As radio waves propagate through space, their energy remains constant. According to the law of conservation of energy, energy cannot be created or destroyed, only transformed or transferred from one form to another.
Radio waves are a form of electromagnetic radiation, which consists of oscillating electric and magnetic fields. These waves are generated by an oscillating current in an antenna or transmitter. Once generated, radio waves propagate outward from the source at the speed of light.
As radio waves travel through space, they spread out and become less concentrated. This spreading is known as "free-space propagation" or "spreading loss." The energy of the radio waves is distributed over an expanding sphere, resulting in a decrease in the power density (energy per unit area) as the distance from the source increases.
Mathematically, the power density of a radio wave decreases with the square of the distance from the source. This phenomenon is described by the inverse-square law. However, it's important to note that the total energy of the radio wave remains constant; it is simply spread out over a larger area as it propagates.
So, while the power density of radio waves decreases as they travel through space, the total energy carried by the waves remains the same. This is why radio signals from distant sources can still be received, albeit with lower intensity, as the energy of the waves is distributed over a larger area.