A direct radio wave is a wave that propagates directly from the source to the receiving point. In a homogeneous or slightly inhomogeneous medium, radio waves reach the receiving point along straight or nearly straight trajectories. Such waves are called direct waves.
Radio communication via a direct wave (or direct beam) is possible only when there is direct (or geometric) line-of-sight between the antennas of the correspondents, as in the case of a radio link with an aircraft or an artificial Earth satellite. When the antennas of the correspondents are located near the Earth's surface, the range of direct visibility is limited by the Earth's curvature and surface irregularities and amounts to several tens of kilometers. This is the propagation mode for very high frequency (VHF) waves. For communication over greater distances, radio waves reach the receiving point due to refraction, diffraction, reflection, and scattering. These phenomena are caused by the influence of the Earth's surface, inhomogeneities in the troposphere (the lower layer of the atmosphere, up to 15 km thick), and the ionosphere (the ionized region of the atmosphere at altitudes of 60–1000 km).
The most effective way to achieve the greatest range for direct wave propagation is to increase the height of the antenna suspension.
Related articles: Very high frequency (VHF, ultra-short waves), Centimeter waves, super high frequencies (SHF), HF, Frequency range, Ground waves, Ionospheric waves.