What propagation phenomenon occurs when radio signals follow the curvature of the Earth?

The correct response identifies groundwave propagation as the phenomenon where radio signals follow the curvature of the Earth. Groundwave propagation is particularly effective at low frequencies, such as those used for AM radio broadcasting, enabling signals to travel over the horizon by bending around the Earth's curvature due to its interaction with the surface of the Earth.

This effect is influenced by several factors, including the frequency of the signal and the conductivity of the ground. A conductive surface enhances the groundwave's ability to propagate, allowing the signal to maintain strength over long distances. This is vital for communications that require coverage over large areas, especially in situations where line-of-sight transmission isn't possible due to obstacles or the Earth’s curvature.

In contrast, line of sight propagation refers to the condition where the transmitting and receiving antennas must be directly visible to each other, making it ineffective over the horizon. Skywave propagation involves the reflection of radio waves off the ionosphere, allowing them to travel large distances but not through the Earth's curvature like groundwaves do. Tropospheric scattering occurs when signals are scattered by irregularities in the atmosphere, which can also extend range but is dependent on weather conditions and is not a direct function of following the Earth's curvature.