FriisĀ“s Formula

Derived by Harald Friis (hence the name). This formula equates the power at the reciver terminals of a receive antenna as the product of power density of the incident wave and the effective aperture of the receiving antenna under idealized conditions given another antenna some distance away transmitting a known amount of power.

However, rather than apertures, it is more convenient to use the antenna gains (directivity)

The Friis Equation Calculator.

Friis Diagram

Design Specification