direct wave

Electromagnetic waves can propagate as ground wave, space wave or direct wave.

While the ground wave follows the curvature of the earth and the space wave is reflected at the ionosphere, the wave propagation of the direct wave is straight between transmitter and receiver. Their behavior is comparable to that of light beams.

Frequencies above 100 MHz show the behavior of direct waves with straight- line wave propagation. Since direct waves are virtually only transmitted via line-of-sight links, the distance can only be increased by antennas mounted on higher transmission towers due to the curvature of the earth.