Satellite Communication Slides of Chap 5 (Polarization) of Dennis Roddy

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Chapter 5. Polarization  Introduction  Antenna

Polarization  Polarization of Satellite Signals  Cross-Polarization Discrimination  Depolarization 5/18/2013

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Introduction 

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In the far field zone of a transmitting antenna, the radiated wave takes on the characteristics of a transverse electromagnetic wave (TEM) Far field zone: At distances greater than 2*D^2/ λ from the antenna, where D is the larger linear dimension of the antenna and λ is the wavelength. For a parabolic antenna of 3 m diameter transmitting a 6 GHz wave (λ = 5 cm), the far field zone begins at approximately 360 m. 5/18/2013

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Transverse electromagnetic wave (TEM): Both the magnetic field H and the electric field E are transverse to the direction of propagation.  TEM wave is a plane at great distance from the transmitting antenna. 

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The direction of the line traced out by the tip of the electric field vector determine the polarization of the wave. 5/18/2013

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Introduction (continued) Electric and magnetic fields are varying as functions of time. The magnetic field varies exactly in phase with the electric field, and its amplitude is proportional to the electric field in this discussion.  Polarization: Linear, elliptical, and circular polarization 

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Linear polarization: The tip of the E vector may be trace out a straight line, in which case the polarization is referred to as linear.  Vertical polarization: the electric field was perpendicular to the earth’s surface or parallel to the earth’s polar axis.  Horizontal polarization: the electric field was parallel to the earth’s surface. 

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Remember that light (and all electromagnetic radiation) can be considered as a wave of oscillating electric and magnetic forces. The electric force and the magnetic force are always perpendicular to each other, and both are perpendicular to the direction of ray travel. The polarization is the direction that the electric field varies in. The following diagram illustrates the electric variation in a linearly polarized wave.

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Circular Polarization : The tip of the resultant electric field vector traces out a circle and resultant wave is said to be circularly polarized.  Right-hand circular polarization : The IEEE defines right-hand circular polarization (RHC) as a rotation in the clockwise direction when viewed from behind. 

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Left-hand circular polarization : The IEEE defines left-hand circular polarization (LHC) as a rotation in the counterclockwise direction when viewed from behind.  IEEE definitions versus Classical optics definitions: they are just opposite. The IEEE definition will be used throughout this text. 

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Antenna Polarization 

Transmitting antenna polarization:  A horizontal dipole would produce a horizontally polarized wave.  Two dipoles mounted close together symmetrically and at right angles to each other would produce a circularly polarized wave if fed current equal in magnitude but differing in phase by 90 degree. 5/18/2013

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Receiving antenna polarization: The polarization of a receiving antenna has to be aligned to that of the wave for maximum power transfer.  A single dipole will always receive a signal from a circularly wave, but at a loss of 3 dB. This is so because the single dipole will respond only to one of the linear components, and hence the received power will be half that of the cross dipoles. 

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Polarization of Satellite Signals 

Satellite linear polarization :  The horizontal polarization is where the electric field vector is parallel to the equatorial plane,  The vertical polarization is where the electric field vector is parallel to the earth’s polar axis. 5/18/2013

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Cross-Polarization Discrimination 

The propagation path between a satellite and earth station passes through the ionosphere, and possibly through layers of ice crystals in the upper atmosphere and rain, all of which are capable of altering the polarization of the wave being transmitted.

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Depolarization: An orthogonal component may be generated from the transmitted polarization, an effect referred to as depolarization.  This can cause interference where orthogonal polarization is used to provided isolation between signals, as in the case of frequency reuse 

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Cross-Polarization Discrimination (continued) 

Two measures are in use to quantify the effects of polarization interference:  Cross-Polarization Discrimination : The most widely used measure is called Cross-Polarization Discrimination (XPD).  Co-polar component of E1 = E11 and cross-polar component of E1 = E12  XPD = 20 log (E11/E12)

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Polarization Isolation : The second measure is polarization isolation (I) that is defined by the ratio of received co-polar power to received cross-polar power and thus takes into account any additional depolarization introduced by the receiving system.  Co-polar component of E2 = E22 and cross-polar component of E2 = E21  I = 20 log (E11/E21) When the transmitted signals have the same magnitudes and where the receiving system introduces negligible depolarization, then I and XPD give identical results. 

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Depolarization 

Ionospheric Depolarization :  One of the effects of the ionosphere is to produce a rotation of the polarization of a signal, an effect known as Faraday rotation. ( Michael Faraday, 1791-1867, )  Faraday rotation is inversely proportional to frequency squared and is not considered to be a serious problem for frequency above about 10 GHz. 5/18/2013

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Maximum values for Faraday rotation are 9 degree at 4 GHz and 4 degree at 6 GHz. In order to counter the depolarizing effects of Faraday rotation, circular polarization may be used.  When a linear polarization is used, a 15 dB amount added to the XPD, where is only 0.13 dB as with circular polarization. 

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Depolarization (continued) 

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Rain Depolarization : The shape of small raindrops is close to spherical, but large drops are better modeled as oblate spheroids .  There will be a difference therefore in the attenuation, termed differential attenuation, and phase shift, termed differential phase shift. Ice Depolarization : Value of 2 dB added to the XPD are suggested for North America and 4 to 5 dB for maritime regions. 5/18/2013

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