What is the primary type of radio signal propagation that requires a direct path between transmitter and receiver?

The primary type of radio signal propagation that necessitates a direct path between the transmitter and receiver is line of sight. This mode of propagation relies on the fact that electromagnetic waves travel in straight lines and are not significantly affected by the atmosphere when the distances involved are relatively short.

In line of sight communication, the transmitter must have a clear path to the receiver, meaning there should be no obstacles, such as buildings or hills, that could interfere with the signal. This is crucial for maintaining a strong and reliable connection, which is particularly important in applications like microwave transmissions and some types of satellite communications.

Other types of propagation, such as ground wave, can bend around obstacles, and sky wave reflects off the ionosphere, allowing signals to reach beyond the visual line of sight. Non-line of sight propagation may involve transmission paths that are indirect and can rely on reflection or diffraction, which do not apply to the strict requirements of a line of sight setup. Thus, the nature of line of sight propagation is distinct and identifies an essential aspect of radio communication technologies.