Transcript Document 7727105

Chapter 6. Antennas
第六章 衛星天線
亞洲大學
資訊工程學系碩士班
呂克明教授
二○○六年十月三十日
5/25/2016
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Chapter 6. Antennas
(第六章衛星天線)
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Introduction (緒言)
Reciprocity Theorem for Antennas (天線的相互依靠定理)
Coordinate System (座標系統)
The Radiated Fields (輻射場)
Power Flux Density (電力流動密度)
The Isotropic Radiator and Antenna Gain (等方性輻射器與天線増益)
Radiation Pattern (輻射圖形)
Aperture Antennas (孔徑天線)
Horn Antennas (角狀天線)
The Parabolic Reflector (拋物面反射器)
The Offset Feed (偏中心饋源)
Double-Reflector Antennas (雙反射器天線)
Homework problems (習題)
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Introduction
(緒言)
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Antennas (天線) can be classified according to function as
transmitting antenna (發射天線) and as receiving antennas (接收天
線) .
Reciprocity theorem (天線的相互依靠定理) : Many of the properties
of an antenna, such as its directional characteristics (天線方向的特
性) , apply equally to both transmitting and receiving modes of
operation.
Two (2) types of antennas in satellite communication: earth
antennas (地面天線) and satellite antennas (衛星天線) .
A feeder (天線饋源) connects the power amplifier to the antenna,
and the net power reach the antenna will be total power minus the
losses in the feeder (天線饋源的損耗) . These losses include
ohmic losses and mismatch losses (天線內部電阻的損耗與阻抗不
匹配的損耗) .
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Reciprocity Theorem for Antenna
(天線的相互依靠定理)
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Reciprocity Theorem (天線的相互依靠定理) : if a current I is
induced in an antenna B (天線B的感應電流) , operated in the
received mode, by an EMF (電磁場) applied at the terminals of
antenna A operated in the transmit mode, then the same EMF
applied to the terminals of B will induce (感應) the same current
at the terminals of A (天線A會有同一電磁場感應下的等值電流) .
This applies to the same directional pattern and antenna
impedance. (同一電磁場方向圖形與天線匹配阻抗)
What is the antenna impedance? There are two (2) typical
impedance: 50 Ω and 75 Ω.
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Coordinate System
(座標系統)
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The coordinate system (座標系統) in common use is
the spherical coordinate system (球體座標系統).
The antenna is imagined to be at the origin of the
coordinates, and a distant point P in space is related
to the origin by the coordinates:
 r is the radius vector, the magnitude of which gives
the distance between point P and the antenna;
 θ is the angle measured from the z axis to r; and
 φ is the angle measured from the x axis to the
projection of r in the x-y plane.
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The Radiated Fields
(輻射場)
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There are three (3) main components to the radiated
electromagnetic fields surrounding an antenna: two near-field
regions (近距輻射場) and a far-field region (遠距輻射場) .
Near-field region I (近距輻射場 I) : R1=0.62*SQRT(D**3/λ)
Near-field region II (近距輻射場 II) : R2=2*D**2/λ
Far-field region (遠距輻射場) : R > R2
Only far-field region is of interest here. In the far-field region, the
radiated field form a transverse electromagnetic (TEM) wave in
which the electric field is at right angles (垂直正交) to the magnetic
field, and both are at right angles to the direction of propagation (同
時又和傳播方向成直角) .
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Power Flux Density
(電力流動密度)
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The power flux density (電力流動密度) of a radio wave
is a quantity used in calculating the performance of
satellite communications links.
The concept can be understood by imagining the
transmitting antennas to be at the center of a sphere.
The power from the antenna radiates outward, normal to
the surface of the sphere, and the power flux density (電
力流動密度) is the power flow per unit surface area (單
位面積內的電力流量).
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The Isotropic Radiator and Antenna Gain
(等方性輻射器與天線増益)
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An isotropic radiator is one which radiates equally in all directions.
It is not real and is hypothetical.
By imagining the isotropic radiator to be at the center of a sphere
of radius r, the power flux density, which is the power flow through
unit area is Flux I = Ps/(the surface of sphere of radius r)
=Ps/(4*π*r**2)
Gain = Flux max/Flux i
The power gain of an antenna may be referred to some “standard”
other than “isotropic”.
The isotropic gain is the most commonly used figure and will be
assumed throughout this text unless otherwise noted.
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Radiation Pattern
(輻射圖形)
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The radiation pattern (輻射圖形) varies with direction θ
and φ.
It may be written as G (θ, φ).
The main lobe (主要的輻射束, 或稱主瓣) represents a
beam of radiation (輻射束) , and the beam width is
specified as the angle subtended by the 3 dB lines.
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Aperture Antennas
(孔徑天線)
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The open end of a waveguide (波導管的開口) is an
example of a simple aperture antenna (孔徑天線).
The most common aperture antennas are
 horn antennas (喇叭天線) and
 reflector antennas (反射器天線) .
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Horn Antennas
(角狀天線)
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The horn antenna (角狀天線) is an example of an
aperture (波導管) antenna which provides a smooth
transition from a waveguide (波導管,或稱導波管) to a
larger aperture that couples more effectively into space.
What is waveguide? (什麼是波導管?)
How many kinds of waveguide? (有多少種波導管? 軟波
導管, 可彎曲式波導管, 與硬波導管)
Three (3) types of horns (三種喇叭天線) :
 Conical horn antenna (圓錐形的喇叭天線)
 Pyramidal horn antenna (金字塔的喇叭天線)
 The parabolic antenna (拋物曲線面天線)
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The Parabolic Reflector
(拋物面反射器)
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f/D (焦距與天線直徑比率, 或稱焦距直徑比) : An
important ratio is that of aperture diameter to focal
length (焦距) .
f/D = 0.25*cotan(ψ/2)
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f/D < 0.25 (焦距小於拋物面天線直徑的四分之一)
f/D = 0.25 (焦距等於拋物面天線直徑的四分之一)
f/D > 0.25 (焦距大於拋物面天線直徑的四分之一)
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The Offset Feed
(偏中心饋源)
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With the center fed (中心饋源) arrangement, the
blockage results typically in a 10 percent reduction in
efficiency and increased radiation in the lobes.
The main disadvantages of the offset feed (偏中心饋源)
are that a strong mechanical support (強而有力的機械支
架) is required to maintain the reflector shape, and
because of the asymmetry (非對稱性) , the crosspolarization (交叉極化) with a linear polarized feed is
worse compared with the center-fed antenna.
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Double-Reflector Antennas
(雙反射器天線)
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Single reflector (單反射器) Antenna versus double
reflector (雙反射器) antenna.
Two (2) types of double reflector antennas:
 Cassegrain antenna (卡塞格倫天線, 消除饋線支架)
 Gregorian antenna (格列高里天線, 由雙反射面天線
組成, 凸雙曲線為副反射面, 拋物面為主反射面)
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Homework Problems
(習題)
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6.1 Explain what is meant by the reciprocity theorem.
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