Editing Radio propagation (section)

==Free space propagation==
{{further|Free-space path loss}}

In [[free space]], all [[electromagnetic wave]]s (radio, light, X-rays, etc.) obey the [[inverse-square law]] which states that the power density <math>\rho\,</math> of an electromagnetic wave is proportional to the inverse of the square of the distance <math>r\,</math> from a [[point source]]<ref name=Westman-1968/>{{rp|page=26‑19}} or:

:<math>\rho \propto \frac{1}{r^2}~.</math>

At typical communication distances from a transmitter, the transmitting antenna usually can be approximated by a point source. Doubling the distance of a receiver from a transmitter means that the power density of the radiated wave at that new location is reduced to one-quarter of its previous value.

The power density per surface unit is proportional to the product of the electric and magnetic field strengths. Thus, doubling the propagation path distance from the transmitter reduces each of these received field strengths over a free-space path by one-half.

Radio waves in vacuum travel at the [[speed of light]]. The Earth's atmosphere is thin enough that radio waves in the atmosphere travel very close to the speed of light, but variations in density and temperature can cause some slight [[refraction]] (bending) of waves over distances.