电子信息与通信工程专业英语lesson4-21(太原理工大学版).ppt

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1、 Chapter 4 Electromagnetic Field and Wave 第第4章章 电磁场与电磁波电磁场与电磁波Outline 大纲大纲l4.1 Electromagnetic Fields Theory 电磁场理论电磁场理论l4.2 Antenna 天线天线1/23/20231电子信息与通信工程专业英语4.2 Antennas 天线天线 Outline 大纲大纲lHow an Antenna radiates 天线辐射的方式天线辐射的方式lBasic antenna parameters 基本天线参数基本天线参数lAntennas Types 天线类型天线类型1/23/20232

2、电子信息与通信工程专业英语Terminologylantenna n.天线；触角lradiate vt.放射，辐射，传播；vi.发光，辐射 adj.辐射状的lelectromagnetic adj.电磁的；电磁学的loscillate vi.振动，振荡；摇摆；(银行利率等)在中间值上下波动lsinusoidal adj.正弦曲线，窦状小管，窦状隙ltangential adj.数切线的，正切的；略为触及的，附带的lconductor n.指挥家，指导者；售票员；导体，导线lperpendicular adj.（常与to连用）成直角的；垂直的；陡的（坡）ldipole n.偶极子，对称振子；偶极

3、天线；双合价lbeamwidth n.波束宽度lhorizontal adj.地平线的；水平的；平放的，横的(与 vertical 相对)ldirectivity n.定向性，指向性，方向性lterminal n.终端，终点；极限；接头，电极，终端设备；终点站 1/23/20233电子信息与通信工程专业英语lresistance n.反抗，抵抗，抵抗力，阻力，电阻，阻抗limpedance n.阻抗；电阻抗；交流电阻lreactance n.反应性；反作用力；电抗lcomponent n.元件，分量lgain n.增益lpolarization n.物偏振(现象)，极化(作用)，两极化，分化l

4、vector n.矢量，向量lelliptical adj.椭圆的；省略的lfrequency n.频率，周率，发生次数ltransport v.传送，运输，流放，放逐ltransmission n.播送,发射,传动,传送,传输,转播lcurrent adj.当前的，通用的，流通的；n.涌流，趋势，电流，水流，气流1/23/20234电子信息与通信工程专业英语lamplitude n.振幅，物理学名词lelement n.要素，元素，成分，元件，自然环境lmobile adj.可移动的，易变的，机动的 n.运动物体lwavelength n.波长linsensitive adj.感觉迟钝的，无

5、感觉的(to)；不机敏的；低灵敏度的lnull adj.无效力的，无效的，无价值的，等于零的 n.零，空lparallel adj.平行的，并联的 n.平行面，相似物v.相应，平行lhelix n.螺旋，螺旋状物，解剖耳轮lwaveguide n.波导laperture n.孔，穴，缝隙，(照相机,望远镜等的)光圈，孔径lrectangular adj.矩形的，长方形的；直角的lgeometry n.几何学 lexcitation n.刺激，激励，鼓舞，物激发，励磁，植激感(现象)ldimension n.尺寸，尺度，维(数)，度(数)，元lIEEE abbr.电气和电子工程师协会1/23/2

6、0235电子信息与通信工程专业英语lAn antenna is a device that converts RF power applied to its feed point into electromagnetic radiation and intercepts energy from a passing electromagnetic radiation,which then appears as RF voltage across the antennas feed point.The official IEEE definition of an antenna as given

7、by Stutzman and Thiele follows the concept:“That part of a transmitting or receiving system that is designed to radiate or receive electromagnetic waves”.An antenna acts as a transitional structure between the guiding device(e.g.waveguide,transmission line)and the free space.Antennas can be consider

8、ed as one of the most important parts of the radio-communication chain.Introduction 介绍Text tour 文章浏览文章浏览1/23/20236电子信息与通信工程专业英语 How an Antenna radiateslA conducting wire radiates mainly because of time-varying current or an acceleration(or deceleration)of charge.If there is no motion of charges in a

9、 wire,no radiation takes place,since no flow of current occurs.Radiation will not occur even if charges are moving with uniform velocity along a straight wire.However,charges moving with uniform velocity along a curved or bent wire will produce radiation.If the charge is oscillating with time,then r

10、adiation occurs even along a straight wire.1/23/20237电子信息与通信工程专业英语lDue to the time varying electric and magnetic fields,electromagnetic waves are created and these travel between the conductors.As these waves approach open space,free space waves are formed by connecting the open ends of the electric

11、 lines.Since the sinusoidal source continuously creates the electric disturbance,electromagnetic waves are created continuously and these travel through the transmission line,through the antenna and are radiated into the free space.Inside the transmission line and the antenna,the electromagnetic wav

12、es are sustained due to the charges,but as soon as they enter the free space,they form closed loops and are radiated.1/23/20238电子信息与通信工程专业英语lAntenna Radiation Patterns 天线方向特性lAntenna radiation patternis a graphic representation of directional properties of the antenna.Directional properties are usua

13、lly expressed in the form of the absolute value of the radio of electric intensity of the radiated wave in a given direction and its maximum value.Basic antenna parameters1/23/20239电子信息与通信工程专业英语lDirectivity pattern is usually drawn for a certain plane.An antenna radiation pattern can be displayed in

14、 two kinds of graphics:those displaying the E-plane(or E-field for Electric field)and the H-plane(or H-field for magnetic field).1/23/202310电子信息与通信工程专业英语lFrom the field pattern,we can get the following parameters of antenna:lHPBW:The half power beamwidth(HPBW)can be defined as the angle subtended by

15、 the half power points of the main lobe.lMain Lobe:This is the radiation lobe containing the direction of maximum radiation.lMinor Lobe:All the lobes other then the main lobe are called the minor lobes.These lobes represent the radiation in undesired directions.The level of minor lobes is usually ex

16、pressed as a ratio of the power density in the lobe in question to that of the major lobe.This ratio is called as the side lobe level(expressed in decibels).lBack Lobe:This is the minor lobe diametrically opposite the main lobe.lSide Lobes:These are the minor lobes adjacent to the main lobe and are

17、separated by various nulls.Side lobes are generally the largest among the minor lobes.1/23/202311电子信息与通信工程专业英语Directivity方向性方向性lThe directivity of an antenna has been defined as“the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all di

18、rections”.In other words,the directivity of a nonisotropic source is equal to the ratio of its radiation intensity in a given direction,over that of an isotropic source.lDirectivity is a dimensionless quantity,since it is the ratio of two radiation intensities.Hence,it is generally expressed in dB.T

19、he directivity of an antenna can be easily estimated from the radiation pattern of the antenna.An antenna that has a narrow main lobe would have better directivity,than the one which has a broad main lobe,hence it is more directive.1/23/202312电子信息与通信工程专业英语lThe input impedance of an antenna is define

20、d as“the impedance presented by an antenna at its terminals or the ratio of the voltage to the current at the pair of terminals or the ratio of the appropriate components of the electric to magnetic fields at a point”.Input impedance输入阻抗输入阻抗1/23/202313电子信息与通信工程专业英语Gain增益增益lAntenna gain is a paramete

21、r which is closely related to the directivity of the antenna.We know that the directivity is how much an antenna concentrates energy in one direction in preference to radiation in other directions.Hence,if the antenna is 100%efficient,then the directivity would be equal to the antenna gain and the a

22、ntenna would be an isotropic radiator.1/23/202314电子信息与通信工程专业英语lPolarization of a radiated wave is defined as“that property of an electromagnetic wave describing the time varying direction and relative magnitude of the electric field vector”.The polarization of an antenna refers to the polarization o

23、f the electric field vector of the radiated wave.In other words,the position and direction of the electric field with reference to the earths surface or ground determines the wave polarization.l The most common types of polarization include the linear(horizontal or vertical),circular(right hand pola

24、rization(RHCP)or the left hand polarization(LHCP),Elliptical Right Hand and Elliptical Left Hand.Antenna Polarization天线极化天线极化 1/23/202315电子信息与通信工程专业英语lPolarization is most important if you are trying to get the maximum performance from the antennas.For best performance you will need to match up the

25、polarization of the transmitting antenna and the receiving antenna.When a signal is transmitted at one polarization but received at a different polarization there exist a great many decibels of loss.1/23/202316电子信息与通信工程专业英语Bandwidth带宽带宽 lThe bandwidth of an antenna is defined as“the range of usable

26、frequencies within which the performance of the antenna,with respect to some characteristic,conforms to a specified standard.”The bandwidth can be the range of frequencies on either side of the center frequency where the antenna characteristics like input impedance,radiation pattern,beamwidth,polari

27、zation,side lobe level or gain,are close to those values which have been obtained at the center frequency.1/23/202317电子信息与通信工程专业英语lThe bandwidth of a broadband antenna can be defined as the ratio of the upper to lower frequencies of acceptable operation.The bandwidth of a narrowband antenna can be d

28、efined as the percentage of the frequency difference over the center frequency.1/23/202318电子信息与通信工程专业英语The Near and Far Field 近场与远场近场与远场 lThe field patterns,associated with an antenna,change with distance and are associated with two types of energy:-radiating energy and reactive energy.Hence,the spa

29、ce surrounding an antenna can be divided into two regions.lThe Near Field is an electromagnetic field that exists within /2 of the antenna.It temporarily stores power and is related to the imaginary term of the input impedance.The Far Field is an electromagnetic field launched by the antenna that extends throughout all space.This field transports power and is related to the radiation resistance of the antenna.1/23/202319电子信息与通信工程专业英语