《通信工程专业英语（第二版）》课件Unit 14

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,andmanyotherapplications.TextWordsNotesDiscussionHome3.Whatistheantennaradiationpattern?Thedirectionalfunctioncharacterizingtherelativedistributionofpowerradiatedbyanantennaisknownastheantennaradiationpattern(orsimplytheantennapattern).TextWordsNotesDiscussionHome4.WhatisFar-FieldRegion?Thewaveradiatedbyapointsourceissphericalinnature,withthewavefrontexpandingoutwardatarateequaltothephasevelocityup(orthevelocityoflightcifthemediumisfreespace).IfR,whichisthedistancebetweenthetransmittingantennaandthereceivingantenna,issufficientlylargeenoughforthewavefrontacrossthereceivingaperturetobeconsideredplanar,thenthereceivingapertureissaidtobeinthefar-field(orfar-zone)regionofthetransmittingpointsource.TextWordsNotesDiscussionHomeThankyou!Unit14TextBTheFive-hundred-meterApertureSphericalradioTelescope(FAST)projectTheFive-hundred-meterApertureSphericalradioTelescope(FAST)project1.EngineeringconceptofFASTFASTisanArecibo-typeantennawiththreeoutstandingaspects:thekarstdepressionusedasthesite,whichislargeenoughtohostthe500-metertelescopeanddeepenoughtoallowazenithangle[1]of40degrees;theactivemainreflectorcorrectingforsphericalaberrationonthegroundtoachieveafullpolarization;andawidebandwithoutinvolvingcomplexfeedsystems.IfweadoptArecibodesign,wewouldneed10000tonsofsteelabovethereflector,whichisimplausible.Wehaveverylightfocalcabinweightinglessthan50tons.Feedcabinissupportedanddrivenbycablesandservomechanism.Insidethecabin,secondaryadjustablesystemismountedtocompensatetheoffsetofthecabinmainlyduetowindimpact.TextTextWordsNotesDiscussionHomeTheoutstandingfeaturesofFASTanditsopticalgeometryareshowninFigure14-4.Mainreflectorisasphericalcapwiththecurvatureofa300mradiusandopeningupto500m.Duringtheobservation,the300milluminatedareaformsaparaboloidtothesourcedirection.Feedmovesonavirtualsphericalcaphalf-wayfromthereflectortotheoriginofthesphere.Thecentralpartofasphericalsurfaceisverynearlyaparaboloidofrevolutionwhenaproperfocallengthischosen.Intheconfigurationshowninthefigure,thepeakdeviationbetweendeformedparaboloid(yellowdashedline)andthesphericalcapwillbeminimizedto0.67mwithinthe300milluminatedaperture.Forthemaximumapparentmotionofcelestialobjects,therateofvariationisalsofoundtobeverysmall,lessthan0.7mm/s,whichenablesastraightforwardsolutionforthemechanicalcontrol.Todeformthereflector,itisnecessarytodivideitintosmallelements.Eachelementisasmallpartofasphericalsurface,andcurvaturesofalltheelementsareidenticaltogetanisotropicgeometryforthereflector.Themaintechnicalspecificationsarelistedintable14-4.TextTextWordsNotesDiscussionHomeTextWordsNotesDiscussionHomeFigure14-4TextWordsNotesDiscussionHomeTable14-1.MaintechnicalspecificationsofFASTTextTextWordsNotesDiscussionHome2.Technicalplan-criticaltechnologiesTheFASTsiteselectionstartedin1994andakarstdepressioncalledDawodanginthesouthernGuizhouProvincewasfinallychosen(seeFigure14-5).Telescopeconstructionisdividedintosixsub-systems,includingsitesurveyandexcavation;activereflectorsystem;feedcabinsuspensionsystem;measurementandcontrolsystem;receiverandbackendsystem;andobservatorybuilding.(1)SitesurveyandexcavationDespiteoftheexcellentfitbetweenthedepressionprofileandthesphericalcapmodel,westillneedtomovesomeonemillioncubemetersofearthfromthekarsthole.Atunneltolinkthesinkholetolowerdepressionisrequiredinordertoguaranteethesafetyofthetelescope.AsofSeptember2012,thesiteexcavationandprotectionofthedangerousrocksandslopesarenearcompletion.Theconstructionofundergroundtunnelof~1.2kmandexternalroadsof~4kminthedepressionhavebeensuccessfullycompleted(seeFigure14-5).TextTextWordsNotesDiscussionHomeTextTextWordsNotesDiscussionHomeFigure14-5TextTextWordsNotesDiscussionHome(2)ActivereflectorsystemMostexpensivepartofthetelescopeisthemainactivereflectorof500mindiameter.Thewholesurfaceistobesupportedbycablenetwork.Morethan2000actuatorsdrivetie-downcablesaccordingtothefeedbackfromthemeasuringsystemtodeformthesurface.Theoptimizationoftheelementsizeisatradeoffbetweenaccuracyandcost.Thewholereflectorconsistsof~4400triangularpanels,whichgiveanRMS[2]errorsmallerthan~5.0mm.Fromtheaforementioneddescription,itcanbeeasilyseenthatthelong-termobservationprocessofFASTisequivalenttolong-termfrequentshape-changingoperation.Earlyresearchresultshaveshownthatsuchshape-changingoperationwouldleadtoabout500MPaofstressrange,whichisnearlytwicetheindustrystandard.TheFASTteamhavecarriedoutextensivenumericalandexperimentalinvestigationtodecreasethestressrangeofthecablerequiredandhavesuccessfullydevelopedanewtypeofsteelstrandwithultrahighfatigueresistantproperty.Allsampleshavepassedthrough2×106fatiguecyclesunder550MPastressrange.Comparedwithfull70000fatiguecyclesrequiredduringFASTlifetimeandthemaximumdemandedstressrangeof450Mpa,enoughreservehasbeenguaranteed(Jiangetal.2011).TextTextWordsNotesDiscussionHomeThesidedimensionofeachreflectorpanelisabout11m,dependingonitslocationonthesurface.Differentkindofprototypinghavebeeninvestigatedandexperimented.Thefinaldesignhasasurfacemadeofthealuminumsheet,backupofaluminumspatialtrussandanadjustablelayerinbetween.TextWordsNotesDiscussionHomeItisextremelyimportanttohavereliableactuatorsconsideringtheirlargeamount.Besideslonglife,easymaintainabilityandlessdependenceonneighboringonesarecrucialfactors.VarioustypesofactuatorshavebeenmanufacturedandtestedattheMiyunstationofNAOCandatthedepressionsite.Thefinalselectionbetweenmechanicalandhydraulicactuatorsisunderway.TextTextWordsNotesDiscussionHome(3)FeedcabinsuspensionsystemFeedcabinsuspensionsystemmightbethemostriskypartofFAST.Thereisnosolidconnectionbetweenthereflectorandthefeedcabinbecauseofthelargedimensionofthetelescope.ThefeedcabinofFASTissupportedanddrivenbycablesandservomechanism.Insideofthecabin,secondaryadjustablesystemisemployedtoachievetherequiredaccuracy.Thedesignsconsistofthreeessentialparts,includingthecablenetworkthatsupportsanddrivesthefeedcabin,thesecondaryadjustabledevicesinsidethecabinthatcarrythemostprecisepartofthereceivers,andthecloseloopcontrol.Numerousmodelsweremadetoverifythefeasibilityoftheconcept.TextTextWordsNotesDiscussionHomeBecauseofthelargedimensionandthecomplicateddynamics,similaritylawsmaynotguaranteeapplyingtheresultsfromdownscaledmodelstorealstructure.Theteamhascarriedoutend-to-endsimulationcooperatedwithMTMechatronicsandTechnicalUniversityDarmstadt.Thisanalysishasconfirmedthefeasibilityoftheconceptandyieldedcriticalresultsforsystemoptimization.Thedisplacementoffeedcabinafterfirstadjustmentcontrolcanbeconstrainedtobewithinafewcentimeters.Theachievableaccuracyoffeedpositionwiththehelpofthesecondarystabilizerturnsouttobeafewmillimeters,whichmeetstherequirementoftelescopepointing.TextTextWordsNotesDiscussionHomeThedetaileddesignandmanufacturehavestartedinaChineseliftfirm.Thedetaileddesignfor6suspensiontowershigherthanahundredmetershasbeenfinishedandthegroundworkhasbegun.Preliminarydesignforfeedcabinhascompleted,anddetaileddesignandconstructionstartedtoacceptbids.TextTextWordsNotesDiscussionHome(4)MeasurementandcontrolsystemMajorpartsofFASTareactive,thatrequiresfastandhighaccuracymeasurementandcontrolinlongdistance.Oneheavytaskisthecontrolof3-Dpositionsandorientationoffocuscabinwhichneedhighaccuracy~2mm,samplingrate>10Hzinalargeworkingrangeupto300m.Anotheroneisthecontroloftheprofileofmainreflector,surveying~1000nodesinilluminatedareainrealtimewithinanintervalofafewminutes.TextTextWordsNotesDiscussionHomeThedatumlineshavebeenestablished,monitoringshowstheaccuracyandstabilitybetterthan1mm.Complexcontrolnetworkunderneaththereflectorisattheconceptdesignstage,dividingthe~2400actuatorsintodistrictsandgroups.MostchallengingissuewouldbetheEMIfromactiveequipmentsoverthedepression.TextTextWordsNotesDiscussionHome(5)Receiverandback-endsystemFASTwillbuildeightsetsofreceivers,coveringafrequencyrangeof70MHz-3GHz.Scientificbackends,timeandfrequencystandard,monitoring/diagnosticsofthereceivershavebeenoptimizedthroughlongterminternationalcooperation.The19-multi-beamreceiveratL-bandisthecorereceiversystemofFAST.AtrilateralcollaborationamongNAOC,JBCAandCSIROhasbeenestablishedforadesigningstudyofthisimportantreceiver.TextTextWordsNotesDiscussionHomeL-bandsinglepixelreceiverhasbeendevelopedinFASTlab,coveringawideband1.1–1.9GHzwithreturnlossbetterthan-22dBandRLisolation<-22dBacrosstheband.Wearenowworkingonthefeasibilityofusinghoneycombandcarbonfibrematerialtoreducetheweight,thereforereducetheloadonthestabilizerinthecabin.TextTextWordsNotesDiscussionHome3.FASTSciencesFASTshouldhavesignificantimpactonastronomyandhasthepotentialtorevolutionizeotherareasofthenaturalsciences.Itsuniquecontributionstosciencemaynotyetbepredictableatpresent.Majorkeyscientificprojectsareplanned(Nanetal.2011):•surveytheGalacticISMinHIataresolutioncomparabletothecurrentlargescaleCOsurveysTextTextWordsNotesDiscussionHome•discover~4000newGalacticpulsarsandsearchforthefirstextragalacticpulsarsbeyondMagellanicclouds•detecthundredsofthousandsofHIgalaxiesanddetectindividualmassivegalaxiesuptoz~1•spectroscopicsurveyofrichGalacticsourceswithcontinuouscoveragebetween70MHzand3GHz•jointheinternationalVLBIexperiments•searchforradiosignalsfromexoplanets•SETITextTextWordsNotesDiscussionHomeDuringtheearlystagesoftelescopeadjustments,wecouldstartfromlowfrequencies.Tworeceiversareplannedforearlysciences.Oneisthe7-multi-beamreceiverwithanoperatingband400MHz-560MHz;andanothersinglepixel270MHz-1450MHz.ThesereceiverswillbeusedforlowfrequencydeeppulsarsurveyintheMilkywayandnearbygalaxies,forsearchingforlargemoleculesinOrionnebula,forstudyingHIdistributionofnearbydiskgalaxies,e.g.M31andM33,andfordetectingdistantOHmega-masers.TextTextWordsNotesDiscussionHomezenith[ˈzenɪθ]n.顶峰；顶点；最高点servomechanism[ˌsɜːvəʊˈmekənɪzəm]n.[自]伺服机构；自动驾驶装置；自动控制装置；跟踪器curvature[ˈkɜːvətʃə(r)]n.弯曲，[数]曲率paraboloid[pəˈræbəlɔɪd]n.抛物面sinkhole[ˈsɪŋkhəʊl]n.污水坑；排水口；阴沟口strand[strænd]n.（绳、线、毛发等的）股，缕；串WordsWordsTextWordsNotesDiscussionHomedatum[ˈdeɪtəm]n.数据，资料；前提，假定；基准点(复数data或datums)honeycomb[ˈhʌnikəʊm]n.蜂巢；蜂窝状结构，蜂巢式排列物Orionnebula[oʊˈraɪənˈnebjələ][天]猎户座星云；[天]猎户星云WordsTextWordsNotesDiscussionHome[1]zenithangle指天顶角，是入射光线、入射光源与地面法线间的夹角。TextWordsNotesDiscussionNotesHome[2]RMS（RootMeanSquare）又名均方根，是指一组数据的平方的平均数的算术平方根。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.

WhatarethethreeoutstandingaspectsofFAST?2.

Whatisthesub-systemsoftelescopeconstruction?3.WhyFeedcabinsuspensionsystemmightbethemostriskypartofFAST?HowtosupportanddrivethefeedcabinofFAST?4.WhateffectsdoesFASThaveonscience?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.WhatarethethreeoutstandingaspectsofFAST?FAST