How does the power density of an electromagnetic wave change as it propagates away from an antenna in free space?

In free space, electromagnetic waves radiate outward in all directions from an antenna, forming an expanding sphere of energy. As the wave travels further away, this fixed amount of energy is distributed over a continuously growing surface area. Because the surface area of a sphere increases with the square of its radius, the energy concentration (power density) follows the inverse-square law. This means the power density decreases as the square of the distance from the antenna. For instance, if you double your distance from the transmitting antenna, the RF power density does not simply halve; it drops to one-quarter of its previous value. This rapid drop-off is a key concept in calculating safe distances for RF exposure.