antenna parameters [ edit]
electrically antennas are characterized by a number of parameters, being the most common described below:
radiation pattern (radiation pattern) [ edit]
is a graphical representation of the radiation characteristics of an antenna. It is customary to represent the magnitude of electric field and the radiated power density, but you can also find diagrams of polarization or phase.
radiation pattern
bandwidth [ edit]
is the frequency range in which the antenna parameters meet certain characteristics. You can define a bandwidth of impedance, polarization, gain or other parameters. Directivity
[edit ]
is the ratio of radiated power density in the direction of maximum radiation at a distance r and the total radiated power divided by the area of \u200b\u200bthe sphere of radius r. Directivity can be calculated from the radiation pattern. The antenna gain is equal to the directivity multiplied by the efficiency.
Gain [edit ]
The ratio of radiated power density in the direction of maximum at a distance r and the total power delivered to the antenna divided by the area of \u200b\u200ba sphere of radius r. The efficiency of an antenna is the relationship between gain and directivity. This relationship coincides with the relation between the total radiated power and power delivered to the antenna.
input impedance [ edit]
is the impedance of the antenna at its terminals. Is the relationship between voltage and current input. . The impedance is complex. The real part of impedance is called resistance antenna and the imaginary part is reactance. The antenna resistance is the sum of radiation resistance and loss resistance. The antennas are called resonant when you cancel your input reactor.
beamwidth [edit ]
radiation is a parameter, linked to the radiation pattern. You can set the beam width at -3 dB, which is the angular range in which the radiated power density is equal to half the maximum. You can also define the beam width between zero, which is the main beam angular range of the radiation between the two zeros adjacent to the fullest. Polarization
[edit ]
antennas create radiated electromagnetic fields. Polarization is defined electromagnetic in a certain direction, as the geometric figure traced by the end of the vector electric field at a certain distance from the antenna by varying the time. Polarization can be linear, circular and elliptical. Linear polarization can take different orientations (horizontal, vertical, +45 °, -45 °). Circular or elliptical polarization may be clockwise or counterclockwise (right-handed or left-handed), depending on the direction of rotation of the field (observed away from the antenna).
diagram is called the co-polar radiation pattern with desired polarization diagram contrapolar (CrossPolo, in English) to the radiation pattern with opposite polarization.
Antenna Types [edit ]
There are two main types of antennas: wire antennas
. Are antennas whose radiating elements are conductive wire having a section negligible compared to the working wavelength. The dimensions are usually a maximum length onda.Se widely used in the bands of MF, HF, HF, VHF and UHF. You can find wire antenna arrays. Examples of wire antennas are vertical monopole
The
The dipole and its evolution, Yagi antenna
The loop The propeller
wire antennas are analyzed from the electric currents of the conductors. Aperture
. The antennas are those that use open areas or openings to direct the electromagnetic beam focused so that the issuance and receipt of your heating system in one direction, solid angles. The best known and is currently used in the dish, both terrestrial radio links such as satellites. The gain of these antennas will be connected to the surface of the parabola, the larger the more collimation of the beam and therefore we will have more gain less angular. The radiating element is the Illuminator, which can directly illuminate the parabola or indirectly through a subreflector, depending on the design of it. The illuminator is usually placed at the focus of the parabola.
In general, one can calculate the directivity of this certain type of antenna, with the following expression, where is the area and is the wavelength:
There are two types of aperture antennas by type of business:
wide
standard synthetic aperture (for software)
Parabolic
Examples of openings are
The horn antenna
The dish
parabolic antenna Doppler Radar reflective surfaces in general
And in linear polarization is vertical and horizontal.
reception antennae [edit ]
Different types of antennas and their radiation.
The electric field of an electromagnetic wave induces a voltage in each small segment of the conductor of an antenna. The current flowing in the antenna has to go through the impedance of the antenna. Using
reciprocity theorem can be proved that the Thevenin equivalent circuit of a receiving antenna is following:
circuit voltage is the Thevenin equivalent.
is the impedance of the Thevenin equivalent circuit and is equal to the impedance of the antenna.
series resistance is the impedance of the antenna.
is the gain of the antenna (the same as in issue) in the direction of where they come from electromagnetic waves.
is the wavelength.
is the electric field of the incident electromagnetic wave.
is the angle that measures the electric field alignment with the antenna. For example, in the case of an antenna formed by a dipole, the induced voltage is maximum when the dipole and the incident electric field are aligned. If they are not, and forming an angle induced voltage is multiplied by.
The equivalent circuit and formula on the right are valid for all types of antenna: it is a simple dipole, a satellite dish antenna or a Yagi-Uda antenna array.
Here are three definitions that are self-explanatory:
The corollary of these definitions is that the maximum power that an antenna can be extracted from an electromagnetic wave depends only on the antenna gain and the square.
Influence of the Earth [ edit]
ground conductivity is a factor in the influence of the earth on the propagation of electromagnetic waves. The conductivity of the earth's surface depends on the frequency of electromagnetic waves that affect her and the material by which it is composed, behaving as a good conductor at low frequencies and reducing its conductivity at higher frequencies.
The ground reflection coefficient is a parameter related to conductivity and reports about how waves are reflected in it. Its value depends on the angle of incidence and the material that makes up the soil, wet soil, dry land, lakes, seas, urban, etc.
For a given reflection coefficient, the reflected energy from the soil increases with increasing the angle of incidence with respect to normal, with most of the energy reflected when the incidence is flush, and having the electric and magnetic fields of the reflected wave almost the same amplitude as the incident wave.
In the case of the antennas in the case of emission or reception usually long distances, often there is a grazing incidence.
The beam reflected by the earth can be modeled, from the standpoint of the receiving antenna as the beam transmitted by a satellite image of the transmitting antenna located under the floor. The reflected beam travels farther than the direct ray.
The appearance of the aerial image is an image mirror the look of real transmitting antenna. In some cases we can consider that the wave transmitted from the real antenna and the wave transmitted from the satellite image have approximately the same extent, in other cases, such as when the ground has irregularities of a similar size or larger than the wavelength, reflection of the incident ray is not net.
The distance traveled by the beam reflected by the earth from the transmitting antenna to receiving antenna is greater than the distance traveled by the direct ray. That difference introduces a gap distance between the two waves (see Radiation of a pair of antennas ).
The right figure represents an angle incidence on the horizontal great when, in reality, the angle is usually very small. The distance between the antenna and its image is.
The reflection of electromagnetic waves depends on the polarization . When the polarization is horizontal, the reflection produces a phase shift of radians, whereas when the polarization is vertical, reflection produces no lag.
The vertical component of the current reflects without changing sign, however, the horizontal component changes sign.
In the case of an antenna that emits a vertical polarization (vertical electric field) the electric field calculation result is the same as in radiation from a pair of antennas . The result is:
Investing sign for the parallel field changes only in one breast cosine:
In these two formulas:
is the electric field of the electromagnetic wave radiated by the antenna had no land.
is the wave number .
is wavelength.
is the height of the antenna.
References [edit ]
Antennas. A. Cardama, L. Jofre, JM Rius, J. Romeu, S. Blanch, M. Ferrando. Edicions UPC http://www.edicionsupc.es
http://www.upv.es/antenas Antennas
Antenna Theory: Analysis and Design (John Wiley & Sons, 2005) by Constantine A. Balan
Introduction to antenna theory http://www.upv.es/antenas/Documentos_PDF/Notas_clase/Tema_1.P
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