EIRP Calculator

EIRP stands for Effective (or Equivalent) Isotropic Radiated Power. It represents the amount of power a theoretical isotropic antenna — one that radiates equally in all directions — would need to emit to produce the same peak power density as your actual antenna system. It is a key metric in link budget calculations and regulatory compliance for wireless systems.

EIRP is calculated using the formula: EIRP (dBm) = Transmitter Power (dBm) − Cable Loss (dB) + Antenna Gain (dBi). You simply take your transmitter output power, subtract any feedline or connector losses, and add the antenna gain. The result gives you the effective radiated power in the direction of maximum antenna gain.

ERP (Effective Radiated Power) is referenced to a half-wave dipole antenna, while EIRP is referenced to a perfect isotropic radiator. Because a dipole has a gain of approximately 2.15 dBi over an isotropic antenna, EIRP is always about 2.15 dB higher than ERP for the same system. To convert: EIRP (dBm) = ERP (dBm) + 2.15 dB.

It depends on your cable losses and antenna gain. For example, if you have 15 dBm transmit power, 1 dB cable loss, and an antenna with 8 dBi gain, the EIRP would be: 15 − 1 + 8 = 22 dBm (approximately 158 mW). Use the calculator above to try different combinations.

EIRP limits vary by region, frequency band, and application. In the US, the FCC limits EIRP for unlicensed 2.4 GHz Wi-Fi point-to-point systems to 36 dBm (4 W), and for 5 GHz systems to 53 dBm in some configurations. The EU (ETSI) and other regions have their own limits. Always check the regulations applicable to your frequency band and country before deployment.

An isotropic antenna is a theoretical point source that radiates power uniformly in all directions with no gain or loss. It does not exist in practice but provides a useful, unambiguous reference for comparing antenna performance. Antenna gain expressed in dBi (decibels relative to isotropic) directly tells you how much more power the antenna focuses in its best direction compared to an isotropic source.

Use the formula: P (W) = 10^((EIRP_dBm − 30) / 10). For example, an EIRP of 35 dBm converts to 10^((35−30)/10) = 10^0.5 ≈ 3.16 W. Conversely, to convert Watts to dBm: EIRP (dBm) = 10 × log10(P_W) + 30.

Cable loss (also called feedline loss) occurs because coaxial cables absorb and dissipate RF energy as heat. Longer cables, thinner cables, and higher frequencies all increase loss. To minimize it, use low-loss coaxial cable (e.g. LMR-400 instead of RG-58), keep cable runs as short as possible, use quality connectors, and consider mounting the power amplifier close to the antenna.