Diffraction of Radio Waves

phenomena that occur when radio waves encounter obstacles. On encountering an obstacle during propagation in a homogeneous medium, a radio wave changes in amplitude and phase and penetrates the shadow zone, deviating from a straight path. This phenomenon is analogous to the diffraction of light. In cases of radio wave propagation in real media, the obstacles may be of arbitrary form and may be either opaque or translucent for radio waves.

The diffraction of radio waves on the spherical surface of the earth is one reason for the reception of radio signals beyond the limits of direct visibility when the transmitter and receiver are separated by the curvature of the earth. The effect of the diffraction penetration of a radio wave into a shadow zone (in the case of visible waves as well) depends on the ratio between the dimensions of the obstacle and the wavelength; the longer the wavelength, the more marked it is. On the other hand, radio waves propagating close to the semiconducting surface of the earth attentuate because of the partial absorption of the wave energy by the earth, the shorter the wave, the more strongly this occurs. Therefore, the range of propagation of a so-called ground wave essentially depends on its length. Sufficiently long waves may be propagated, owing to the diffraction of radio waves, over great distances, which sometimes reach several thousand kilometers.

The diffraction of radio waves by individual buildings and prominences in the terrain, such as mountains, that lie along the line of propagation can also play a useful role; it effects redistribution of the wave energy and may lead to “reinforcement” of the radio signal beyond the obstacle.

Diffraction plays a special role in the propagation of radio waves in media that contain local nonuniformities, such as the ionosphere, where a radio wave encounters a multitude of randomly located obstacles (for example, various kinds of clouds with various electric properties). These continuously changing and moving nonuniformities cause changes in the signal energy at the point of reception—the so-called diffraction fading of the radio wave.

Diffraction phenomena may be significant in the emission of radio waves by directional antennas and in the radar detection of complex objects.