EIRP is the power required when the antenna is replaced by an isotropic antenna, and can be calculated from the transmit power, feed line loss, and gain of the transmit antenna.

This article explains the meaning of EIRP and how to calculate it.

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## What is EIRP?

EIRP stands for Equivalent Isotropically Radiated Power or Effective Isotropically Radiated Power. The antenna has a large radio strength in a given direction.

Antennas have the property of increasing radio wave strength in a certain direction (directivity).

However, it is inconvenient to compare the strength of radio waves because the strength of directivity differs from antenna to antenna.

Therefore, we consider isotropic antennas.

An isotropic antenna is a theoretical point that radiates radio waves with the same strength in all directions.

n other words, isotropic antennas are not directional.

The EIRP is the power required to reproduce the field strength in the direction of maximum antenna radiation (main lobe) with an isotropic antenna.

The solid circle represents the antenna pattern of an isotropic antenna.

The absolute gain of the antenna is calculated with respect to the isotropic antenna, so the absolute gain of the isotropic antenna is 0 dBi.

The horizontal extension is the main lobe of the antenna, and the EIRP is the power fed to the isotropic antenna to account for this gain.

ERP (Equivalent Radiated Power or Effective Radiated Power), in which the reference antenna for EIRP is a half-wavelength dipole antenna, is also available.

Since the absolute gain of the half-wavelength dipole antenna is 2.14 dBi, the EIRP can be expressed as

EIRP can be expressed as

## How to calculate EIRP?

If EIRP \(P_t[\mathrm{dBW}]\) is the transmitting power, the feeder loss between the transmitter and the transmitting antenna, and the gain of the transmitting antenna, then EIRP is

\[\mathrm{EIRP} = P_t-L_{ft}+G_t\]

## Example of EIRP calculation

Assuming that the transmitting power is 200 mW (-7 dBW), the feeder loss between the transmitter and the transmitting antenna is 1 dB, and the gain of the transmitting antenna is 2.14 dBi (half-wavelength dipole antenna)

\[\mathrm{EIRP} = -7\mathrm{dBW}-1\mathrm{dB}+2.14\mathrm{dBi} = -5.86\mathrm{dBW}\]

The result is as follows. As shown in this result, the EIRP can be negative depending on the unit.