电子信息与通信工程专业英语lesson4-2.2(太原理工大学版).ppt

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1、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.Dipolescanbeshorterorlongerthanh

2、alfthewavelength,butatradeoffexistsintheperformanceandhencethehalfwavelengthdipoleiswidelyused.Thedipoleantennaisfedbyatwowiretransmissionline,wherethetwocurrentsintheconductorsareofsinusoidaldistributionandequalinamplitude,butoppositeindirection.Hence,duetocancelingeffects,noradiationoccu

3、rsfromthetransmissionline.Thecurrentsinthearmsofthedipoleareinthesamedirectionandtheyproduceradiationinthehorizontaldirection.Thus,foraverticalorientation,thedipoleradiatesinthehorizontaldirection.《电子信息与通信工程专业英语》Monopoleantenna单极天线Themonopoleantennaresultsfromapplyingtheimagetheorytothedip

4、ole.Accordingtothistheory,ifaconductingplaneisplacedbelowasingleelementoflengthL/2carryingacurrent,thenthecombinationoftheelementanditsimageactsidenticallytoadipoleoflengthLexceptthattheradiationoccursonlyinthespaceabovetheplane.《电子信息与通信工程专业英语》Forthistypeofantenna,thedirectivityisdoubledan

5、dtheradiationresistanceishalvedwhencomparedtothedipole.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Theloopantennaisaconductorbentintotheshape

6、ofaclosedcurvesuchasacircleorasquarewithagapintheconductortoformtheterminals.Therearetwotypesofloopantennas-electricallysmallloopantennasandelectricallylargeloopantennas.Ifthetotalloopcircumferenceisverysmallascomparedtothewavelength(L<<<λ),thentheloopantennaissaidtobeelectricallysmall.Ane

7、lectricallylargeloopantennatypicallyhasitscircumferenceclosetoawavelength.Thefar-fieldradiationpatternsofthesmallloopantennaareinsensitivetoshape.《电子信息与通信工程专业英语》Helicalantenna螺旋形天线Ahelicalantennaorhelixisoneinwhichaconductorconnectedtoagroundplane,iswoundintoa