Equivalent isotropic radiated power meaning

In telecoms regulation and practice, equivalent isotropic radiated power (EIRP) means the maximum effective radiated power in a particular direction from a radio antenna, combining transmitter output, antenna gain and feeder losses. Regulators (Ofcom in the UK and ComReg in Ireland) use EIRP to set spectrum licence power limits, equipment compliance conditions and for interference and EMF assessments. EIRP is the power a hypothetical isotropic (omnidirectional) antenna would need to radiate to match the field strength produced by the actual antenna in its strongest direction (the main beam). Because antennas focus energy, the highest power appears in a specific direction, often near the horizontal or with slight downtilt; power in other directions, including vertically downwards, is usually much lower. EIRP is typically expressed in watts or dBm. EIRP is a descriptive engineering measure referenced across legislation, licences, standards (e.g. ETSI) and contracts, rather than a standalone statutory definition. Usage and meaning are consistent across England & Wales, Scotland, Northern Ireland and Ireland. Typical drafting sets an “EIRP not to exceed” figure per band or site. In practice: EIRP = transmitter power at the antenna port − cable/combiner losses + antenna gain (dBi). Breaches can attract regulatory enforcement.