电子信息与通信工程专业英语lesson4-2.ppt

2020 3 20 可编辑 1 Chapter4ElectromagneticFieldandWave Outline4 1ElectromagneticFieldsTheory4 2Antenna 2020 3 20 可编辑 2 4 2Antennas OutlineHowanAntennaradiatesBasicantennaparametersAntennasTypes 2020 3 20 可编辑 3 Terminology antennan 天线 触角radiatevt 放射 辐射 传播 vi 发光 辐射adj 辐射状的electromagneticadj 电磁的 电磁学的oscillatevi 振动 振荡 摇摆 银行利率等 在中间值上下波动sinusoidaladj 正弦曲线 窦状小管 窦状隙tangentialadj 数 切线的 正切的 略为触及的 附带的conductorn 指挥家 指导者 售票员 导体 导线perpendicularadj 常与to连用 成直角的 垂直的 陡的 坡 dipolen 偶极子 对称振子 偶极天线 双合价beamwidthn 波束宽度horizontaladj 地平线的 水平的 平放的 横的 与vertical相对 directivityn 定向性 指向性 方向性terminaln 终端 终点 极限 接头 电极 终端设备 终点站 2020 3 20 可编辑 4 resistancen 反抗 抵抗 抵抗力 阻力 电阻 阻抗impedancen 阻抗 电阻抗 交流电阻reactancen 反应性 反作用力 电抗componentn 元件 分量gainn 增益polarizationn 物 偏振 现象 极化 作用 两极化 分化vectorn 矢量 向量ellipticaladj 椭圆的 省略的frequencyn 频率 周率 发生次数transportv 传送 运输 流放 放逐transmissionn 播送 发射 传动 传送 传输 转播currentadj 当前的 通用的 流通的 n 涌流 趋势 电流 水流 气流 2020 3 20 可编辑 5 amplituden 振幅 物理学名词elementn 要素 元素 成分 元件 自然环境mobileadj 可移动的 易变的 机动的n 运动物体wavelengthn 波长insensitiveadj 感觉迟钝的 无感觉的 to 不机敏的 低灵敏度的nulladj 无效力的 无效的 无价值的 等于零的n 零 空paralleladj 平行的 并联的n 平行面 相似物v 相应 平行helixn 螺旋 螺旋状物 解剖 耳轮waveguiden 波导aperturen 孔 穴 缝隙 照相机 望远镜等的 光圈 孔径rectangularadj 矩形的 长方形的 直角的geometryn 几何学excitationn 刺激 激励 鼓舞 物 激发 励磁 植 激感 现象 dimensionn 尺寸 尺度 维 数 度 数 元IEEEabbr 电气和电子工程师协会 2020 3 20 可编辑 6 AnantennaisadevicethatconvertsRFpowerappliedtoitsfeedpointintoelectromagneticradiationandinterceptsenergyfromapassingelectromagneticradiation whichthenappearsasRFvoltageacrosstheantenna sfeedpoint TheofficialIEEEdefinitionofanantennaasgivenbyStutzmanandThielefollowstheconcept Thatpartofatransmittingorreceivingsystemthatisdesignedtoradiateorreceiveelectromagneticwaves Anantennaactsasatransitionalstructurebetweentheguidingdevice e g waveguide transmissionline andthefreespace Antennascanbeconsideredasoneofthemostimportantpartsoftheradio communicationchain Introduction Texttour 2020 3 20 可编辑 7 HowanAntennaradiates Aconductingwireradiatesmainlybecauseoftime varyingcurrentoranacceleration ordeceleration ofcharge Ifthereisnomotionofchargesinawire noradiationtakesplace sincenoflowofcurrentoccurs Radiationwillnotoccurevenifchargesaremovingwithuniformvelocityalongastraightwire However chargesmovingwithuniformvelocityalongacurvedorbentwirewillproduceradiation Ifthechargeisoscillatingwithtime thenradiationoccursevenalongastraightwire 2020 3 20 可编辑 8 Duetothetimevaryingelectricandmagneticfields electromagneticwavesarecreatedandthesetravelbetweentheconductors Asthesewavesapproachopenspace freespacewavesareformedbyconnectingtheopenendsoftheelectriclines Sincethesinusoidalsourcecontinuouslycreatestheelectricdisturbance electromagneticwavesarecreatedcontinuouslyandthesetravelthroughthetransmissionline throughtheantennaandareradiatedintothefreespace Insidethetransmissionlineandtheantenna theelectromagneticwavesaresustainedduetothecharges butassoonastheyenterthefreespace theyformclosedloopsandareradiated 2020 3 20 可编辑 9 AntennaRadiationPatternsAntennaradiationpatternisagraphicrepresentationofdirectionalpropertiesoftheantenna Directionalpropertiesareusuallyexpressedintheformoftheabsolutevalueoftheradioofelectricintensityoftheradiatedwaveinagivendirectionanditsmaximumvalue Directivitypatternisusuallydrawnforacertainplane Anantennaradiationpatterncanbedisplayedintwokindsofgraphics thosedisplayingtheE plane orE fieldforElectricfield andtheH plane orH fieldformagneticfield Basicantennaparameters 2020 3 20 可编辑 10 Fromthefieldpattern wecangetthefollowingparametersofantenna HPBW Thehalfpowerbeamwidth HPBW canbedefinedastheanglesubtendedbythehalfpowerpointsofthemainlobe MainLobe Thisistheradiationlobecontainingthedirectionofmaximumradiation MinorLobe Allthelobesotherthenthemainlobearecalledtheminorlobes Theselobesrepresenttheradiationinundesireddirections Thelevelofminorlobesisusuallyexpressedasaratioofthepowerdensityinthelobeinquestiontothatofthemajorlobe Thisratioiscalledasthesidelobelevel expressedindecibels BackLobe Thisistheminorlobediametricallyoppositethemainlobe SideLobes Thesearetheminorlobesadjacenttothemainlobeandareseparatedbyvariousnulls Sidelobesaregenerallythelargestamongtheminorlobes 2020 3 20 可编辑 11 Directivity Thedirectivityofanantennahasbeendefinedas theratiooftheradiationintensityinagivendirectionfromtheantennatotheradiationintensityaveragedoveralldirections Inotherwords thedirectivityofanonisotropicsourceisequaltotheratioofitsradiationintensityinagivendirection overthatofanisotropicsource Directivityisadimensionlessquantity sinceitistheratiooftworadiationintensities Hence itisgenerallyexpressedindB Thedirectivityofanantennacanbeeasilyestimatedfromtheradiationpatternoftheantenna Anantennathathasanarrowmainlobewouldhavebetterdirectivity thantheonewhichhasabroadmainlobe henceitismoredirective 2020 3 20 可编辑 12 Theinputimpedanceofanantennaisdefinedas theimpedancepresentedbyanantennaatitsterminalsortheratioofthevoltagetothecurrentatthepairofterminalsortheratiooftheappropriatecomponentsoftheelectrictomagneticfieldsatapoint Inputimpedance 2020 3 20 可编辑 13 Gain Antennagainisaparameterwhichiscloselyrelatedtothedirectivityoftheantenna Weknowthatthedirectivityishowmuchanantennaconcentratesenergyinonedirectioninpreferencetoradiationinotherdirections Hence iftheantennais100 efficient thenthedirectivitywouldbeequaltotheantennagainandtheantennawouldbeanisotropicradiator 2020 3 20 可编辑 14 Polarizationofaradiatedwaveisdefinedas thatpropertyofanelectromagneticwavedescribingthetimevaryingdirectionandrelativemagnitudeoftheelectricfieldvector Thepolarizationofanantennareferstothepolarizationoftheelectricfieldvectoroftheradiatedwave Inotherwords thepositionanddirectionoftheelectricfieldwithreferencetotheearth ssurfaceorgrounddeterminesthewavepolarization Themostcommontypesofpolarizationincludethelinear horizontalorvertical circular righthandpolarization RHCP orthelefthandpolarization LHCP EllipticalRightHandandEllipticalLeftHand AntennaPolarization 2020 3 20 可编辑 15 Polarizationismostimportantifyouaretryingtogetthemaximumperformancefromtheantennas Forbestperformanceyouwillneedtomatchupthepolarizationofthetransmittingantennaandthereceivingantenna Whenasignalistransmittedatonepolarizationbutreceivedatadifferentpolarizationthereexistagreatmanydecibelsofloss 2020 3 20 可编辑 16 Bandwidth Thebandwidthofanantennaisdefinedas therangeofusablefrequencieswithinwhichtheperformanceoftheantenna withrespecttosomecharacteristic conformstoaspecifiedstandard Thebandwidthcanbetherangeoffrequenciesoneithersideofthecenterfrequencywheretheantennacharacteristicslikeinputimpedance radiationpattern beamwidth polarization sidelobelevelorgain areclosetothosevalueswhichhavebeenobtainedatthecenterfrequency Thebandwidthofabroadbandantennacanbedefinedastheratiooftheuppertolowerfrequenciesofacceptableoperation Thebandwidthofanarrowbandantennacanbedefinedasthepercentageofthefrequencydifferenceoverthecenterfrequency 2020 3 20 可编辑 17 TheNearandFarField Thefieldpatterns associatedwithanantenna changewithdistanceandareassociatedwithtwotypesofenergy radiatingenergyandreactiveenergy Hence thespacesurroundinganantennacanbedividedintotworegions TheNearFieldisanelectromagneticfieldthatexistswithin 2oftheantenna Ittemporarilystorespowerandisrelatedtotheimaginarytermoftheinputimpedance TheFarFieldisanelectromagneticfieldlaunchedbytheantennathatextendsthroughoutallspace Thisfieldtransportspowerandisrelatedtotheradiationresistanceoftheantenna 2020 3 20 可编辑 18 AntennasTypes Antennascanbedividedintotwocategories wireantennasandapertureantennas Someofthecommonlyusedantennasarebrieflydescribedbelow 2020 3 20 可编辑 19 HalfwavedipoleThelengthofthisantennaisequaltohalfofitswavelengthasthenameitselfsuggests Dipolescanbeshorterorlongerthanhalfthewavelength butatradeoffexistsintheperformanceandhencethehalfwavelengthdipoleiswidelyused Thedipoleantennaisfedbyatwowiretransmissionline wherethetwocurrentsintheconductorsareofsinusoidaldistributionandequalinamplitude butoppositeindirection Hence duetocancelingeffects noradiationoccursfromthetransmissionline Thecurrentsinthearmsofthedipoleareinthesamedirectionandtheyproduceradiationinthehorizontaldirection Thus foraverticalorientation thedipoleradiatesinthehorizontaldirection 2020 3 20 可编辑 20 MonopoleantennaThemonopoleantennaresultsfromapplyingtheimagetheorytothedipole Accordingtothistheory ifaconductingplaneisplacedbelowasingleelementoflengthL 2carryingacurrent thenthecombinationoftheelementanditsimageactsidenticallytoadipoleoflengthLexceptthattheradiationoccursonlyinthespaceabovetheplane Forthistypeofantenna thedirectivityisdoubledandtheradiationresistanceishalvedwhencomparedtothedipole Thus ahalfwavedipolecanbeapproximatedbyaquarterwavemonopole L 2 4 Themonopoleisveryusefulinmobileantennaswheretheconductingplanecanbethecarbodyorthehandsetcase 2020 3 20 可编辑 21 LoopantennaTheloopantennaisaconductorbentintotheshapeofaclosedcurvesuchasacircleorasquarewithagapintheconductortoformtheterminals Therearetwotypesofloopantennas electricallysmallloopantennasandelectricallylargeloopantennas Ifthetotalloopcircumferenceisverysmallascomparedtothewavelength L thentheloopantennaissaidtobeelectricallysmall Anelectricallylargeloopantennatypicallyhasitscircumferenceclosetoawavelength Thefar fieldradiationpatternsofthesmallloopantennaareinsensitivetoshape 2020 3 20 可编辑 22 HelicalantennaAhelicalantennaorhelixisoneinwhichaconductorconnectedtoagroundplane iswoundintoahelicalshape Theantennacanoperateinanumber