通信及基础工程17

Unit14TextAOverviewofAntennaTechnologyOverviewofAntennaTechnologyAnantennaisatransducerthatconvertsaguidedwavepropagatingonatransmissionlineintoanelectromagneticwavepropagatinginanunboundedmedium(usuallyfreespace),orviceversa.Figure14-1showshowawaveislaunchedbyahornlikeantennawiththehornactingasthetransitionsegmentbetweenthewaveguide[1]andfreespace.TextTextWordsNotesDiscussionHomeAntennasaremadeinvariousshapesandsizes(seeFigure14-2)andareusedinradioandtelevisionbroadcastingandreception,radio-wavecommunicationsystems,cellulartelephones,radarsystems,anticollisionautomobilesensors,andmanyotherapplications.Theradiationandimpedancepropertiesofanantennaaregovernedbyitsshape,size,andmaterialproperties.Thedimensionsofanantennaareusuallymeasuredinunitsofλofthewaveitislaunchingorreceiving;a1-mlongdipoleantennaoperatingatawavelengthλ=2mexhibitsthesamepropertiesasa1-cmlongdipoleoperatingatλ=2cm.Hence,inmostofourdiscussionsinthischapter,werefertoantennadimensionsinwavelengthunits.TextTextWordsNotesDiscussionHome1.ReciprocityThedirectionalfunctioncharacterizingtherelativedistributionofpowerradiatedbyanantennaisknownastheantennaradiationpattern(orsimplytheantennapattern).Anisotropicantennaisahypotheticalantennathatradiatesequallyinalldirections,anditisoftenusedasareferenceradiatorwhendescribingtheradiationpropertiesofrealantennas.▶Mostantennasarereciprocaldevices,exhibitingthesameradiationpatternfortransmissionasforreception.◀TextWordsNotesDiscussionHomeReciprocitymeansthat,ifinthetransmissionmodeagivenantennatransmitsindirectionA100timesthepowerittransmitsindirectionB,thenwhenusedinthereceptionmodeitis100timesmoresensitivetoelectromagneticradiationincidentfromdirectionAthanfromB.AlloftheantennasshowninFig.14-2obeythereciprocitylaw,butnotallantennasarereciprocaldevices.Reciprocitymaynotholdforsomesolid-stateantennascomposedofnonlinearsemiconductorsorferritematerials.Suchnonreciprocalantennasarebeyondthescopeofthischapter;hence,reciprocityisassumedthroughout.Thereciprocitypropertyisveryconvenientbecauseitallowsustocomputetheradiationpatternofanantennainthetransmissionmode—evenwhentheantennaisintendedtooperateasareceiver.TextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeTofullycharacterizeanantenna,oneneedstostudyitsradiationpropertiesandimpedance.Theradiationpropertiesincludeitsdirectionalradiationpatternandtheassociatedpolarizationstateoftheradiatedwavewhentheantennaisusedinthetransmissionmode,whichisalsocalledtheantennapolarization.TextTextWordsNotesDiscussionHomeFigure14-1▶Beingareciprocaldevice,anantenna,whenoperatinginthereceivingmode,canextractfromanincidentwaveonlythatcomponentofthewavewhoseelectricfieldmatchestheantennapolarizationstate.◀TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeThesecondaspect,theantennaimpedance,pertainstothetransferofpowerfromageneratortotheantennawhentheantennaisusedasatransmitterand,conversely,thetransferofpowerfromtheantennatoaloadwhentheantennaisusedasareceiver.Itshouldbenotedthatthroughoutourdiscussionsinthischapteritwillbeassumedthattheantennaisproperlymatchedtothetransmissionlineconnectedtoitsterminals,therebyavoidingreflectionsandtheirassociatedproblems.TextTextWordsNotesDiscussionHomeFigure14-22.RadiationSourcesRadiationsourcesfallintotwocategories:currentsandaperturefields.Thedipoleandloopantennas(Fig.14-2(a)and(c))areexamplesofcurrentsources;thetime-varyingcurrentsflowingintheconductingwiresgiverisetotheradiatedelectromagneticfields.Ahornantenna(Fig.14-2(g))isanexampleofthesecondgroupbecausetheelectricandmagneticfieldsacrossthehorn’sapertureserveasthesourcesoftheradiatedfields.Theaperturefieldsarethemselvesinducedbytimevaryingcurrentsonthesurfacesofthehorn’swalls.Therefore,allradiationultimatelyisduetotime-varyingcurrents.Thechoiceofcurrentsoraperturesasthesourcesismerelyacomputationalconveniencearisingfromthestructureoftheantenna.Wewillexaminetheradiationprocessesassociatedwithbothtypesofsources.TextWordsNotesDiscussionHome3.Far-FieldRegionThewaveradiatedbyapointsourceissphericalinnature,withthewavefrontexpandingoutwardatarateequaltothephasevelocityup(orthevelocityoflightcifthemediumisfreespace).IfR,whichisthedistancebetweenthetransmittingantennaandthereceivingantenna,issufficientlylargeenoughforthewavefrontacrossthereceivingaperturetobeconsideredplanar(seeFigure14-3),thenthereceivingapertureissaidtobeinthefar-field(orfar-zone)regionofthetransmittingpointsource.Thisregionisofparticularsignificancebecause—formostapplications—thelocationoftheobservationpointisindeedinthefar-fieldregionoftheantenna.Thefar-fieldplane-waveapproximationallowstheuseofcertainmathematicalapproximationsthatsimplifythecomputationoftheradiatedfieldand,conversely,provideconvenienttechniquesforsynthesizingtheappropriateantennastructurethatwouldgiverisetothedesiredfar-fieldantennapattern.TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeFigure14-34.AntennaArrays[2]

Whenmultipleantennasoperatetogether,thecombinationiscalledanantennaarray(Fig.14-2(i)),andthearrayasawholebehavesasifitwereasingleantenna.Bycontrollingthemagnitudeandphaseofthesignalfeedingeachantenna,itispossibletoshapetheradiationpatternofthearrayandtoelectronicallysteerthedirectionofthebeamelectronically.TextTextWordsNotesDiscussionHomewaveguide[ˈweɪvˌɡaɪd]n.[电子]波导anticollision[ˌæntɪkəˈlɪʒən]n.防撞击reciprocity[ˌresɪˈprɒsəti]n.互惠，互换isotropic[ˌaɪsəʊˈtrɒpɪk]adj.[物][数]各向同性的；等方性的hypothetical[ˌhaɪpəˈθetɪk(ə)l]adj.（基于）假设的，假定的ferrite[ˈferaɪt]n.铁酸盐；铁素体WordsWordsTextWordsNotesDiscussionHome[1]waveguide指波导，是用来定向引导电磁波的结构。在电磁学和通信工程中，波导这个词可以指在它的端点间传递电磁波的任何线性结构，最初和最常见的意思是指用来传输无线电波的空心金属管。波导主要用作微波频率的传输线，在微波炉、雷达、通讯卫星和微波无线电链路设备中用来将微波发送器和接收机与它们的天线连接起来。常见的波导结构主要有平行双导线、同轴线、平行平板波导、矩形波导、圆波导、微带线、平板介质光波导和光纤。从引导电磁波的角度看，它们都可分为内部区域和外部区域，电磁波被限制在内部区域传播（要求在波导横截面内满足横向谐振原理）。TextWordsNotesDiscussionNotesHome[2]AntennaArrays指天线阵，就是把许多辐射单元按一定方向排列所构成的辐射系统。构成天线阵的辐射单元称为阵元。而且此阵元必须是相似元或相同元，所谓相似元或相同元是指所有阵元必须结构形状相同、尺寸相同。而且排列方向也相同，即具有相同的方向图因子。阵元可以是对称振子。也可以是其他形式的天线。天线阵的辐射场是各个天线元所产生电磁场的矢量叠加。为了增加天线的方向性或得到所需的辐射方向，采用天线阵。天线阵的作用就是增强天线的方向性。提高天线的增益系数。或者为了得到所需的方向特性。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.

Whatisanantenna?2.

Howcanantennasbeapplied?3.Whatistheantennaradiationpattern?4.WhatisFar-FieldRegion?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.Whatisanantenna?Anantennaisatransducerthatconvertsaguidedwavepropagatingonatransmissionlineintoanelectromagneticwavepropagatinginanunboundedmedium(usuallyfreespace),orviceversa.TextWordsNotesDiscussionHome2.Howcanantennasbeapplied?Antennasaremadeinvariousshapesandsizesandareusedinradioandtelevisionbroadcastingandreception,radio-wavecommunicationsystems,cellulartelephones,radarsystems,anticollisionautomobilesensors,andmanyot