Friis transmission equation

Friis' original idea behind his transmission formula was to dispense with the usage of directivity or gain when describing antenna performance.

In their place is the descriptor of antenna capture area as one of two important parts of the transmission formula that characterizes the behavior of a free-space radio circuit.

[2] This leads to his published form of his transmission formula: where: Friis stated the advantage of this formula over other formulations is the lack of numerical coefficients to remember, but does require the expression of transmitting antenna performance in terms of power flow per unit area instead of field strength and the expression of receiving antenna performance by its effective area rather than by its power gain or radiation resistance.

is the wavelength representing the effective aperture area of the receiving antenna, and

[1] To use the equation as written, the antenna gains are unitless values, and the units for wavelength (

To calculate using decibels, the equation becomes: where: The simple form applies under the following conditions: The ideal conditions are almost never achieved in ordinary terrestrial communications, due to obstructions, reflections from buildings, and most importantly reflections from the ground.