電子信息與通信工程專業(yè)英語(yǔ)lesson4-2

2024/9/16可編輯1

Chapter4

ElectromagneticFieldandWave

Outline4.1ElectromagneticFieldsTheory

4.2Antenna2024/9/16可編輯24.2AntennasOutlineHowanAntennaradiatesBasicantennaparametersAntennasTypes2024/9/16可編輯3Terminologyantennan.天線；觸角radiatevt.放射，輻射，傳播；vi.發(fā)光，輻射adj.輻射狀的electromagneticadj.電磁的；電磁學(xué)的oscillatevi.振動(dòng)，振蕩；搖擺；(銀行利率等)在中間值上下波動(dòng)sinusoidaladj.正弦曲線，竇狀小管，竇狀隙tangentialadj.〈數(shù)〉切線的，正切的；略為觸及的，附帶的conductorn.指揮家，指導(dǎo)者；售票員；導(dǎo)體，導(dǎo)線perpendicularadj.（常與to連用）成直角的；垂直的；陡的（坡）dipolen.偶極子，對(duì)稱振子；偶極天線；雙合價(jià)beamwidthn.波束寬度horizontaladj.地平線的；水平的；平放的，橫的(與vertical相對(duì))directivityn.定向性，指向性，方向性terminaln.終端，終點(diǎn)；極限；接頭，電極，終端設(shè)備；終點(diǎn)站2024/9/16可編輯4resistancen.反抗，抵抗，抵抗力，阻力，電阻，阻抗impedancen.阻抗；電阻抗；交流電阻reactancen.反應(yīng)性；反作用力；電抗componentn.元件，分量gainn.增益polarizationn.[物]偏振(現(xiàn)象)，極化(作用)，兩極化，分化vectorn.矢量，向量ellipticaladj.橢圓的；省略的frequencyn.頻率，周率，發(fā)生次數(shù)transportv.傳送，運(yùn)輸，流放，放逐transmissionn.播送,發(fā)射,傳動(dòng),傳送,傳輸,轉(zhuǎn)播currentadj.當(dāng)前的，通用的，流通的；n.涌流，趨勢(shì)，電流，水流，氣流2024/9/16可編輯5amplituden.振幅，物理學(xué)名詞elementn.要素，元素，成分，元件，自然環(huán)境mobileadj.可移動(dòng)的，易變的，機(jī)動(dòng)的n.運(yùn)動(dòng)物體wavelengthn.波長(zhǎng)insensitiveadj.感覺(jué)遲鈍的，無(wú)感覺(jué)的(to)；不機(jī)敏的；低靈敏度的nulladj.無(wú)效力的，無(wú)效的，無(wú)價(jià)值的，等于零的n.零，空paralleladj.平行的，并聯(lián)的n.平行面，相似物v.相應(yīng)，平行helixn.螺旋，螺旋狀物，解剖]耳輪waveguiden.波導(dǎo)aperturen.孔，穴，縫隙，(照相機(jī),望遠(yuǎn)鏡等的)光圈，孔徑rectangularadj.矩形的，長(zhǎng)方形的；直角的geometryn.幾何學(xué)excitationn.刺激，激勵(lì)，鼓舞，[物]激發(fā)，勵(lì)磁，植]激感(現(xiàn)象)dimensionn.尺寸，尺度，維(數(shù))，度(數(shù))，元IEEEabbr.電氣和電子工程師協(xié)會(huì)2024/9/16可編輯6AnantennaisadevicethatconvertsRFpowerappliedtoitsfeedpointintoelectromagneticradiationandinterceptsenergyfromapassingelectromagneticradiation,whichthenappearsasRFvoltageacrosstheantenna’sfeedpoint.TheofficialIEEEdefinitionofanantennaasgivenbyStutzmanandThielefollowstheconcept:“Thatpartofatransmittingorreceivingsystemthatisdesignedtoradiateorreceiveelectromagneticwaves”.Anantennaactsasatransitionalstructurebetweentheguidingdevice(e.g.waveguide,transmissionline)andthefreespace.Antennascanbeconsideredasoneofthemostimportantpartsoftheradio-communicationchain.IntroductionTexttour2024/9/16可編輯7

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

2024/9/16可編輯8Duetothetimevaryingelectricandmagneticfields,electromagneticwavesarecreatedandthesetravelbetweentheconductors.Asthesewavesapproachopenspace,freespacewavesareformedbyconnectingtheopenendsoftheelectriclines.Sincethesinusoidalsourcecontinuouslycreatestheelectricdisturbance,electromagneticwavesarecreatedcontinuouslyandthesetravelthroughthetransmissionline,throughtheantennaandareradiatedintothefreespace.Insidethetransmissionlineandtheantenna,theelectromagneticwavesaresustainedduetothecharges,butassoonastheyenterthefreespace,theyformclosedloopsandareradiated.2024/9/16可編輯9AntennaRadiationPatterns

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

Basicantennaparameters2024/9/16可編輯10Fromthefieldpattern,wecangetthefollowingparametersofantenna:HPBW:Thehalfpowerbeamwidth(HPBW)canbedefinedastheanglesubtendedbythehalfpowerpointsofthemainlobe.MainLobe:Thisistheradiationlobecontainingthedirectionofmaximumradiation.MinorLobe:Allthelobesotherthenthemainlobearecalledtheminorlobes.Theselobesrepresenttheradiationinundesireddirections.Thelevelofminorlobesisusuallyexpressedasaratioofthepowerdensityinthelobeinquestiontothatofthemajorlobe.Thisratioiscalledasthesidelobelevel(expressedindecibels).BackLobe:Thisistheminorlobediametricallyoppositethemainlobe.SideLobes:Thesearetheminorlobesadjacenttothemainlobeandareseparatedbyvariousnulls.Sidelobesaregenerallythelargestamongtheminorlobes.2024/9/16可編輯11DirectivityThedirectivityofanantennahasbeendefinedas“theratiooftheradiationintensityinagivendirectionfromtheantennatotheradiationintensityaveragedoveralldirections”.Inotherwords,thedirectivityofanonisotropicsourceisequaltotheratioofitsradiationintensityinagivendirection,overthatofanisotropicsource.Directivityisadimensionlessquantity,sinceitistheratiooftworadiationintensities.Hence,itisgenerallyexpressedindB.Thedirectivityofanantennacanbeeasilyestimatedfromtheradiationpatternoftheantenna.Anantennathathasanarrowmainlobewouldhavebetterdirectivity,thantheonewhichhasabroadmainlobe,henceitismoredirective.2024/9/16可編輯12Theinputimpedanceofanantennaisdefinedas“theimpedancepresentedbyanantennaatitsterminalsortheratioofthevoltagetothecurrentatthepairofterminalsortheratiooftheappropriatecomponentsoftheelectrictomagneticfieldsatapoint”.Inputimpedance2024/9/16可編輯13GainAntennagainisaparameterwhichiscloselyrelatedtothedirectivityoftheantenna.Weknowthatthedirectivityishowmuchanantennaconcentratesenergyinonedirectioninpreferencetoradiationinotherdirections.Hence,iftheantennais100%efficient,thenthedirectivitywouldbeequaltotheantennagainandtheantennawouldbeanisotropicradiator.

2024/9/16可編輯14Polarizationofaradiatedwaveisdefinedas“thatpropertyofanelectromagneticwavedescribingthetimevaryingdirectionandrelativemagnitudeoftheelectricfieldvector”.Thepolarizationofanantennareferstothepolarizationoftheelectricfieldvectoroftheradiatedwave.Inotherwords,thepositionanddirectionoftheelectricfieldwithreferencetotheearth’ssurfaceorgrounddeterminesthewavepolarization.Themostcommontypesofpolarizationincludethelinear(horizontalorvertical),circular(righthandpolarization(RHCP)orthelefthandpolarization(LHCP)),EllipticalRightHandandEllipticalLeftHand.AntennaPolarization

2024/9/16可編輯15Polarizationismostimportantifyouaretryingtogetthemaximumperformancefromtheantennas.Forbestperformanceyouwillneedtomatchupthepolarizationofthetransmittingantennaandthereceivingantenna.Whenasignalistransmittedatonepolarizationbutreceivedatadifferentpolarizationthereexistagreatmanydecibelsofloss.2024/9/16可編輯16BandwidthThebandwidthofanantennaisdefinedas“therangeofusablefrequencieswithinwhichtheperformanceoftheantenna,withrespecttosomecharacteristic,conformstoaspecifiedstandard.”Thebandwidthcanbetherangeoffrequenciesoneithersideofthecenterfrequencywheretheantennacharacteristicslikeinputimpedance,radiationpattern,beamwidth,polarization,sidelobelevelorgain,areclosetothosevalueswhichhavebeenobtainedatthecenterfrequency.Thebandwidthofabroadbandantennacanbedefinedastheratiooftheuppertolowerfrequenciesofacceptableoperation.Thebandwidthofanarrowbandantennacanbedefinedasthepercentageofthefrequencydifferenceoverthecenterfrequency.2024/9/16可編輯17TheNearandFarField

Thefieldpatterns,associatedwithanantenna,changewithdistanceandareassociatedwithtwotypesofenergy:-radiatingenergyandreactiveenergy.Hence,thespacesurroundinganantennacanbedividedintotworegions.TheNearFieldisanelectromagneticfieldthatexistswithin~λ/2oftheantenna.Ittemporarilystorespowerandisrelatedtotheimaginarytermoftheinputimpedance.TheFarFieldisanelectromagneticfieldlaunchedbytheantennathatextendsthroughoutallspace.Thisfieldtransportspowerandisrelatedtotheradiationresistanceoftheantenna.2024/9/16可編輯18AntennasTypesAntennascanbedividedintotwocategories:wireantennasandapertureantennas.Someofthecommonlyusedantennasarebrieflydescribedbelow..

2024/9/16可編輯19Halfwavedipole

Thelengthofthisantennaisequaltohalfofitswavelengthasthenameitselfsuggests.Dipolescanbeshorterorlongerthanhalfthewavelength,butatradeoffexistsintheperformanceandhencethehalfwavelengthdipoleiswidelyused.Thedipoleantennaisfedbyatwowiretransmissionline,wherethetwocurrentsintheconductorsareofsinusoidaldistributionandequalinamplitude,butoppositeindirection.Hence,duetocancelingeffects,noradiationoccursfromthetransmissionline.Thecurrentsinthearmsofthedipoleareinthesamedirectionandtheyproduceradiationinthehorizontaldirection.Thus,foraverticalorientation,thedipoleradiatesinthehorizontaldirection.2024/9/16可編輯20MonopoleantennaThemonopoleantennaresultsfromapplyingtheimagetheorytothedipole.Accordingtothistheory,ifaconductingplaneisplacedbelowasingleelementoflengthL/2carryingacurrent,thenthecombinationoftheelementanditsimageactsidenticallytoadipoleoflengthLexceptthattheradiationoccursonlyinthespaceabovetheplane.Forthistypeofantenna,thedirectivityisdoubledandtheradiationresistanceishalvedwhencomparedtothedipole.Thus,ahalfwavedipolecanbeapproximatedbyaquarterwavemonopole(L/2=λ/4).Themonopoleisveryusefulinmobileantennaswheretheconductingplanecanbethecarbodyorthehandsetcase.2024/9/16可編輯21LoopantennaTheloopantennaisaconductorbentintotheshapeofaclosedcurvesuchasacircleorasquarewithagapintheconductortoformtheterminals.Therearetwotypesofloopantennas-electricallysmallloopantennasandelectricallylargeloopantennas.Ifthetotalloopcircumferenceisverysmallascomparedtothewavelength(L<<<λ),thentheloopantennaissaidtobeelectricallysmall.Anelectricallylargeloopantennatypicallyhasitscircumferenceclosetoawavelength.Thefar-fieldradiationpatternsofthesmallloopantennaareinsensitivetoshape.2024/9/16可編輯22HelicalantennaAhelicalantennaorhelixisoneinwhichaconductorconnectedtoagroundplane,iswoundintoahelical