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 12/2/2018

Chapter 6. Antennas (第六章衛星天線) 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) 12/2/2018

Introduction (緒言) 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) . 12/2/2018

Reciprocity Theorem for Antenna (天線的相互依靠定理) 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. 12/2/2018

Coordinate System (座標系統) 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. 12/2/2018

The Radiated Fields (輻射場) 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) . 12/2/2018

Power Flux Density (電力流動密度) 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). 12/2/2018

The Isotropic Radiator and Antenna Gain (等方性輻射器與天線増益) 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. 12/2/2018

Radiation Pattern (輻射圖形) 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. 12/2/2018

Aperture Antennas (孔徑天線) 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) . 12/2/2018

Horn Antennas (角狀天線) 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) 12/2/2018

The Parabolic Reflector (拋物面反射器) 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) 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) 12/2/2018

The Offset Feed (偏中心饋源) 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 cross-polarization (交叉極化) with a linear polarized feed is worse compared with the center-fed antenna. 12/2/2018

Double-Reflector Antennas (雙反射器天線) 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) 12/2/2018

Homework Problems (習題) 6.1 Explain what is meant by the reciprocity theorem. 12/2/2018