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Jean-Pierre BLOT
EUROSATCOM, Les Ulis, France
In radioelectricity, an antenna (using the IEEE Standard 145-1993 definition2 ) is a device which makes it possible to radiate (transmitter) or to collect (receiver) electromagnetic waves in an efficient way. Heinrich Hertz (1857-1894) used antennas for the first time in 1889, to demonstrate the existence of electromagnetic waves, predicted by James Clerk Maxwell's theory. He used doublet antennas for both reception and transmission. He even installed a dipole transmitter at the focal point of a parabolic reflector. The work and drawings of the invention were published in the Annalen der Physik und Chemie3. The term "antenna" was first used by Guillermo Marconi (1874-1937).
Today, the technological evolution of antennas is inseparable from that of satellites. It was during the Second World War that intensive development was undertaken in this field for radar, and is behind many technologies used today. It was with the invention of triode tube generators, in the 1920s, then magnetrons and klystrons, operating at 1 gigahertz for radar, that it all started: antennas with reflectors, lenses, networks of slotted waveguides, wire antennas, etc. It gained a new impetus with the advent of space exploration, which started with the launch of Sputnik in 1957, and which did not stop evolving with the demand for new telecommunications services, which were first fixed, then mobile, with geostationary satellites that were initially passive and then active and low-orbiting.
The evolution of new technologies, such as reconfigurable printed antennas, has made it possible to produce satellites to travel towards high, medium, and low orbits (Galileo, Iridium, Global Star, etc.). These new-generation satellites are light and less expensive and launched and in batches, by launchers which are also lighter. At the same time, demands for new frequency bands are gradually moving towards higher and higher frequencies.
The possibilities offered by the propagation of electromagnetic waves (EM) in the natural environment are utilized for multiple purposes, including civil and military purposes: broadcasting, television, radar, telecommunications, radio navigation, etc. In all these applications, the antenna is the essential component for radiating and capturing waves. They were, therefore, particularly suitable for radiating wire structures, used by the pioneers of radiotelegraphy since the end of the 19th Century. Since then, antenna technologies have evolved with the multiplication of communication needs, and have diversified to take into account both the discovery of new terrestrial propagation modes as well as constant technological progress towards increasingly higher frequencies.
The antenna designation prevails over a large part of the electromagnetic wave spectrum, which ranges from wavelengths in kilometers to submillimeter wavelengths which are close to infrared (Table 1.1). At higher wavelengths, in the field of optics, the universal term of "antenna" is not used, allowing for a more precise denomination of the various constituent elements (lenses, for example). The field of antennas concerned here is limited technologically, and in an internationally accepted manner, to that of the classification and main characteristics of the electromagnetic waves given in Table 1.1: we will discuss waves and microwaves below infrared, and radiation above this range that is no longer the field of telecommunications. Technologies in the field of optics will be covered in more specific works on photonics.
The two aspects of antenna function, transmiting and receiving, are closely related. It is the electronic equipment, associated with an antenna, that defines its possible specificity. In almost all cases, the same antenna is used for transmission as for reception. This is a consequence of the reciprocity theorem. In some cases, when the antennas contain non-linear materials, they are not reciprocal. Because of the reciprocity, there will hardly ever be a difference between the radiation in emission or reception. The qualities stated for an antenna below will be so in both operating modes, without this being specified.
For its purpose, the radiation of an antenna is reduced, by its illumination, its emitted power, and the sensitivity of its receiver, to a problem of covering the surrounding or geographical space at small, medium, or large distances: this may mean covering the surrounding space evenly, or a very localized area in this space. Such a concern allows the first functional classification, to distinguish antennas with little or no directionality from directional antennas..
Quickly, with the multiplication of applications, be they spatial, terrestrial or wireless, rules for coexistence, coverage and frequency management need to be established. Frequency may physically be the opposite of time (? = 1/t), it is an economically rare and exhaustible good that cannot be produced. In addition, it is a heterogeneous good; the "low" frequencies (below 1 gigahertz, see Figure 1.1) have a use value that is higher than that of the "high" frequencies, because their property of good propagation allows for less expensive coverage in lightly-populated areas and better penetration into buildings in urban areas. To best manage the scarcity and heterogeneity of the hertzian spectrum, it should be allocated in the most dynamic and flexible way possible.
Firstly, it should be allocated in a dynamic way because the sustained innovation and technological progress that drive the electronic communications sector, as well as the rapid growth of the markets, transform and renew the range of spectrum use. Some technologies are expected to decline and die out in the medium to long term, such as the GSM or analog television, while others emerge or develop in favor of everything digital, such as UMTS, WiMax, or TNT.
Table 1.1. Official ITU nomenclature of frequency bands
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