Transcript Document 7429725

Chapter 5. Polarization
第五章 極化
亞洲大學
資訊工程學系碩士班
呂克明教授
二○○六年十月十六日
5/23/2016
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Chapter 5. Polarization
(第五章極化)
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Introduction (緒言)
Antenna Polarization (天線極化角)
Polarization of Satellite Signals (衛星訊號的極化)
Cross-Polarization Discrimination (交叉極化的區隔)
Depolarization (去極化)
Homework problems (習題)
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Introduction
(緒言)
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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, 橫向電磁波).
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.
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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Introduction
(緒言)(continued)
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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.
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 (線性極化,
橢圓極化,與圓形極化).
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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Introduction
(緒言)(continued)
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Circular Polarization (圓形極化): 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 (右旋圓形極化): The IEEE defines
right-hand circular polarization (RHC) as a rotation in the clockwise
direction when viewed from behind.
Left-hand circular polarization (左旋圓形極化): 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
(天線極化角)
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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.
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
(衛星訊號的極化)
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Satellite linear polarization (衛星線性極化):
 The horizontal polarization (水平極化) is where the electric field
vector is parallel to the equatorial plane (赤道面),
 The vertical polarization (垂直極化) is where the electric field
vector is parallel to the earth’s polar axis (地極軸面).
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Cross-Polarization Discrimination
(交叉極化的區隔)
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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.
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
(頻率重複使用).
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Cross-Polarization Discrimination
(交叉極化的區隔)(continued)
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Two measures are in use to quantify the effects of polarization interference:
 Cross-Polarization Discrimination (交叉極化區隔度): 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)
 Polarization Isolation (極化隔離度): 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
(去極化)
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Ionospheric Depolarization (電離層之去極化):
 One of the effects of the ionoshpere is to produce a rotation of
the polarization of a signal, an effect known as Faraday rotation.
(法拉第旋轉, Michael Faraday, 1791-1867, 英國物理學家,化
學家)
 Faraday rotation is inversely proportional to frequency squared
(跟頻率的平方成反比) and is not considered to be a serious
problem for frequency above about 10 GHz.
 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
(去極化) (continued)
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Rain Depolarization (雨滴之去極化): The shape of small raindrops is
close to spherical, but large drops are better modeled as oblate
spheroids (扁圓球體).
 There will be a difference therefore in the attenuation, termed
differential attenuation (衰減損耗差), and phase shift, termed
differential phase shift. (相位偏移差)
Ice Depolarization (冰之去極化): Value of 2 dB added to the XPD are
suggested for North America and 4 to 5 dB for maritime regions.
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Homework Problems
(習題)
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5.1 Explain what is meant by a plane TEM wave.
5.15 Explain what is meant by vertical polarization of a satellite
signal.
5.16 Explain what is meant by horizontal polarization of a satellite
signal.
5.19 Explain what is meant by cross-polarization discrimination.
5.20 Explain the difference between cross-polarization
discrimination and polarization isolation.
5.22 Why is Faraday rotation of no concern with circular polarized
wave?
5.23 Explain how depolarization is caused by rain.
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