Use this calculator to find the Effective (or Equivalent) Isotropic Radiated Power EIRP in dBm, dBW and Watt.
- Transmit Power (dBm)
- Antenna gain (dBi)
- Cable Loss (dB)
EIRP = PTx – LC + GAnt
Effective Isotropic Radiated Power is the power that would have to be radiated by an isotropic antenna to provide the same signal level as the actual source antenna in the direction of the antenna’s strongest beam.
An antenna’s dBi value is a measure of its gain compared to an isotropic antenna. Isotropic antennas radiate signal equally in all directions. An antenna with a higher dBi value has more gain than an antenna with a lower dBi value. Antenna Gain can be calculated from Antenna Factor and Frequency of operation.
Cable Loss LC reduces the output signal level. It is expressed in deciBel or dB.
The transmitted power from an amplifier for instance is entered in dBm. Use this calculator to convert from Watt to dBm.
EIRP vs Transmit Power
In general EIRP is not the same as transmit power. This can be seen in the equation: EIRP = PTx – LC + GAnt
Transmit power PTx is measured at the output connector. Whereas EIRP includes the effect of antenna gain and cable losses.
Assuming cable losses are negligible, the EIRP is greater than the transmit power. The higher the antenna gain, the larger the difference between EIRP and transmit power.
How to measure EIRP and Transmit Power?
A transmit power measurement can simply be made by connecting the output of the transmitter to a Signal Generator or Spectrum Analyzer*. The power can be measured in dBm (and converted to Watt).
EIRP on the other hand cannot be directly measured. The antenna is a radiating element and has no output connector. As a result it cannot be connected to an instrument directly. EIRP is determined using the calculator on this page.
*make sure that the Tx power output doesn’t exceed the damage level of the instrument.
Example Use Cases
Some Wi-Fi access points have external antennas like the one shown below.
In these devices, a user can attach their own antenna. For instance, a high dBi gain* directional antenna might be preferred instead of the default omni directional antenna.
Users are required to enter the antenna type and gain in the dashboard. The access point will compute the transmit power taking this into account.
For instance, if the max EIRP allowed at 2.4 GHz in a region is +36 dBm and the antenna gain is 0 dBi, then the max transmit power will be configured by the AP to be +36 dBm.
On the other hand, if the user attaches a 10 dBi antenna, then the AP will determine the max transmit power is +26 dBm. It uses the same calculation provided above. Access points have a Auto Adjust to EIRP Limit feature to account for different antenna gains.