EIRP

EIRP stands for effective isotropic radiated power. It is the power a theoretical isotropic antenna would need to radiate in a given direction to produce the same power flux density as the actual antenna in that direction. In practice, EIRP is calculated as the transmitter power delivered to the antenna multiplied by the antenna gain in the direction of interest.

The standard formula is EIRP = Pt × Gt, where Pt is the transmitter power delivered to the antenna (input or output depending on definitions) and Gt is the antenna gain relative to an isotropic radiator (linear). When expressed in decibels, EIRP(dBW) = Pt(dBW) + Gt(dBi), or EIRP(dBm) = Pt(dBm) + Gt(dBi). If there are losses in the feedline or other components, these losses can be subtracted from Pt before applying the gain to reflect the actual radiated power.

EIRP is widely used in link-budget calculations, coverage predictions, and regulatory compliance to assess how much power is effectively radiated in a given direction. It provides a common basis for comparing different transmitter/antenna configurations, regardless of the transmitter’s nominal output power.

EIRP is related to ERP, the effective radiated power relative to a half-wave dipole. The relationship is EIRP(dBi) = ERP(dBi) + 2.15 dB, and EIRP = ERP × 1.64 in linear terms.

Example: a transmitter output of 1 W (0 dBW) with a 3 dBi antenna yields an EIRP of 3 dBW, about 2 W (33 dBm) in the main lobe.