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Skywave is the propagation of electromagnetic waves bent (refracted) back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station (or during sporadic E season, a low band TV station) can sometimes be heard as clearly as local stations. (This is distinct from "groundwave" propagation, which is direct from transmitter to radio). Most long-distance HF radio communication (between 3 and 30 MHz) is a result of skywave propagation. Since the early 1920s amateur radio operators, limited to lower transmitter power than commercial radio, have taken advantage of skywave for long distance or DX communication.

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Role of the ionosphere

The ionosphere is a region of the upper atmosphere, where neutral air is ionized by solar photons and cosmic rays. When radio waves reach the ionosphere at oblique incidence they are bent downwards (refracted) in the ionized layer. If, compared to the wave frequency, the midlayer ionisation is important enough, the ray leaves the bottomside of the layer earthwards as if it were reflected from a mirror. If the ionization is not great enough the wave path is first bent downwards, but then (above the layer peak) upwards so that it leaves the topside of the layer with some displacement only. Skywave propagation occurs in the waveeguide formed by the reflectors ground and ionosphere. With a one hop path distances up to 3500 km may be reached. Transatlantic connections are mostly obtained with two or three hops.[1]

The structure of the ionospheric plasma is disturbed particularly by gravity waves so that the surfaces of equal ionization may be wavy. When signals have "bounced" off this irregular surface, they may fade in and out and have the "phasing", "flanging" or "fluttery" character familiar to listeners of shortwave music broadcasts.

Depending on the transmitting antenna, signals below approximately 10 MHz during the day and 5 MHz at night may reach the ionosphere at a steep angle (vertical incidence) and be reflected straight back down to Earth. Alternately, if the antenna "aims" the signal close to the horizon; the signal reaches the ionosphere at a shallow angle and returns to Earth at a medium to long distance.

The Earth's surface (ground or water) reflects the incoming wave back toward the ionosphere again. As a result, like a rock "skipping" across water, the wave may actually "bounce" or "skip" between the earth and ionosphere two or more times (multihop propagation). Since at shallow incidence losses remain quite small, signals of only a few watts can sometimes be received many thousands of miles away as a result.

  1. ^ K.Rawer:Wave Propagation in the Ionosphere.Kluwer Acad.Publ.,Dordrecht 1993. ISBN 0-7923-0775-5.
Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including radio waves.

Other considerations

VHF signals with frequencies above about 30 MHz usually penetrate the ionosphere and are not returned to the Earth's surface. E-skip is a notable exception, where VHF signals including FM broadcast and VHF TV signals are frequently reflected to the Earth during late Spring and early Summer. E-skip rarely affects UHF frequencies, except for very rare occurrences below 500 MHz.

Frequencies below approximately 10 MHz (wavelengths longer than 30 meters), including broadcasts in the mediumwave and shortwave bands (and to some extent longwave), propagate most efficiently by skywave at night. Frequencies above 10 MHz (wavelengths shorter than 30 meters) typically propagate most efficiently during the day. Frequencies lower than 3 kHz have a wavelength longer than the distance between the Earth and the ionosphere. The maximum usable frequency for skywave propagation is strongly influenced by sunspot number.

Skywave propagation is usually degraded—sometimes seriously—during geomagnetic storms. Skywave propagation on the sunlit side of the Earth can be entirely disrupted during sudden ionospheric disturbances.

Because the lower-altitude layers (the E-layer in particular) of the ionosphere largely disappear at night, the refractive layer of the ionosphere is much higher above the surface of the Earth at night. This leads to an increase in the "skip" or "hop" distance of the skywave at night.

See also

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