Radioelectric Antenna - SpeedyLook encyclopedia

Principles of operation

All the radioelectric antennas, in particular the doublet , are electric Dipôle S behaving like resonant circuits.

Waveband of use

The Fréquence of Résonance of an antenna depends initially on its dimensions clean, but also on the elements which are added to him. Compared to the frequency of central resonance of the antenna, one tolerates a weakening of 3 decibels, weakening which determines the frequency minimum and the maximum frequency of use; the difference between these two frequencies corresponds to the Band-width.

Especially in reception, it is frequent that an antenna is used largely apart from its band-width. it is the case of the antennas of car radio whose frequency of resonance is often at more than 200 MHz and which one uses for the listening of the band of broadcasting “FM” (band of the Ondes Short Extremists, band OUC) towards 100 MHz, even the range of the long waves not exceeding a few hundreds of Kilohertz with a wavelength about the Kilomètre.

Certain antennas known as multibandes can function correctly on discontinuous segments of waveband without particular device. Others require the use of a circuit Transformer aerial matching to function correctly.

Polarization

The polarization of an antenna is that of the Electric field E of the wave which it emits. A horizontal dipole half-wave thus has a horizontal polarization. Certain antennas have an elliptic or circular polarization like the antenna propeller or theYagi one whose plans are perpendicular. The fact of using two antennas of different polarizations to carry out a connection introduces important additional losses.

See also the polarization and the transmitting television

Directivity and diagram of radiation

The isotropic Radiator, i.e. radiating in the same way in all the directions, is an unrealizable ideal model in practice. Actually, the energy radiated by an antenna is distributed unequally in space, certain directions being privileged: they are the lobes of radiation. The diagram of radiation of an antenna makes it possible to visualize these lobes in three dimensions, the horizontal plane or the vertical plan including the most important lobe. The proximity and the conductibility of the ground or the conducting masses surrounding the antenna can have an important influence on the diagram of radiation.

Directivity

The directivity of the antenna in the horizontal plane is an important characteristic in the choice of an antenna.

a équidirective antenna or omnidirectional rayon in the same way in all the directions of the horizontal plane.

a directing antenna has one or two lobes definitely more important than the others than one names principal lobes . It also comprises secondary lobes which one tries to minimize. It will be all the more directing as the most important lobe will be narrow. If the collected radio station is not always in the same direction it can be necessary to direct the antenna, manually or by making it turn with an engine of positioning.

The antennas of satellite continuation of S are directional in Azimut (direction in the plan Horizontal) and in site (Height above the horizon). The Radiogoniométrie uses directing antennas to determine the direction of a transmitter. The Radar S use antennas whose owner of emission and reception is highly directional.

In the same way certain antennas specific to the reception are they also sensitive to the Orientation: the antennas “tallies” granted by a Capacitator variable or, those consisted of a bar of Ferrite around whose a coil is wound. They are directing, their use requires a correct orientation, (axis of winding parallel with the magnetic field of the electromagnetic wave).

Profit

The profit of an antenna compared to the isotropic radiator is what characterizes the principal lobe. It is due to the fact that energy is focused in a direction, as the luminous energy of a candle can be concentrated thanks to a Miroir and/or a convergent lens. It is expressed in dBi (Décibel S compared to the isotropic Radiator). For an antenna, the mirror can be made up by an element reflectors (plane or parabolic screen) while a directing element (in an antenna Yagi, for example) will play the part of the lens. Measurements on the antennas are taken in free space or anechoic Chambre.

A direction where the profit is weak can be made profitable to eliminate an awkward signal (in reception) or to avoid radiating in an area where there could be interference with other transmitters.

Forms and dimensions

The shape and dimensions of an antenna are extremely variable: that of a cellphone is sometimes invisible because inside the case or limits itself to a small outgrowth on the case of the apparatus while the parabola of the Radiotélescope of Arecibo exceeds 300 m in diameter. Very coarsely one can say that for the same frequency of use, dimensions of an antenna will be all the more large as its profit will be high, because of the use of reflecting elements like that of the parabolic aerial, for example.

The antenna half-wave or doublet , as its name indicates it, has a length almost equal to half the wavelength for which it was manufactured.

The electric length of the element of an antenna can be increased while inserting a reel in series with the discussion thread or adding a reinforcement of condenser at the end of this one.

Types of antennas

The basic antenna is the dipolar Antenne. A good part of the other types of antenna (but not all) are variations or of combination of dipoles.

There exist tens of the types of antennas, different by their operation, their geometry, their technology…
Some examples:

Antenna of interior for TV, apartment or attic TV;
aerial umbrella or in tablecloth for kilometric waves;
Antenne buckles various forms, round in theory, but with parts being able to be rectilinear), vertical or horizontal;
dipolar Antenna , basic telegraphic antenna;
telegraphic antenna doublet for decametric waves;
Yagi-uda Antenna with parasitic elements, very directive and with important profit. It is the rake used for the reception of the analogical or numerical TV;
antenna quarter of vertical wave omnidirectional for very high frequencies ( THF or VHF);
antenna curtain or colinéaire with very marked directivity;
Antenne tallies magnetic , of reduced size, often comprising a bar of Ferrite;
dielectric radiator or by waves of surface ;
Antenna propeller for decimetre waves, very directive;
parabolic Aerial usable with the top of GHz (ultra high frequencies) Wifi, ISM, transmissions by satellite, bands S, C, Ku, Ka;
Antenna with slits Wifi applications, ISM, bands S, C, Ku, Ka;
Antenna quad applications Wifi, ISM band of the 2.4 GHz, TNT short waves Band L (antenna cubical quad )
Antenna horn starting from the band S;
Antenna horn-funnel starting from the band S;

Antenna patch from 1.2 GHz;

dihedral Antenna , from 1.2 GHz;

isotropic Radiator , an ideal model which is used as reference;

network of antennas , antenna formed by a network of antennas;

Antenne network with ordering of phase which is made of a group of independent transmitters that one can feed in a variable way so that the total owner of emission is directional without having to move the antenna. Multiple military, civil and space uses of which the radars with electronic sweeping;

Antenna flexible whip bit ¼ or ½ wave especially for the VHF, e.g.: CB and UHF (e.g.: TNT).

Note: in the field of the general public, the antennas for the reception of analogical television TAT and numerical (TNT) as well as FM broadcasting terrestrial, are more known under the usual name of antenna “rake” (in fact of the Yagi-uda antennas VHF or UHF). The antenna known as parabola like, more rarely, the Antenna punt, are intended for the Satellite television, telecommunications, the radars… Television by MMDS (rare) requires antennas Yagi or parabolic standard.

Mode of food

The antenna is generally deployed outside, even fixed at the top of a mast. To forward to the antenna the high frequency energy provided by the transmitter or in opposite direction to bring the signal collected by the antenna to the entry of the receiver, one uses a two-wire line or a coaxial cable. To obtain an optimal operation, the impedance at the point must be of the same order as the impedance characteristic of the feeder. The order of magnitude of the impedances met is of a few tens (50 or 75 ohms for the coaxial cable) and a few hundreds of Ohm S (300 ohms for a two-wire line). In addition to the adaptation of the impedances, it is desirable to supply a symmetrical antenna (like the doublet half-wave using a symmetrical line (like the Two-wire line) and an asymmetrical antenna like the vertical antenna (ground-plane Antenne) with an asymmetrical line: a Coaxial cable, for example. To pass from a symmetrical line to an asymmetrical line, one often uses a Balun.

On centimetric and shorter waves, one uses waveguides, kinds of tubes of rectangular or elliptic section in which the waves circulate. Certain antennas, like the parabolic aerials of reception of satellite television, incorporate an electronic device of amplification and conversion of the received frequency.

Use in emission

Generally, an antenna can be used as well in emission as in reception. However certain antennas used in reception have a very weak output in emission (Beverage antenna) or could not support a power of important emission because of the losses or too high overpressures which could deteriorate them.

Reception antennas

The electric field of an electromagnetic wave induced a tension in each small segment of any electric driver. The induced tension depends obviously on the value of the electric field and length of the segment. But the tension also depends on the orientation of the segment compared to the electric field.

These small tensions induce currents and these currents which circulate cross each one a small portion of the impedance of the antenna. The result is that the diagram are equivalent of Thévenin of an antenna is not immediate.

By using the Théorème of reciprocity one can show that the diagram are equivalent of Thévenin of a reception antenna is the following:

$V_a= {\ sqrt {R_aG_a} \, \ lambda \ cos \ psi \ over \ sqrt {\ pi Z_0}} E_b$

$\ scriptstyle {V_a}$ is the tension of the equivalent diagram of Thévenin.
$\ scriptstyle {Z_0} = \ sqrt {\ mu_0 \ over \ varepsilon_0} =377 \ Omega$ is the intrinsic impedance of the vacuum.
$\ scriptstyle {Z_a}$ is the impedance of the equivalent diagram of Thévenin and is equal to the impedance of the antenna.
$\ scriptstyle {R_a}$ is resistance series of the impedance of $\ scriptstyle {Z_a} \,$ of the antenna.
$\ scriptstyle {G_a}$ is the profit of the antenna (the same one as in emission) in the direction from which come the electromagnetic waves.
$\ scriptstyle {\ lambda}$ is the wavelength.
$\ scriptstyle {E_B}$ is the electric field of the incidental electromagnetic wave.
$\ scriptstyle {\ psi}$ is the angle misalignment of the electric field with the antenna. For example, if the antenna is a dipole, the induced tension will be maximum when the electric field is aligned with the driver. If it is not the case, and that they form an angle of $\ scriptstyle {\ psi}$ , the induced tension will be multiplied by $\ scriptstyle {\ cos \ psi}$ .

The equivalent diagram and the formula on the right are valid for any type of antenna. It can be a dipolar Antenne, a parabolic Aerial, a Yagi-uda Antenne or a network of antennas.

Here three definitions which speak about themselves:

\begin{matrix}
\ mathit {effective Length \ \ of \ the antenna} &=& \ textstyle \ \
 & & \ \
\ mathit {maximum Power \ \ available} &=& \ textstyle \ \
 & & \ \
\ mathit {effective Surface \ \ or \ effective section \} &=& \ textstyle \ \
\end{matrix}

The corollary of these definitions is that the maximum capacity that an antenna can extract from an electromagnetic wave depends only on the profit of the antenna and square of $\ scriptstyle {\ lambda}$ .

Mutual impedance and interaction between antennas

The current which circulates in each antenna induced of the currents in all the others. As well if the others are fed or not. One can postulate a mutual impedance which will play the same part in the antennas as the mutual inductance in the coupled reels. The mutual impedance $\ scriptstyle {Z_ {12}}$ between two antennas is defined like

\ textstyle {Z_ {12} = {v_2 \ over i_1}}

where

\ textstyle {i_ {1}}

is the current one which circulates in antenna 1 (inductive) and

\ textstyle {v_2}

is the tension which it would be necessary to apply to antenna 2 (induced) - with removed antenna 1 - to obtain the same current in antenna 2 as that induced by the current of antenna 1.

With this definition, the whole of currents and tensions of a whole of antennas are connected by the system of equations according to:

\ begin {matrix} v_1&=&i_1Z_ {11} &+&i_2Z_ {12} &+& \ cdots &+& i_nZ_ {1n} \ \

v_2&=&i_1Z_ {21} &+& i_2Z_ {22} &+& \ cdots&+&i_nZ_ {2n} \ \ \ vdots & & \ vdots & & \ vdots & & & & \ vdots \ \ v_n&=&i_1Z_ {n1} &+&i_2Z_ {N2} &+& \ cdots&+&i_nZ_ {N} \ end {matrix} where:

$\ scriptstyle {v_i}$ is the tension applied to the antenna $\ scriptstyle {I}$
$\ scriptstyle {Z_ {II}}$ is the impedance of the antenna $\ scriptstyle {I}$
$\ scriptstyle {Z_ {ij}}$ is the mutual impedance between the antennas $\ scriptstyle {I}$ and $\ scriptstyle {J}$

Notice that, as for mutual inductances:

\ scriptstyle {Z_ {ij} \, = \, Z_ {ji}}

If some elements are not fed (there is a short-circuit in the place of the electric cable), as it is the case in the antennas Yagui-Uda the corresponding

\ textstyle {v_i}

are zero. These elements receive the unjust name of parasitic elements . In certain geometrical configurations, the mutual impedance between two antennas can be zero. It is the case for dipoles laid out in cross (in the antennas with emission or circularly polarized reception). These null impedances facilitate calculation and especially, the realization.

Mechanical realization

According to whether an antenna is intended for the reception of television large-public or a telecommunications satellite, the quality (and the cost) of the realization will not be the same one. The wind resistance and to the bad weather must be particularly neat to obtain a great reliability and stability, it is the parabolic case of the antennas with reflectors. In altitude it is not rare that an antenna is coated with ice, the elements must support this overload without becoming deformed. To avoid the problems of oxidation and water infiltration, the fed elements are often protected by an insulating case. A Radôme is an impermeable protective shelter used to protect an antenna.

See too

External bonds

Books concerning the design of antenna

Be-X-old: Антэна Simple: Antenna

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