Radio-wave propagation

(electromagnetism) The transfer of energy through space by electromagnetic radiation at radio frequencies.

The means by which radio signals are transported through space from a transmitting antenna to a receiving antenna. See also Radio.

The frequencies around 20 kHz can be received reliably at distances of thousands of miles but are limited to telegraph-type signals and require very large transmitting antennas. Higher frequencies are needed for voice, and still higher frequencies for television transmission. As the frequency increases, the transmission range tends to decrease. Frequencies above 100 MHz can transmit wide-band signals, but they are limited to approximately line-of-sight distances with the usual type of equipment. However, distances of 200 mi (320 km) or more are possible by the use of high power and large antennas to provide narrow “searchlight” beams.

Reflections from the ionosphere (ionized layers 50–250 mi or 80–400 km above the Earth's surface) provide a useful but variable long-distance service at frequencies less than about 30 MHz. These reflections account for the long-range broadcast coverage at night and for the shortwave intercontinental communication. See also Radio broadcasting.

The principal components of the received radio signal are shown symbolically in the illustration. The vector sum of the direct, reflected, and surface waves has been called the space wave, ground wave, or tropospheric transmission to differentiate it from the ionospheric reflections. The ionospheric and surface waves are the principal components at frequencies below 10– 30 MHz. The direct and reflected rays are the principal factors at frequencies above 30–50 MHz. Although the ionospheric, direct, and ground-reflected waves can be easily visualized as rays, the surface wave is more difficult to understand; it originates at the air-Earth boundary because the Earth is not a perfect reflector.

Possible transmission paths between antennas.

Variations in signal level with time are caused by changing atmospheric conditions. The severity of the fading usually increases as either the frequency or path length increases. Most fading is temporary diversion of energy to some direction other than the intended location, associated with refraction or interference, but absorption effects are important in the microwave region.

The dielectric constant of the atmosphere normally decreases gradually with increasing altitude. The result is that on the average the radio ray is bent or refracted toward the Earth so that the distance to the radio horizon is slightly greater than to the optical horizon. The amount of refraction is variable, and exceptionally long-range transmission may occur occasionally. Conversely, when the radio energy is bent away from the Earth (upward bending), the transmission loss is increased. See also Refraction of waves.