Use this tool to find the **EIRP** (Effective Isotropic Radiated Power) from **ERP** (Effective Radiated Power).

**Formula**

**EIRP = 1.64 * ERP **

**EIRP _{dBm} = ERP_{dBm} + 2.15**

**Background**

**ERP** is the power that would have to be radiated by a **half-wave dipole antenna** to provide the same signal level as the actual source antenna in the direction of the antenna’s strongest beam.

**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.

**A half-wave dipole can be realized practically**. The rabbit ears antenna on older television sets is an example. Use this calculator to determine the dimensions of a half wave dipole.

**An isotropic antenna cannot be built practically. **It is a hypothetical or an ideal antenna type and radiates energy equally in all directions.

**Equations**

In linear terms, the ratio of the gain of a half-wave dipole antenna to that of an isotropic antenna is given by the following relationship:

**G _{D}/G_{I} = 0.6098**

Therefore on the Log scale **10*Log _{10}(G_{D}/G_{I})** =

**10*Log**

_{10}(0.6098) = -2.15or

**EIRP (dBm) = ERP (dBm) +**

**2.15**

**dB**

The way to understand this intuitively: **An isotropic antenna would have to radiate 2.15 dB more energy to provide the same signal level as a half-wave dipole.**

**Notes**

[1] In this publication the FCC has provided guidelines on how to determine the EIRP and ERP practically.

[2] ERP can be specified in Watt. Use this tool to convert Watt to dBm for the above calculations.