dBm to EIRP Calculator (with Examples)

Introduction

This tool converts Transmit Power (expressed in dBm) to the Effective Isotropic Radiated Power (EIRP).

Calculator

Enter the Transmit Power, Cable Loss and Antenna Gain.

Formula

EIRP = PTx – LC + GAnt

The variables in the equation above are described below.

EIRP 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.

Antenna Gain GAnt, in dBi is its gain compared to that of 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 and adapter Insertion 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.

Why do you need to know EIRP?

Spectrum regulatory agencies like the FCC and ISED in Canada stipulate limits on the EIRP.

RF designers are able to measure the output power at the transmit port of a wireless product (like a Wi-Fi access point or a LoRa transciever). This is the quantity PTx in the equation EIRP = PTx – LC + GAnt

Cable and adapter Insertion Loss LC reduce the effective transmit power. Antenna gain GAnt can be added to see if the combination of transmitter and antenna exceed the EIRP limit.

Calculation Example

If the output of a transmitter is +20 dBm and the antenna gain is 10 dBi with a cable loss of 3 dB, the EIRP can be calculated as +27 dBm. The next question is, should the antenna gain be reduced to comply with the FCC specification for instance. Alternatively the transmitter could use an attenuator pad at the final output stage.

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