Using a gain aerial on a transmitter the ERP can be several times the power consumed by the transmitter, From wikipedia to save me typing "" Effective radiated power (ERP) is used when calculating station coverage, even for most non-broadcast stations. It is the TPO, minus any attenuation or radiated loss in the line to the antenna, multiplied by the gain (magnification) which the antenna provides toward the horizon. This antenna gain is important, because achieving a desired signal strength without it would result in an enormous electric utility bill for the transmitter, and a prohibitively expensive transmitter. For most large stations in the VHF- and UHF-range, the transmitter power is no more than 20% of the ERP. "" For smaller stations the aerial gain can be 20 or more.