Ionospheric Propagation

One invites ionospheric propagation the property of the electromagnetic waves to traverse distances larger than the simple line of sight, by reflection on the ionized layers of the atmosphere.

The ionospheric propagation depends on the hour and the solar cycle.

All those which have a radio equipped with the small waves (PO) or (SW) could note this phenomenon: of day, few audible stations; on the other hand at night one can hear much more stations.

Description of the phenomenon

The wave propagation radios is influenced by certain high layers of the atmosphere, of which the Ionosphère, located between 10 and 150 km.

The ionization of the air by the solar particles electrically charged gives him its name.

Ionization varies according to the hour of the Journée, the Saison, and according to one approximate duration a solar cycle from 10 to 11 years. The last peaks took place in 1981, 1991 and 2001.

The ionosphere is made up of several underlayers identified with the letters of the alphabet. Most important are D, E, F1, and F2, these two last become only one, the layer F, during the night.

the layer D absorbs much the bands in lower parts of 10 MHz. It appears at dawn and disappears the night. The bands in lower parts of 10 MHz are called bands of night , because it is during the night, at the time of the absence of the layer D, which they can reach the layer F. Its behavior, or rather its disappearance, explains all these radio stations which appear out of PO and OM the night.
the layer E, unstable and erratic, is excellent reflectors, but because of its unstable character it cannot be used for reliable communications. It is responsible for the propagation of the high bands and the VHF, known as " sporadic Propagation E ".
the layers F1 and F2, become layer F the night, are also excellent reflectors. They allow the reflection of the low layers during the night, and of the high layers during the day.

Technological applications of the ionospheric propagation

Thanks to the ionosphere, the Ondes radios can rebound on certain layers of the ionosphere, typically the layers E, F1, and F2, and thus traverse very long distances.

In addition, the waves radios can be absorptive by these layers ionized in certain cases.

The Fréquence of emission influences the ionospheric propagation much.

the low frequencies, lower than 30 MHz are supported by this phenomenon,
the high frequencies, higher than 30 MHz are not concerned, however that can arrive but in a more sporadic way, often with the rising and laying down sun.

The ionospheric propagation makes it possible to carry out contacts to thousands of kilometers with a power of emission lower than 100 W, to see 10 W and 5 W. the waves move then by successive jumps between which zones of Silence S are created, it is then possible to hear a station located at 2000km but another not located at 200km.

However that is especially possible in summer, or at the time of a peak of solar activity.

The Radioamateur S use this phenomenon on their frequencies; most adequate are those of 3.5MHz, 7MHz, and 14MHz, where the propagation is with go throughout the year and even to the hollow of the solar cycles.

See also the site of Denis Auquebon F6CRP, in the chapter of the Course of Radioelectricity dedicated to the Propagation.

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