电子信息与通信工程专业英语lesson4-2001

2024/7/5可编辑1

Chapter4

ElectromagneticFieldandWave

Outline4.1ElectromagneticFieldsTheory

4.2Antenna2024/7/5可编辑24.2AntennasOutlineHowanAntennaradiatesBasicantennaparametersAntennasTypes2024/7/5可编辑3Terminologyantennan.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2024/7/5可编辑4resistancen.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2024/7/5可编辑5amplituden.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2024/7/5可编辑6AnantennaisadevicethatconvertsRFpowerappliedtoitsfeedpointintoelectromagneticradiationandinterceptsenergyfromapassingelectromagneticradiation,whichthenappearsasRFvoltageacrosstheantenna’sfeedpoint.TheofficialIEEEdefinitionofanantennaasgivenbyStutzmanandThielefollowstheconcept:“Thatpartofatransmittingorreceivingsystemthatisdesignedtoradiateorreceiveelectromagneticwaves”.Anantennaactsasatransitionalstructurebetweentheguidingdevice(e.g.waveguide,transmissionline)andthefreespace.Antennascanbeconsideredasoneofthemostimportantpartsoftheradio-communicationchain.IntroductionTexttour2024/7/5可编辑7

HowanAntennaradiatesAconductingwireradiatesmainlybecauseoftime-varyingcurrentoranacceleration(ordeceleration)ofcharge.Ifthereisnomotionofchargesinawire,noradiationtakesplace,sincenoflowofcurrentoccurs.Radiationwillnotoccurevenifchargesaremovingwithuniformvelocityalongastraightwire.However,chargesmovingwithuniformvelocityalongacurvedorbentwirewillproduceradiation.Ifthechargeisoscillatingwithtime,thenradiationoccursevenalongastraightwire.

2024/7/5可编辑8Duetothetimevaryingelectricandmagneticfields,electromagneticwavesarecreatedandthesetravelbetweentheconductors.Asthesewavesapproachopenspace,freespacewavesareformedbyconnectingtheopenendsoftheelectriclines.Sincethesinusoidalsourcecontinuouslycreatestheelectricdisturbance,electromagneticwavesarecreatedcontinuouslyandthesetravelthroughthetransmissionline,throughtheantennaandareradiatedintothefreespace.Insidethetransmissionlineandtheantenna,theelectromagneticwavesaresustainedduetothecharges,butassoonastheyenterthefreespace,theyformclosedloopsandareradiated.2024/7/5可编辑9AntennaRadiationPatterns

Antennaradiationpatternisagraphicrepresentationofdirectionalpropertiesoftheantenna.Directionalpropertiesareusuallyexpressedintheformoftheabsolutevalueoftheradioofelectricintensityoftheradiatedwaveinagivendirectionanditsmaximumvalue.Directivitypatternisusuallydrawnforacertainplane.Anantennaradiationpatterncanbedisplayedintwokindsofgraphics:thosedisplayingtheE-plane(orE-fieldforElectricfield)andtheH-plane(orH-fieldformagneticfield).

Basicantennaparameters2024/7/5可编辑10Fromthefieldpattern,wecangetthefollowingparametersofantenna:HPBW:Thehalfpowerbeamwidth(HPBW)canbedefinedastheanglesubtendedbythehalfpowerpointsofthemainlobe.MainLobe:Thisistheradiationlobecontainingthedirectionofmaximumradiation.MinorLobe:Allthelobesotherthenthemainlobearecalledtheminorlobes.Theselobesrepresenttheradiationinundesireddirections.Thelevelofminorlobesisusuallyexpressedasaratioofthepowerdensityinthelobeinquestiontothatofthemajorlobe.Thisratioiscalledasthesidelobelevel(expressedindecibels).BackLobe:Thisistheminorlobediametricallyoppositethemainlobe.SideLobes:Thesearetheminorlobesadjacenttothemainlobeandareseparatedbyvariousnulls.Sidelobesaregenerallythelargestamongtheminorlobes.2024/7/5可编辑11DirectivityThedirectivityofanantennahasbeendefinedas“theratiooftheradiationintensityinagivendirectionfromtheantennatotheradiationintensityaveragedoveralldirections”.Inotherwords,thedirectivityofanonisotropicsourceisequaltotheratioofitsradiationintensityinagivendirection,overthatofanisotropicsource.Directivityisadimensionlessquantity,sinceitistheratiooftworadiationintensities.Hence,itisgenerallyexpressedindB.Thedirectivityofanantennacanbeeasilyestimatedfromtheradiationpatternoftheantenna.Anantennathathasanarrowmainlobewouldhavebetterdirectivity,thantheonewhichhasabroadmainlobe,henceitismoredirective.2024/7/5可编辑12Theinputimpedanceofanantennaisdefinedas“theimpedancepresentedbyanantennaatitsterminalsortheratioofthevoltagetothecurrentatthepairofterminalsortheratiooftheappropriatecomponentsoftheelectrictomagneticfieldsatapoint”.Inputimpedance2024/7/5可编辑13GainAntennagainisaparameterwhichiscloselyrelatedtothedirectivityoftheantenna.Weknowthatthedirectivityishowmuchanantennaconcentratesenergyinonedirectioninpreferencetoradiationinotherdirections.Hence,iftheantennais100%efficient,thenthedirectivitywouldbeequaltotheantennagainandtheantennawouldbeanisotropicradiator.

2024/7/5可编辑14Polarizationofaradiatedwaveisdefinedas“thatpropertyofanelectromagneticwavedescribingthetimevaryingdirectionandrelativemagnitudeoftheelectricfieldvector”.Thepolarizationofanantennareferstothepolarizationoftheelectricfieldvectoroftheradiatedwave.Inotherwords,thepositionanddirectionoftheelectricfieldwithreferencetotheearth’ssurfaceorgrounddeterminesthewavepolarization.Themostcommontypesofpolarizationincludethelinear(horizontalorvertical),circular(righthandpolarization(RHCP)orthelefthandpolarization(LHCP)),EllipticalRightHandandEllipticalLeftHand.AntennaPolarization

2024/7/5可编辑15Polarizationismostimportantifyouaretryingtogetthemaximumperformancefromtheantennas.Forbestperformanceyouwillneedtomatchupthepolarizationofthetransmittingantennaandthereceivingantenna.Whenasignalistransmittedatonepolarizationbutreceivedatadifferentpolarizationthereexistagreatmanydecibelsofloss.2024/7/5可编辑16BandwidthThebandwidthofanantennaisdefinedas“therangeofusablefrequencieswithinwhichtheperformanceoftheantenna,withrespecttosomecharacteristic,conformstoaspecifiedstandard.”Thebandwidthcanbetherangeoffrequenciesoneithersideofthecenterfrequencywheretheantennacharacteristicslikeinputimpedance,radiationpattern,beamwidth,polarization,sidelobelevelorgain,areclosetothosevalueswhichhavebeenobtainedatthecenterfrequency.Thebandwidthofabroadbandantennacanbedefinedastheratiooftheuppertolowerfrequenciesofacceptableoperation.Thebandwidthofanarrowbandantennacanbedefinedasthepercentageofthefrequencydifferenceoverthecenterfrequency.2024/7/5可编辑17TheNearandFarField

Thefieldpatterns,associatedwithanantenna,changewithdistanceandareassociatedwithtwotypesofenergy:-radiatingenergyandreactiveenergy.Hence,thespacesurroundinganantennacanbedividedintotworegions.TheNearFieldisanelectromagneticfieldthatexistswithin~λ/2oftheantenna.Ittemporarilystorespowerandisrelatedtotheimaginarytermoftheinputimpedance.TheFarFieldisanelectromagneticfieldlaunchedbytheantennathatextendsthroughoutallspace.Thisfieldtransportspowerandisrelatedtotheradiationresistanceoftheantenna.2024/7/5可编辑18AntennasTypesAntennascanbedividedintotwocategories:wireantennasandapertureantennas.Someofthecommonlyusedantennasarebrieflydescribedbelow..

2024/7/5可编辑19Halfwavedipole

Thelengthofthisantennaisequaltohalfofitswavelengthasthenameitselfsuggests.Dipolescanbeshorterorlongerthanhalfthewavelength,butatradeoffexistsintheperformanceandhencethehalfwavelengthdipoleiswidelyused.Thedipoleantennaisfedbyatwowiretransmissionline,wherethetwocurrentsintheconductorsareofsinusoidaldistributionandequalinamplitude,butoppositeindirection.Hence,duetocancelingeffects,noradiationoccursfromthetransmissionline.Thecurrentsinthearmsofthedipoleareinthesamedirectionandtheyproduceradiationinthehorizontaldirection.Thus,foraverticalorientation,thedipoleradiatesinthehorizontaldirection.2024/7/5可编辑20MonopoleantennaThemonopoleantennaresultsfromapplyingtheimagetheorytothedipole.Accordingtothistheory,ifaconductingplaneisplacedbelowasingleelementoflengthL/2carryingacurrent,thenthecombinationoftheelementanditsimageactsidenticallytoadipoleoflengthLexceptthattheradiationoccursonlyinthespaceabovetheplane.Forthistypeofantenna,thedirectivityisdoubledandtheradiationresistanceishalvedwhencomparedtothedipole.Thus,ahalfwavedipolecanbeapproximatedbyaquarterwavemonopole(L/2=λ/4).Themonopoleisveryusefulinmobileantennaswheretheconductingplanecanbethecarbodyorthehandsetcase.2024/7/5可编辑21LoopantennaTheloopantennaisaconductorbentintotheshapeofaclosedcurvesuchasacircleorasquarewithagapintheconductortoformtheterminals.Therearetwotypesofloopantennas-electricallysmallloopantennasandelectricallylargeloopantennas.Ifthetotalloopcircumferenceisverysmallascomparedtothewavelength(L<<<λ),thentheloopantennaissaidtobeelectricallysmall.Anelectricallylargeloopantennatypicallyhasitscircumferenceclosetoawavelength.Thefar-fieldradiationpatternsofthesmallloopantennaareinsensitivetoshape.2024/7/5可编辑22HelicalantennaAhelicalantennaorhelixisoneinwhichaconductorconnectedtoagroundplane,iswoundintoahelica