Diffraction is the spatial and temporal redistribution of a wave field when radio waves encounter obstacles. Such obstacles may include irregularities within volumetric resonators, inconsistencies in coaxial and waveguide transmission lines, elements of receiving and transmitting antennas, inhomogeneities (disturbances) in the atmosphere, the Earth's surface and its irregularities (mountains, trees, buildings, sea waves, etc.), as well as aircraft, satellites, and other objects.
When propagating in a homogeneous medium and encountering an obstacle, a radio wave changes in amplitude and phase and penetrates into the shadow region, deviating from a straight-line path.
The effect of diffraction penetration of a radio wave into the shadow region depends on the ratio between the size of the obstacle and the wavelength, and is more pronounced the longer the wavelength. On the other hand, radio waves propagating near the semiconductive surface of the Earth attenuate due to partial absorption of wave energy by the Earth, with the attenuation being stronger for shorter wavelengths. Therefore, the range of propagation of an earth wave is significantly dependent on its wavelength. Diffraction plays a special role in the propagation of radio waves in media containing local inhomogeneities, for example, in the ionosphere, where the radio wave encounters numerous chaotically distributed obstacles — clouds of various shapes differing in electrical properties. Continuous changes and movements of these inhomogeneities cause variations in signal energy at the reception point — the so-called diffraction-induced radio wave fading.
Related articles: Radio wave interference, Signal fading, Interference noise.