电子信息与通信工程专业英语lesson422太原理工大学版

《电子信息与通信工程专业英语》AntennasTypes天线类型Antennascanbedividedintotwocategories:wireantennasandapertureantennas.Someofthecommonlyusedantennasarebrieflydescribedbelow..《电子信息与通信工程专业英语》Halfwavedipoleantenna半波振子天线Thelengthofthisantennaisequaltohalfofitswavelengthasthenameitselfsuggests.Dipolescanbeshorterorlongerthanhalfthewavelength,butatradeoffexistsintheperformanceandhencethehalfwavelengthdipoleiswidelyused.Thedipoleantennaisfedbyatwowiretransmissionline,wherethetwocurrentsintheconductorsareofsinusoidaldistributionandequalinamplitude,butoppositeindirection.Hence,duetocancelingeffects,noradiationoccursfromthetransmissionline.Thecurrentsinthearmsofthedipoleareinthesamedirectionandtheyproduceradiationinthehorizontaldirection.Thus,foraverticalorientation,thedipoleradiatesinthehorizontaldirection.《电子信息与通信工程专业英语》Monopoleantenna单极天线Themonopoleantennaresultsfromapplyingtheimagetheorytothedipole.Accordingtothistheory,ifaconductingplaneisplacedbelowasingleelementoflengthL/2carryingacurrent,thenthecombinationoftheelementanditsimageactsidenticallytoadipoleoflengthLexceptthattheradiationoccursonlyinthespaceabovetheplane.《电子信息与通信工程专业英语》Forthistypeofantenna,thedirectivityisdoubledandtheradiationresistanceishalvedwhencomparedtothedipole.Thus,ahalfwavedipolecanbeapproximatedbyaquarterwavemonopole(L/2=λ/4).Themonopoleisveryusefulinmobileantennaswheretheconductingplanecanbethecarbodyorthehandsetcase.《电子信息与通信工程专业英语》Loopantenna回路天线Theloopantennaisaconductorbentintotheshapeofaclosedcurvesuchasacircleorasquarewithagapintheconductortoformtheterminals.Therearetwotypesofloopantennas-electricallysmallloopantennasandelectricallylargeloopantennas.Ifthetotalloopcircumferenceisverysmallascomparedtothewavelength(Lλ),thentheloopantennaissaidtobeelectricallysmall.Anelectricallylargeloopantennatypicallyhasitscircumferenceclosetoawavelength.Thefar-fieldradiationpatternsofthesmallloopantennaareinsensitivetoshape.《电子信息与通信工程专业英语》Helicalantenna螺旋形天线Ahelicalantennaorhelixisoneinwhichaconductorconnectedtoagroundplane,iswoundintoahelicalshape.Theantennacanoperateinanumberofmodes,howeverthetwoprincipalmodesarethenormalmode(broadsideradiation)andtheaxialmode(endfireradiation).Whenthehelixdiameterisverysmallascomparedtothewavelength,thentheantennaoperatesinthenormalmode.However,whenthecircumferenceofthehelixisoftheorderofawavelength,thenthehelicalantennaissaidtobeoperatingintheaxialmode.《电子信息与通信工程专业英语》Hornantenna号角天线Hornantennasareusedtypicallyinthemicrowaveregion(gigahertzrange)wherewaveguidesarethestandardfeedmethod,sincehornantennasessentiallyconsistofawaveguidewhoseendwallsareflaredoutwardstoformamegaphonelikestructure.Hornsprovidehighgain,lowVSWR,relativelywidebandwidth,lowweight,andareeasytoconstruct.Theapertureofthehorncanberectangular,circularorelliptical.Thehornbasicallyactsasatransitionfromthewaveguidemodetothefree-spacemodeandthistransitionreducesthereflectedwavesandemphasizesthetravelingwaveswhichleadtolowVSWRandwidebandwidth.Thehorniswidelyusedasafeedelementforlargeradioastronomy,satellitetracking,andcommunicationdishes.《电子信息与通信工程专业英语》EXERCISES练习《电子信息与通信工程专业英语》Theendoflesson4.2Thanks!Seeyounexttime!