, may be arithmetically added and subtracted. For example, in an RF transmission system, if a known amount of RF power, is fed into the system, and the losses, in dB, of each component (for example, lengths of feeder coax, connectors, duplexers, etc.) are known, the overall system loss may be quickly calculated with simple addition and subtraction.