Power
The power is expressed either in watts or so in relative units in decibels relative to milliwatts (dBm).
Correspondence between wattage (W) and power in decibels "milliwatts (dBm):
dBm: Watts:
(dBm= 10*log10(P ⁄ 0.001))
Loss in a coaxial cable at 2.45 GHz
Typical values for some common coaxial cables:
Choosing the type of cable :
Length (meters): DB loss (negative value)
Antenna
Antenna gain is normally given in decibels isotropic [dBi]. This is the power gain over an isotropic antenna (antenna radiating the same power in all directions .... a This antenna does not exist in reality !).
Some antennas have their gain given in [dBd], it is the gain relative to a half-wave (dipole). In this case it will add 2.14 dB for isotropic gain [dBi].
More antenna has gain more it is directive (energy in a preferred direction).
supplied antennas generally have a base of very low gain (2.14 dBi).
Antenna gain is the same at the reception and transmission.
Satellite TV
The parabolic reflector is frequency independent, it only affects the gain of the antenna. This means that you can re-use your satellite TV to WiFi.
higher the gain, the higher the directivity is high and therefore the larger the antenna must be pointed accurately.
The difficulty lies in the illumination of the dish. If the lighting is too large or too concentrated, there will be a loss of earnings
Here the maximum gain of a parabolic reflector according to its diameter and its frequency:
Frequency band:
Diameter of the antenna in meters: Maximum gain dB:
Radiated power
The radiated power (power emitted by the antenna) is calculated very simply in dBm:
Radiated power [dBm] = transmitter power [dBm] - cable loss [dB] + antenna gain [dBi]
The legal limit of radiated power (EIRP), set by OFCOM for WLAN 100mW ( + 20 dBm =). Similarly to most other countries.
Do not confuse this with the boundary limits for radiation in dwellings (DRB). The latter specifies that the electric field should not exceed 6 volts per meter in the home. It comes from fear of the effect of waves on health. In the case of WLAN there is very little risk of exceeding this value unless the fields are used to power amplifiers.
Free space loss
calculation of loss generated by the wave propagation in free space (without obstacles).
Correspondence between attenuation in decibels (dB) and distance in kilometers (km):
Frequency band:
Loss in dB (negative value !): kilometers:
(Friis formula)
Receiving Sensitivity
The receiver card has a low threshold of sensitivity to determine the minimum power to be received (the connector of the card) for a certain data rate. If the received power is below the threshold, the data rate will be reduced to regain acceptable performance. So it is advantageous to use maps with thresholds from receiver sensitivity as low as possible.
Signal ⁄ Noise
the reception sensitivity is not everything, it must also consider the power ratio SNR. This is the minimum difference between the signal power that is sought to be received and the noise (thermal noise, industrial noise due for example to microwave ovens, noise due to other WLAN working on the same band). It is defined by:
Signal to Noise Ratio [dB] = 10 * Log10 (Signal Power [W] ⁄ Power Noise [W])
If the signal is stronger than the noise, the signal to noise ratio (abbreviated as S ⁄ N) will be positive if the signal is buried in the noise report will be negative. To operate at a data rate, the system will need an S ⁄ N minimum:
If the noise level is very low, the system will be limited rather by the minimum sensitivity of receipt. By cons, if the noise level is high it is more the signal to noise ratio will matter rather than the receiver sensitivity for a given flow.
If the noise level is high, it will need more power received. Under normal conditions, without any WLAN on the same frequency, without industrial noise, the noise level is around-100dBm. Example: For a rate of 11Mbps with a orinoco will therefore require a signal of more than 16 dB (SNR) so -100 +16 = -84 dBm, but this level is below the receiver sensitivity minimum is -82 dBm, so it’s receiving sensitivity, which limits the system in this case.
Review link
The link budget is the computation of the entire chain of transmission is (without transmission constraints):
Issue [dBm]: Transmitter [dBm]-cable loss [dB] + antenna gain [dBi]
Propagation [dB]: Free space loss [dB].
Receive [dBm]: antenna gain [dBi] - cable loss [dB] - receiver sensitivity [dB]
Propagation: Fresnel ellipsoid
A quick and easy way to explain the role of Fresnel ellipsoids radio propagation is to see this as a "pipe" in which virtual moves the majority of energy between a transmitter and a receiver. This means that to avoid losses should not be an obstacle that ale in the area corresponding to the "pipe" (forbidden region). Indeed a barrier will disrupt the "flow of energy. " (Explanation really easy!)
Spread: Diffraction
when an obstacle between the transmitter and receiver, some energy will always happen to pass. This thanks to the phenomenon of diffraction on the top of the obstacle. The higher the frequency is high over the energy loss will be great.
Propagation: Polarisation
The polarization of a wave depends on the type of antenna used and its orientation (radiator) to the ground. For example, a whip antenna (telescope) will give a vertical polarization when placed vertically (|) and a horizontal polarization if the layer (--). The same is valid for an antenna Yago (|-|-|-|). The helical antennas produce a circular polarization.
Virtually the transmission antennas and reception should have the same polarization for best performance. (But as the polarization changes with diffraction and reflections, this rule is not always valid). Vertical polarization is preferred for long distance coverage because the effect of soil strongly attenuates the signal in the horizontal case from a distance.
A transmission system using circularly polarized antennas can mitigate the effect of reflections (eg principle used for GPS).
Reflections and spread over time (delay spread)
waves have the ability to reflect on the obstacles they face. At the reception they received while at the same time the direct wave and reflected waves. This causes cancellations at certain frequencies, but also a lag time between the different components which received the signal is spread over time.
The effect on the system is harmful and does decrease performance (transmission errors). To reduce this effect the receiver usually has an equalizer that balances these defects. This however has a limited capacity and manufacturers give limit values averaging time for a given error rate mimimum and function of data rate:
This shows that for high speed it is better to minimize the rate of reflections. The lag time of reflection is calculated easily knowing that the waves propagate at the speed of light (300,000 km ⁄ s):
Offset time [s] = length difference between direct and reflected path [m] ⁄ 300’000’000
Thus a time difference of 50 ns corresponds to a path difference of 15 meters. To minimize the rate of reflections must use directional antennas, have a direct view, pay attention to the evolution of the antenna. We may also use circular polarized antennas (helix antenna) that cancel fairly early reflections.
Reflections can also be caused within the entire coaxial cable-connector-antenna if they are poorly constructed and poorly adapted (low impedance, untuned antenna, standing waves) and therefore cause transmission errors.
note that the value of the averaging time is a calculation that takes into account the level and offset of each component:
Spreading time = sum over all components of { (component weight) * (offset component)}

polarization wave

Formation of the electromagnetic wave in a dipole
Two distinct electrical phenomena combine in a perfect dipole isolated in space to give rise to the formation of an electromagnetic wave:
currents flowing in the strands of the antenna produce a magnetic field around each conductor. At each location of the wire, the field lines grow in a plane perpendicular to the driver.
Potential differences between the two strands of the dipole cause the appearance of an electric field whose field lines are distributed intersecting planes in which the dipole is on the line of intersection.
If the currents and voltages were constant amplitude, there would be no formation of a radio wave. They are extremely fast changes and reversing each period currents and voltages in the strands of the antenna that make local vibrations of electric and magnetic fields propagate through space. Very high frequencies, propagation speeds of current and waves are not infinite, varying electric fields that cause the appearance of identical frequency magnetic fields and vice versa, a whole host of complex phenomena that contribute to expulsion to infinity of energy packets from the transmitter.
Plane wave
In a homogeneous isotropic space, the speed of propagation of a wave is constant and the attenuation distance is itself constant in all directions. After a few periods, the wave front (the start of the disturbance of electric and magnetic fields) has the shape of a sphere, like a giant soap bubble swelling at the speed of light. The radius of this sphere is so large that one can consider that over a limited area, the wave front is plane. This approximation of little consequence facilitates the understanding of what follows.
orientation of the electric and magnetic wave
Until a barrier does not disturb the movement of the wave, the orientation of electric and magnetic fields that make up the radio wave remain constant. If one takes a few hundred wavelengths from the antenna, it will be seen as a small dot in the center of the sphere representing the wave front.
Consider the surface of the sphere an area S 1 meter squared. Relative to a horizontal dipole considered, we notice that the surface S:
- the electric field lines are horizontal
- the magnetic field lines are vertical
Definition:
The polarization of a radio wave is the electric field that composes it.
The electromagnetic field near the antenna does not meet the same laws as the far field, located a few tens of wavelengths of the antenna. For example, in the far field the ratio between the amplitude of electric field and magnetic field amplitude is a constant. Thus simply measure to assess one another.
linear polarization and circular polarization
At time t, the electric field can be represented by a vector perpendicular to the direction of wave propagation. The magnetic field, too, is a vector perpendicular to the electric vector and perpendicular to the direction of propagation. If the direction of the electric field vector is constant (as above) the polarization of the wave is called linear. Some antennas radiate elliptically polarized waves, ie with the electric vector E rotates around the axis of propagation.
elliptical polarization can be right (if the vector rotates in a clockwise turning his back on the air) or left in the opposite direction. If the maximum amplitude of the electric field is the same regardless of its direction, polarization is called circular, a special case of elliptical polarization.
The actual polarization of a wave
In practice, the polarization of a radio wave does not stay long as the antenna that it has printed. The slightest reflection on a obstable affects and is found in finding that the VHF wave emitted by a tag with the antenna is vertically polarized vertically remains as the beacon is in sight or the signal is generally stronger polarization the horizontal and vertical polarization.
polarization rotation
Refraction and reflection are among the phenomena that can cause a change in the direction of polarization of a wave. In optics, the image of an object reflected in a mirror inclined at 45 degrees rotated 90 degrees. A vertical object is seen in the horizontal mirror.

Antenne Tesla

Voici an important aspect and often neglected Tesla.La resonator winds secondary of a Tesla resonator behaves like an antenna radio frequency.Under these conditions the electromagnetic waves move in the antenna and are considered in the reel (standing waves). Like shows it the following diagram:
On notes that the maximum potential will be acquired when the length of winding is equal to the distance covered by the wave during a quarter of the wave. Knowing the frequency of resonance of the resonator Tesla (Tesla Coil) one can deduce the length (L) from it:Distance Covered by the wave during a quarter of wave: l = CT ⁄ 4 (C being speed of light, and T the period) Concretely, to check if the secondary reel is quarter of wave, it is enough to know the length of wire and the frequency to which she works.Here an example drawn from our TC20:It is necessary to calculate the quarter of the time of period of the frequency considérée.100kHz gives T = 10us (f=1/T) that one divides by 4Le quarter of the wave is thus of 2,5us.This value is the time which makes it possible to traverse the total distance from wire at speed of light.In 2,5us the light traverses 750 metres.This value is almost our length of wire on TC20.La tension in the reel will be defined by:
Ux = U2max sin (π ⁄ 2 * x ⁄ h)
h represents the height of reel X represents the distance starting from the beginning of the U2maxreprésente reel the maximum tension with the sortie

Antenna curtain HF of telecommunication

Reception antennas of television

Assembly of an antenna of earth station
History
Heinrich Hertz used for the first time, in 1888, of the antennas to show the existence of the electromagnetic waves predicted by the theory of Maxwell. It used antennas doublet, both for the reception the emission. It installed even the transmitting dipole with the hearth of reflectors parabolic. Work and the drawings of the installation were published in Annalen der Physik und Chemie (vol. 36,1889). The antenna term was used by Marconi.
General theory
Very generally, a radioelectric antenna converts the existing electric quantities in a conductor or a line of transmission (tension and current) into electromagnetic fields in space (electric field and magnetic field), this in emission and conversely in reception. Into emission, the electric output is converted into electromagnetic power and it is the reverse in reception.
Operation in emission
emission of the elementary electric doublet

Geometrical diagram of an elementary antenna.
An elementary antenna in emission, also called electric doublet, consists of a small length Δl«λ of conductor (small in front of the wavelength λ) in which circulates a AC current I:
I = Ioejωt
In which ω = 2πƒ is the pulsation and ƒ is the frequency. J² = -1 is the imaginary number. (This notation, employing the complex numbers corresponds to the complex notation of the impedances).
That is to say a reference mark whose origin is placed in the centre of the antenna, and whose ordinate merges with the antenna, the field Εr,θ created by this elementary antenna in a point of polar coordinates (R, θ, such as R >> λ
Er,θ = (-jIοsinθ) ⁄ 2εοcr) * Δl ⁄ λ * ej(ωt - kr)
In which:
Εr,θ is the amplitude of the electric field at the point R, θ
εο is the dielectric permittivity of the vacuum.
C is speed of light in the vacuum.
R is the distance between the dipole and the point where the Εθ field is evaluated.
K is the number of wave, k=2π ⁄ λ
The remote electric field Εθ of the electromagnetic wave is coplanar with the conductor and perpendicular to the line which connects the point where it is evaluated with the conductor. If we imagine the antenna elementary in the centre of a sphere and parallel with the North-South axis, the electric field of the radiated electromagnetic wave will be parallel to the meridian lines and the magnetic field of the wave will have the same direction as the geographical parallels.
Such an elementary antenna does not exist. An unspecified real telegraphic antenna will be regarded as constituted by the juxtaposition of elementary antennas, and its characteristics will be obtained by the integration of the elementary fields, knowing that the characteristics of the current in each element are different in amplitude and phase. An antenna this time real, that one can thus build, is the antenna doublet half-wave, still called dipole half-wave.
If P is the power radiated by the elementary antenna, the electric field radiated in a point located at a distance R of this antenna, is maximum in a direction perpendicular to the conductor of the antenna (direction of the vector of Poynting). The maximum amplitude of this field is given by the relation:
E = √kP ⁄ r
E in V ⁄ m, P out of W, R in Mr. With K = 90 for an elementary electric doublet.
emission of the isotropic radiator
The isotropic radiator is an artificial aerial which would radiate the same field in all the directions. It is used as reference to define the gain of the antennas (see further) the preceding formula applies to find the amplitude maximum of the field E, by making K = 60 (one finds this formula by considering the flow of the vector of Poynting on the surface of a sphere of radius R).
If the effective field is considered, one will make K = 30.
emission of the dipole half-wave
If one considers the maximum amplitude of the field E in a mediating direction of the dipole, the preceding formula applies by making K = 98 (2,15 dB more than the isotropic radiator).
If the effective field is sought, one will make K = 49
Operation in reception
The electric field of an electromagnetic wave induced a tension in each small segment of any electric conductor. The induced tension depends 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 to calculate.
By using the theorem of reciprocity one can show that the diagram are equivalent of Thévenin of a reception antenna is the following:

Vα =(√RαGαλcosψ) ⁄ √πZ0
Vα is the tension of the equivalent diagram of Thévenin.
Z0 = √µ0 ⁄ δ0 = 377 Ω is the intrinsic impedance of the vacuum.
Zα is the impedance of the equivalent diagram of Thévenin and is equal to the impedance of the antenna.
Rα is resistance series of the impedance of Zα antenna.
Gα is the gain of the antenna (the same one as in emission) in the direction from which come the electromagnetic waves.
λ is the wavelength.
EB is the electric field of the incidental electromagnetic wave.
ψ is the angle of misalignment of the electric field with the antenna.
The equivalent diagram and the formula on the right are valid for any type of antenna. It can be a dipolar antenna, a parabolic aerial, a Yagi-Uda antenna or a network of antennas.
Concepts relating to the reception antennas: the three following definitions all rise from the formula of the preceding paragraph.
Effective length of the antenna : (√RαGαλcosψ) ⁄ √πZ0
Maximum capacity available : Gαλ² ⁄ 480π² * E²b
Effective surface or cross section : λ² ⁄ 4π * Gα
Characteristics
The main features of an antenna are:
frequencies of use
the diagram of radiation
impedance of antenna
polarisation
the output
maximum power tolerated in emission
mechanical obstruction
Frequency of use
An antenna is used in general with signals around a given frequency for which the antenna has optimal capacities to emit or receive corresponding electromagnetic energy in surrounding space. The frequency of resonance of an antenna depends initially on its own dimensions, but also on the elements which are added to him. Compared to the frequency of central resonance of the antenna, an attenuation of 3 dB determines the frequencies minimum and maximum of use, the difference between these two frequencies corresponds to the band-width.
In practise, and for the raised frequencies, the diameter of the conductor is not negligible any more compared to the wavelength, which increases its band-width considerably. In general:
The band-width of an antenna decreases if the antenna becomes small compared to the half-wave: there do not exist antennas broad band and compact. At least with reasonable losses.
The band-width of a telegraphic antenna increases if the diameter of the conductor increases.
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.
Impedance of antenna
The impedance of antenna is the generalisation of the concept of impedance used for the other passive components with the antennas. It is thus about the complex relationship observed between the tension and the current at the entry of a sending antenna. The utility of this concept is important to ensure the best transfers of energy between the antennas and the devices which are connected there thanks to the techniques of adaptation.
An antenna taken between its two terminals of access thus constitutes a dipole having an impedance complexes R + jX where R and X respectively represent the resistance and the reactance of the antenna. The resistance of antenna R is itself the sum of two types of resistance which represent the various energy utilisations absorptive: first RP is resistance related to the losses by Joule effect in the antenna while second Rr is the resistance of radiation related to the useful energy radiated by the antenna in the space which surrounds it. One says of an antenna which it resounds on a frequency so at this frequency the imaginary term jX is null. The power absorptive by the antenna is the power absorptive by resistance R. Rr resistance is sometimes described as fictitious, because it is not subjected to the law of Joule: indeed, the power absorptive by this resistance is, unlike a true resistance, transformed into electromagnetic radiation.
Very often, the manufacturers of the antennas seek to obtain a pure resistance R= 50 Ohms, and X= 0 in order to be able to supply this antenna by a line 50 Ohms (or more rarely 300 or 600 Ohms). Indeed, ideally, the antenna must present to its feeder a pure resistance equal to the impedance characteristic of this line. The feeder will function then “in travelling wave”. This condition is practically always required at the frequencies beyond 30 MHz, because it optimises the transfer of energy and does not impose conditions over the length of this line. The measurement of the report ⁄ ratio of standing wave makes it possible to make sure that the line functions in travelling wave.
However, for the low frequencies, it is sometimes impossible to obtain a resistive impedance of 50 Ohms. One must then intercalate between the antenna and the feeder a transformer of impedance the purpose of which will be to transform the complex impedance of the antenna into a pure resistance, generally of 50 Ohms. It is a device of adaptation or adaptor of antenna. The device of adaptation is sometimes consisted the line itself. Line length becomes critical then, and the report ⁄ ratio of standing wave is high.
Polarisation
Optical polarisation and electromagnetic field.
The polarisation 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 polarisation, other antennas have an elliptic or circular polarisation.
Accordingly of terrestrial reception one can consider that the Yagi standard antenna attenuates the signal of a factor 10 is (20 dB) during its rotation of the mode of horizontal reception to the mode of vertical polarisation for the same transmitter.
In reception, the difference between received polarisation and that of the antenna create an attenuation being able to be total if polarisation is perpendicular. Circular polarisation is used if the sending antennas and reception are directed in a random way, for example for the or not stabilised ravelling satellites.
Diagram of radiation
The isotropic radiator, that is to say radiating in the same way in all the directions, is an unrealizable theoretical framework in practise. 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 visualise 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. Measurements on the antennas are taken in free space or anechoic chamber.
The diagram of complete radiation can be summarised in some useful parameters:
Directivity
Various send-out charts of antennas
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 principal lobes are named. It will be all the more directing as the most important lobe will be narrow. Directivity corresponds to the width of the principal lobe, between the angles of attenuation to 3 dB.
For all the antennas, dimension constitutes a fundamental parameter to determine directivity. The antennas with high directivity and gain will be always large compared to the wavelength. There exist indeed mathematical relations (transformation of Fourier) between the space characteristics and the diagram of radiation.
Gain
The gain defines the increase in power emitted or received in the principal lobe. It is due to the fact that energy is focused in a direction, as luminous energy can be concentrated thanks to a convergent mirror and/or a lens. It is expressed in dBi (decibels 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 Yagi aerial, for example) will play the part of the lens.
Lobes and zero secondaries
With the angles close to the principal lobe, an antenna present of the relative minima and maximum called secondary lobes which one tries to minimise. The antennas with great directivity also present weak and irregular lobes in all the other angles, called diffuse lobes.
The general level of these secondary lobes describes the sensitivity of the antenna to jamming (in telecommunications) or the smoothness of imagery (out of radar). A direction where the gain 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.
Vertical starting angle
In the case of an antenna close to the ground, in particular in high frequency and intermediate frequency, the vertical diagram depends on the distance of the ground. It results a loss from it from gain in the horizontal plane. The angle of the principal lobe in the vertical plan (starting angle) defines the performances of an antenna with respect to the ionospheric modes of propagation.
Output
The sum of the power output in all the directions defines the actually radiated power. The relationship with the power provided by the line of transmission defines its output. The resistance presented by the antenna has two origins:
the resistance of radiation. The energy absorptive by the resistance of radiation is in fact the energy radiated by the antenna.
the resistance of losses. The energy absorptive by this resistance is dissipated in heat by the antenna, Joule effect in resistances or losses in the dielectric ones.
The output is function of the relationship between these two resistances. An antenna will have a good output if the resistance of losses is low in front of the resistance of radiation. The antennas of the type dipole half-wave or monopoly have in general resistances of radiation much higher than their resistance of losses and their output thus remains good. On the other hand, if the antenna has low dimensions compared to the dipole half-wave, its resistance of radiation will decrease. At this point in time really the difficulty of the output will arise and which will have to be sought to also reduce the resistance of losses.
If one considers the power applied to the input of the line of transmission, the output is obviously weaker, since part of energy is dissipated in this line. A line is characterised by the losses in dB per unit of length, for a given frequency. But if the line is the seat of standing waves because of loss of adaptability, the losses in the line will be still higher.
Maximum power in emission
The output defines the actually radiated power, the not radiated power is dissipated thermically is in wire, connexions, screws and bolts, etc, which limits the tolerated average power. The power tolerated maximum peak depends on the electric field before starting in each point of the antenna, in the lines, points, guides, supports, insulators. The most critical point is in general the line of transmission, coaxial or guides: its diameter must be adapted, like its dielectric.
Forms and dimension

An antenna multibandes HF of the Yagi type rotary printing-press
The shape and dimensions of an antenna are extremely variable: that of a cellphone is sometimes invisible because inside the case or limiting itself to a small outgrowth on the apparatus, while the parabola of the radio telescope 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 gain will be high and its narrower principal lobe.
The directing antennas can be fixed for the point-to-point connexions, or rotary printing-presses in mobile telecommunications. The antennas of continuation of the satellites are directional in azimuth (direction in the horizontal plane) and in site (height above the horizon).
Types
The shapes of antennas are multiple, but can be gathered in families.
Elementary antennas

Antenna dipole half-wave (on the left)
and antenna quarter of wave (on the right).
The elementary antennas can be used separately or as elements of networks, or source of a system with reflectors or parasitic elements. These antennas allow only one linear polarisation.
The isotropic radiator is an unrealizable theoretical reference, which would also radiate in all the directions. It is used only as reference to the evaluation of the gain.
The dipolar antenna or dipole half-wave or doublet half-wave consist of a conducting element length equal to the half wavelength. Its characteristic impedance is resistive and close to 73 ohms for a dipole isolated in space.
The antenna monopoly or quarter of wave consists of an element length equal to the quarter wavelength, perpendicular to a conducting plan. It behaves like a half dipole, the conducting plan acting as mirror. Its characteristic impedance is half of that of the dipole is approximately 37 ohms. Its form depends on the frequencies, since the umbrella aerial in VHF or tablecloth for kilometric waves.
Radiant slits: at the high frequencies (ultra high frequencies), the waves are easier to handle than the currents and tensions, the radiant slit attacked by a guide of wave is the equivalent of a dipole attacked by a symmetrical line (duality).
The loop is the base unit of the antennas quad or frameworks.
Antennas in networks
The elementary antennas can be assembled in networks with one or two dimensions, thus increasing the gain and directivity. The diagram of an antenna network can be modulated by modifying phase and amplitude of the individual excitations.
The antenna curtain or colinéaire comprises in VHF ⁄ UHF several dipoles supplied with a parallel line, in general in front of reflectors. In low frequency they are monopolies or multiple dipoles supplied with independent coaxial lines.
The antenna candle, is omnidirectional in the horizontal plane. It is made up of several dipoles half-wave supplied in order to radiate in phase. These dipoles are laid out end to end vertically the ones above the others, and are coated in a shell. The more important the number of dipoles is, the more the antenna will be long, the more important its gain will be and its directivity in the vertical plan high.
The Yagi-Uda antenna with parasitic elements, is most known of the public: it is the rake used for the reception of terrestrial analogical television or numerical. Its gain and its directivity depend on the number of elements (thus its length). It is an alternative of antenna network, the parasitic elements being fed by coupling, thanks to the choice their length.
The radiant panels in ultra high frequency comprise many elementary antennas, in general antenna patch (or plane or planar), on a plane support.
hybrid antennas (planar + elements) more known in TNT under compact antenna
Antennas with reflectors
In ultra high frequencies, the antennas can use assemblies similar to optics, with plane or parabolic reflectors.
The parabolic aerial is most known for its use in satellite television.
The antennas of very large diameters used in space transmissions or radioastronomy use also assemblies Cassegrain type similar to the telescopes.
Antennas for circular polarisation
A combination of two cross elementary antennas makes it possible to emit or receive in circular polarisation. Other principles are specific to circular polarisation.
The cross Yagi aerial combines two Yagi aerials attacked with a dephasing of 90°.
The antenna propeller monofilaire, of form corkscrew makes it possible to carry out a narrow diagram, adapted for example to the continuation of satellites.
The antenna propeller quadrifilaire makes it possible to carry out a diagram supporting the side angles (used in communications space with the ravelling satellites.
Guide antennas of wave

Antenna with slits (standard guide of wave) for wave of 10 GHZ
The antenna horn used in ultra high frequency is a radiant opening excited by a guide or a monopoly, rectangular in linear polarisation, circular in circular polarisation.
The networks of radiant slits are networks of dipoles open on a guide. Their geometry makes it possible to define the beam and polarisation (antenna with slits).
Active antennas
An active antenna incorporates a circuit of amplification directly at the boundaries of the elementary antenna, either in reception to adapt the impedance (in BF for example), or in emission to allow the creation of complex diagrams in an assembly in radiant panel. These antennas network with ordering of phase are used for the airborne radars of space observation or, the strategic radars of detection, and can comprise a thousand of active elements.
Shortened antennas
One of the antennas most used in the portable equipment is the antenna quarter of wave. It uses the mobile equipment as plan of mass and its theoretical length is of a quarter wavelength. In practise, one can still reduce his length by intercalating an inductance to his base. Another more recent and more effective technique consists in carrying out the conductor using a tightened rolling up, in the shape of spring. The unit is made rigid while surrounding this rolling up with a plastic membrane. One obtains the antenna thus known as “roll”, used in the portable equipment. One can thus shorten the antenna of a factor four. This reduction of the size is paid by an important reduction of the band-width.
Broad-band aerials

Dipole ultra high frequency with broad band
An elementary antenna has a frequency of resonance and a bandwidth related to his length ratio/diameter, By increasing this ratio it is possible to obtain a band-width of 50%. A dipole with broad band resembles then a haltère in ultra high frequency, or a double telegraphic cone in high frequency.
To go beyond, the special antennas functioning over one decade or more, are of the type antenna log-periodical or comparable like the antenna discone, the antenna helicoid punt.
Antennas patch
With the miniaturization of the systems of radiocommunication, one needed to create antennas the least cumbersome possible, but of sufficiently high output. These are the antennas patch, of which there exists a great diversity.
In general, an antenna patch is made up of a resonant element placed above a metal plan.
Antennas frameworks and loops
When the wavelength is too large compared to possible dimensions of the antenna, one uses the antennas frameworks or loops. One speaks about antenna tallies if there are several whorls, and of loop if there is only one of them. These antennas are in fact of the resonant circuits which one increases to the maximum to obtain a radiation. As dimensions remain small compared to the wavelength, the resistance of radiation remains very weak, often lower than the ohm. The output is then reduced, because ohmic resistance can be higher than the resistance of radiation.
To support the output, ohmic resistance must be minimised, the over-tension coefficient is then raised and the antenna has a low bandwidth.
One uses these antennas in systems RFID, the readers of radio smartcards, in low-size remote controls etc…

Antenna ferrite GO
If one places a ferrite stick in an antenna tallies, it is not necessary any more to increase the reel size physically, it is the ferrite which concentrates the field H: there are then the antennas used on the radio receiver on average frequency.
Mode of feeding
The antenna is generally deployed outside, even fixed at the node 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 input of the receiver, a line of transmission or a guide of wave is used.
To obtain an optimal operation, the impedance at the point must be equal to 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 Ohms (300 ohms for a two-wire line). In addition to the adaptation of the impedances, a symmetrical antenna (as the doublet half-wave) must be supplied with a symmetrical line (like the two-wire line) or by a system making the feeding symmetrical (balun) and an asymmetrical antenna like the vertical antenna by an asymmetrical line.
An antenna can also be supplied with a line of transmission with high impedance, consisted of two parallel wire in the air, of characteristic impedance 600 Ohms. The adaptation to a line of traditional transmission is done then at its end. This assembly is frequent to feed the individual elements of an antenna curtain.
In ultra high frequencies one uses also waveguides, kinds of tubes of rectangular or elliptic section in which the waves circulate. The guides of wave make it possible to convey the waves with minimal losses and support high powers.
To allow the operation of an elementary antenna on a broad waveband, a system adaptor of antenna can be inserted, adapting for each frequency the complex impedance of the antenna to the line of transmission.
It should be noted that the EBU supports by its regulations a supply voltage of 5V for the feeding of (pre) amplifying external this in order to provide for protection with the report ⁄ ratio signal noise (S/N) by the increase with the intensity with the current with power supply the amplifier in the coaxial line and filtering (dimensions of the capacitors)
Reception antennas
Any sending antenna is adapted to the 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.
The reception antennas known as active incorporate an preamplifier-adaptor between the element of antenna and the line of transmission. This active element in the case of comprises moreover the satellite aerials, a change of frequency to reduce the losses of distribution.
In broadcasting PO or GO, the antennas tallies on ferrite allow a reception with an installation more compact than an antenna telegraphic and less sensitive to the parasites. These antennas present an angle of cancellation and must possibly be directed.
In reception, it is frequent that an antenna is used largely apart from its frequency of agreement. it is the case of the antennas of car radio to which the frequency of resonance is close to the tape of broadcasting FM (tape of the Short Waves Extremists, tape OUC) towards 100 MHz and that one uses in small waves or even long waves to a few hundred kilocycles with a wavelength about the kilometer.
Fields around an antenna
An antenna, used in emission, creates a plane wave only at one certain distance. One can distinguish four zones in the environment from the antenna, as one moves away from this one:
Zone of reactive fields: Very near to the elements composing the antenna, one finds fields E and fields H, function of the tensions and currents on these conductors. Near a raised tension, one will find primarily a field E and near the currents, primarily a field H.
Zone of Rayleigh: One finds a zone where the power per unit of area decrease little according to the distance, although report ⁄ ratio E ⁄ H is already close to 377 Ohms. This zone, especially identifiable for the antennas with gain, extends until a distance equal squared from dimension from the antenna (measured in a direction perpendicular to the direction considered), divided by lambda ⁄ 2 (cf Example Ci below)
Zone of Fresnel. Beyond the zone of Rayleigh, one notes that report ⁄ ratio E ⁄ H balanced with 377 ohms. But one observes important variations of the fields and even of the undulations if the antenna is of great dimension. One cannot still make measurement of the gain of the antenna in this zone. In the direction of the maximum of radiation, the various supposed parts of the antenna to radiate in phase ad infinitum, do not radiate yet in phase.
Zone of Fraunhofer. It in the following way is characterised: In this zone, if one moves away indefinitely in the same direction, one notes that the difference of the distances between the points of the antenna does not vary any more. In the direction of the maximum of radiation, the various supposed parts of the antenna to radiate in phase ad infinitum, radiate well in phase. In this zone, which extends until the infinite one, one can consider that one has a plane wave, the fields decrease in 1 ⁄ r and one can measure the gain of the antenna. It is as only in this zone as the diagram of radiation is valid. This zone starts at a distance equal to twice the square of greatest dimension perpendicular to the direction considered, divided by lambda. This distance can be very large for the antennas with great gain.
To measure the gain of an antenna with great gain, it is thus important to know to define the zone of Fraunhofer. For example, in the axis of a parabola of 1 m diameter and on 10 GHZ, the zone of Fraunhofer starts with more than 60 Mr.
Disturbance of an antenna by its immediate environment
The environment close to an antenna is not always released. Whereas the fixed antennas at the high frequencies are generally well disengaged from the surrounding obstacles, it is not the same antennas of the mobile devices, often built-in broader systems. It is for example the case of the small antennas quarter of wave incorporated in portable systems of radiocommunication, or many antennas of the modems radio operator associates to the computing systems, often assembled in exiguous spaces. In addition, the antennas for the average frequencies and low, because of their dimensions, will be influenced by the ground.
The metal objects located at a distance about the wavelength will be able to produce an effect of shade in the direction considered, if their dimension is itself about the wavelength or more, but they are there rather phenomena of “mask” that disturbances themselves.
One can voluntarily modify the characteristics of radiation of an aerial element, by the addition of conductors near this element. On the other hand, of the disturbances this nondesired time of the operation even of the antenna will appear by the presence of conducting bodies, in the immediate environment of the antenna. In general, the frequency of resonance of an antenna depends on the capacity of the antenna compared to its environment, especially around the bellies of tension. Thus, if a conducting body is close to the end of the antenna (belly of tension), one will observe a reduction in the frequency of resonance. If this body of large dimensions and is connected on the ground or the mass, one will have in more one collapse of the resistance of radiation, because the lines of electric field will join the mass by a short path, instead of spreading itself in space.
The frequency of resonance of an antenna depends in addition on the inductance of the parts subjected to a belly of current. Thus, if a conductor is placed parallel to a belly of current and if this conductor is sufficiently long to be able to be the seat of induced currents, the inductance of the antenna will decrease and its frequency of resonance increases.
That explains why, for example for an antenna quarter of wave, the close conductors will not have the same effect if they are close to the node (belly of tension) or close to the base (belly of current).
If it is the whole of a telegraphic antenna which is parallel to a conducting plan or a metal mass, the two effects quoted above will be compensated: the frequency of resonance will be modified little. On the other hand this conducting plan parallel with the antenna will influence the resistance of radiation. This influence will become very important if the distance to the plan is much lower than the quarter of wave: in this case, there is not any more one antenna, but a line and the radiation will crumble. For the antennas of low frequencies, parallel on the ground, it is of course the ground which will represent this conducting plan. Generally, one will almost always seek sufficiently to maintain an antenna far from the plan of mass or the ground, in order to prevent that the resistance of radiation does not crumble. One can certainly envisage a readjustment of the antenna, but the band-width of the antenna will be in any case weaker and if the resistance of radiation is not large any more in front of ohmic resistance, the output will drop. Sometimes one seeks to reduce the obstruction of an antenna, by maintaining it relatively close to a metal plan.
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 realisation will not be the same one. The wind resistance and to the bad weather must be particularly neat to obtain a greater 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 radome is an impermeable protective shelter used to protect an antenna.

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