# How to: Calculating antenna received power (Friis Transmission Equation)

The Friis Transmission Equation is a fundamental equation in antenna theory. It’s used to identify the correct antenna to be used in a situation when the distance between antennas is known.

Calculators exist online, and can be used without having a full understanding of the formula, an example can be found here

The formula begins by calculating the power density. The variables are defined below

```
Pt = Output power from transmitting antenna
R = Distance between antennas
```

This is the basis of the Friis Transmission Equation. What is added next is the gain of the transmitting and receiving antenna, and the wavelength.

The final formula looks like this:

```
Pt = Output power from transmitting antenna
R = Distance between antennas
Gt = Gain of the transmitting antenna
Gr = Gain of the receiving antenna
λ = The wavelength in meters
```

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