00Satellite Comms Antennas | Antenna (Radio)

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antenas de satelite compendio manual
  Antenna ByKamranAhmed Antenna  Antennas form a very important element in communication system, either terrestrial or extra terrestrial, depending on the mission type and requirements  That part of a transmitting or receiving system which is designed to radiate or to receive electromagnetic waves .  we use antennas to overcome our inability to lay a physical interconnection between two remote locations or an antenna can also be viewed as a transitional structure (transducer) between free-space and a transmission line (such as a coaxial line).  Antennas cannot add power, instead they can only focus and shape the radiated power in spacee.g. it enhances the power in some wanted directions and suppresses the power in other directions Some Basic Definitions  Suppose we have an antenna located at the srcin of a spherical co-ordinate system, further assume that the antenna is transmitting and the observations are made for a very large distance;  Let P o (Watts) be the accepted power in the antenna and P r  (Watts) be the radiated power, then the radiating efficiency ή as; ã  ή = P r  / P o zr Pyzç θ AntLocation Radiation Intensity  We define Radiation Intensity f ( θ , Ф ) or Θ ( θ , Ф ) (watts/steradians)  The Average radiation intensity is; Θ avg = P r  / 4 π P r  =  Antenna Directivity (Measure of the focusing property of an antenna)  The directivity of an antenna is defined as the ratio of the radiation intensity in a given direction from the antenna, to the radiation intensity averaged over all directions.  This average radiation intensity is equal to the total power of the antenna divided by (4 pi). If the direction is not specified, the directivity refers to the direction of maximum radiation intensity . D ( θ , Ф ) = { Θ ( θ , Ф ) / Θ avg }or D ( θ , Ф ) = 4 π { Θ ( θ , Ф ) / P r  } θ is the elevation angle φ is the azimuth  where D is the directivity. Generally D > 1, except in the case of an isotropic antenna for which D = 1. An antenna with directivity D >> 1 is called a directive antenna. Cont…Gain (Measure of Directivity)  The Gain G( θ , ф ) is the ability to concentrate the power accepted by the antenna in a particular direction. It is related to the Directivity and Power Radiation efficiency or in other words Power Radiation Intensity as follow;G( θ , ф )= ή D( θ , ф ) for loss less antenna ή =1 G( θ , ф )=4 π { Θ ( θ , Ф ) / P r  }  With respect to the antenna's dimensions,G= ή {4 π A / λ 2} A is the aperture area of the antenna λ is the wavelength of the operational frequency η is the antenna efficiency (usually between 50%and 70%) Cont…  Basically there are only two types of antennas: ã dipole antenna (Hertzian) ã vertical antenna (Marconi)  All antennas can be broken down to one of these types (although some say that there is only one -the dipole)  In addition to this we have a theoretical perfect antenna (non-existent) that radiates equally in all directions with 100% efficiency. This antenna is called an isotropic radiator .  Cont… (Basic Antenna types) The gain can also be presented as a 3D gain. The radius of the spheroid is proportional to the antenna gain. Gain presented as 3D gainGain in theory  Since all real antennas will radiate more in some directions than in others, you can say that gain is the amount of power you can reach in one direction at the expense of the power lost in the others. When talking about gain it is always the main lobe that is discussed  Gain may be expressed as dBior dBd. The first is gain compared to the isotropic radiator and the second gain is compared to a half-wave dipole in free space (0 dBd=2.15 dBi) Power Density  The power density P( θ , ф ) is related to radiation intensity as follows; P ( θ , ф )= { Θ ( θ , Ф ) / r  2 }or  P ( θ , ф )= {G( θ , Ф ) P o  / 4 π r  2 }  The factor P o  / 4 π r  2 represent the power density that results if the power accepted by the antenna were radiated by loss-less isotropic antenna   The maximum power flux density at some distance “r”from a transmitting antenna of gain “G”is;  An isotropic radiator with input power equal to GP S would produce the same flux density. Hence, Equivalent Isotopic Radiated Power (EIRP) Antenna Effective Area  Measure of the effective absorption area presented by an antenna to an incident plane wave.  Depends on the antenna gain and wavelength  Aperture efficiency: a = A e  / A A: physical area of antenna’s aperture, (m 2 ) ][m ),( 4 22 ϕ ϑ π λ  G A e = Transmission losses  Free Space Transmission [FSL]  – More to follow  Feeder Losses [RFL]  – Between the receive antenna and the receive proper   Antenna Misalignment Losses [AML]  Fixed Atmospheric & Ionospheric Losses  – Absorption losses  – Depolarization losses Power transfer between two antennas  For two antennas in free space separated by large distance R  The received power is equal to a product of power density of the incident wave and the effective aperture area of the receiving antennasPr = PA e or Pr = {(G t P t G r  λ 2 ) / (16 π 2 R 2 )}
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