Free-space loss

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In telecommunication, free-space loss is the loss in signal strength (see discussion) that would result if all absorbing, diffracting, obstructing, refracting, scattering, and reflecting influences were sufficiently removed so as to have no effect on its propagation.

Note: Free-space loss is primarily caused by beam divergence, i.e., signal energy spreading over larger areas at increased distances from the source, and by the inverse square law of electromagnetic radiation.

The equation for free-space loss is below where <math>\lambda</math> is the signal wavelength, f is the signal frequency, R is the distance or radius of the signal from the transmitter, and c is the speed of light in the signal transmission medium. Note that the units used should be consistent, e.g., <math>\lambda</math> and R in meters, and c in meters per second.
<math>free\ space\ loss = \left ( \frac{4\pi R}{\lambda} \right )^2 = \left ( \frac{4\pi R f}{c} \right )^2</math>
Source: from Federal Standard 1037C and from MIL-STD-188