EIRP Calculator — Effective Isotropic Radiated Power

Effective Isotropic Radiated Power (EIRP) represents the total power that a theoretical isotropic antenna would emit to produce the peak power density observed in the direction of the antenna's maximum gain. The mathematical foundation of EIRP aggregates all gains and losses along the RF path, typically measured in decibels relative to one milliwatt (dBm) or decibels relative to an isotropic radiator (dBi). The core formula is: EIRP = Output Power - Cable Loss - Connector Loss + Antenna Gain. Cable loss is calculated by multiplying the physical length of the cable by the attenuation factor specific to your operating frequency per unit length, while connector losses account for discrete insertion attenuations at patch panels and physical junctions.

Imagine configuring a 2.4 GHz wireless transmitter system with the following specifications: a transmitter output power of 20 dBm, 50 feet of coaxial cable with an attenuation of 0.05 dB per foot at 2.4 GHz, 2 physical RF connectors each introducing an insertion loss of 0.5 dB, and an antenna with a gain of 12 dBi. First, calculate the total cable loss: 50 ft multiplied by 0.05 dB/ft equals 2.5 dB. Next, compute the total connector loss: 2 connectors multiplied by 0.5 dB equals 1.0 dB. Finally, assemble the EIRP equation: EIRP = 20 dBm (Output) - 2.5 dB (Cable Loss) - 1.0 dB (Connector Loss) + 12 dBi (Antenna Gain). This results in an EIRP of 28.5 dBm.

Always verify the frequency-dependent attenuation rating of your coaxial cables, as high frequencies like 2.4 GHz and 5 GHz experience significantly higher signal degradation per foot than sub-GHz frequencies. Minimize jumper cable lengths and keep physical connections to an absolute minimum to preserve precious decibels before the signal reaches the antenna. Finally, always cross-reference your calculated EIRP against local regulatory power spectral density limits to ensure full legal compliance for your specific radio band.