High frequency

The field of the High frequencies ( High Frequencies in English, shortened in HF ), indicates the waves radio whose Fréquence lies between 3 MHz and 30 MHz. They are also called “decametric waves”, i.e. them Wavelength lies between 10 and 100 Mètre S. the term “short waves” is also often used, though less precis within its limits.

History and topicality

Until the medium of the Years 1920, the range of the short waves was neglected by the authorities and the scientists with the profit of the lower frequencies whose propagation is more stable. Following work of the Radioamateur S of the time which carried out connections around the sphere, the development of the use of the decametric spectrum was fast for the Radiodiffusion and the official traffic.

The high frequencies were used a long time for many uses: Military S, maritime, air, and diplomatic. Currently, with the development of the connections by satellite and terrestrial relays, the waves HF are given up little by little by the official services and the Radiodiffusion. They remain however impossible to circumvent for the maritime and air services in particular to guarantee the safety of the oceanic connections, for the fixed or mobile connections in zones without infrastructure, or in help in the event of natural disaster.

The wavebands are allocated by ITU. Broadcasting accounts for 13% of the spectrum, the sea traffic 20%, the air traffic 10% and them bands amateurs 12%, the remainder of the spectrum is used by fixed or mobile services.

Use

Broadcasting

Broadcasting in short waves uses specific bands between 2Mhz and 25Mhz, located by their historical denomination wavelength (example: " bandage 41m"). Each program is emitted on several frequencies for permettere its reception according to the propagation. From the simplicity and the low costs of the receivers, it is used mainly like bond with the expatriates, like media of influence or cultural radiation.

Governments and services

The fixed communications or mobiles in high frequencies are used like means of connection of help by the embassies, the news agencies, the organizations first-aid, the soldiers, etc. like like bond with the points isolated not - covers by ground networks or satellite: isolated polar regions, zones. Their advantage is autonomy with respect to the infrastructures, in case for example of conflict or catastrophe.

Maritime

For the sea traffic, the connections HF use bands distributed on the spectrum, the band of 1,6Mhz with 3Mhz out of the spectrum “HF” strict, is often called MHF (between average and high frequency!), or “bandages marine”. The frequency 2,182Mhz is the frequency of call and international maritime security. The communications are more and more numerically, but BLU remains used for the specific safety and contacts. Many coastal stations transmit the evolution of the weather reports and circulation and ensure the communications of the ships with the ground.

Aeronautics

These frequencies are used for the flights long mails, but the use is very difficult. The range being several thousands of Kilometer S, the collected Communication S are very numerous. Certain phenomena, like the solar cycles, can make any communication impossible during several hours, that is to say thus, on thousands of kilometers.

Thus, all the links, for example, between Paris and New York make the object of an air control ensured by voice via regional centres on both sides of the Atlantic (Shanwick, Santa Maria, Gander…). Many stations VOLMET continue to provide weather forecasting for the majority of the large airports of the various continents.

To make this technology exceeded less painful, a system of call SELCAL, emitting a light signal and sound, makes it possible the pilot to be informed call of the station ground, and thus to inform him to establish the radio operator contact.

Amateurs

The Radio ham S and the Cibiste S continue to use the high frequencies for contacts with long distances (often intercontinental).

Mode of propagation

The decametric waves are propagated by reflection S successive between the ground or the sea and the layers E and F (F1 and F2) of the Ionosphère and by the wave of ground. They can thus be received at a long distance from the transmitter, even in the presence of obstacles (relief) or even when the curve of terrestrial surface prevents a connection in direct sight between the transmitting station and the receiving station.

The wave propagation HF thus depends strongly on the reflection on the layers of the Ionosphère. If this one is more ionized by the solar radiation, it ensures a better decametric wave propagation.

Also alternation day-night has it a notable influence on the quality of propagation decametric waves, as well as the solar Cycle.

Antennas

Public uses

The materials used by the public in these frequencies have specific assignments:

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