24G高增益天线论文定稿英文翻译.docx

24G高增益天线论文定稿英文翻译.docx

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24G高增益天线论文定稿英文翻译

学位论文

中北大学分校学位论文英文翻译

作者姓名：

李雁鹏

学科、专业：

通信工程

学号：

******

指导教师：

王章勉副教授

完成日期：

2010-6-10

中北大学分校

英文原文：

PROPERTIESOFANTENNAS

Oneapproachtoanantennabookstartswithadiscussionofhowantennasradiate.BeginningwithMaxwell’sequations,wederiveelectromagneticwaves.Afterthatlengthydiscussion,whichcontainsalotofmathematics,wediscusshowthesewavesexcitecurrentsonconductors.Thesecondhalfofthestoryisthatcurrentsradiateandproduceelectromagneticwaves.Youmayalreadyhavestudiedthatsubject,orifyouwishtofurtheryourbackground,consultbooksonelectromagnetics.Thestudyofelectromagneticsgivesinsightintothemathematicsdescribingantennaradiationandprovidestherigortopreventmistakes.Weskipthediscussionofthoseequationsandmovedirectlytopracticalaspects.

Itisimportanttorealizethatantennasradiatefromcurrents.Designconsistsofcontrollingcurrentstoproducethedesiredradiationdistribution,calleditspattern.Inmanysituationstheproblemishowtopreventradiationfromcurrents,suchasincircuits.Wheneveracurrentbecomesseparatedindistancefromitsreturncurrent,itradiates.Simplystated,wedesigntokeepthetwocurrentsclosetogether,toreduceradiation.Somediscussionswillignorethecurrentdistributionandinstead,considerderivedquantities,suchasfieldsinanapertureormagneticcurrentsinaslotoraroundtheedgesofamicrostrippatch.Youwilldiscoverthatweuseanyconceptthatprovidesinsightorsimplifiesthemathematics.

Anantennaconvertsboundcircuitfieldsintopropagatingelectromagneticwavesand,byreciprocity,collectspowerfrompassingelectromagneticwaves.Maxwell’sequationspredictthatanytime-varyingelectricormagneticfieldproducestheoppositefieldandformsanelectromagneticwave.Thewavehasitstwofieldsorientedorthogonally,anditpropagatesinthedirectionnormaltotheplanedefinedbytheperpendicularelectricandmagneticfields.Theelectricfield,themagneticfield,andthedirectionofpropagationformaright-handedcoordinatesystem.Thepropagatingwavefieldintensitydecreasesby1/Rawayfromthesource,whereasastaticfielddropsoffby1/

.Anycircuitwithtime-varyingfieldshasthecapabilityofradiatingtosomeextent.

Weconsideronlytime-harmonicfieldsandusephasornotationwithtimedependence

.Anoutward-propagatingwaveisgivenby

wherek,thewavenumber,isgivenby2π/λ.λisthewavelengthofthewavegivenbyc/f,wherecisthevelocityoflight（3×

m/sinfreespace）andfisthefrequency.Increasingthedistancefromthesourcedecreasesthephaseofthewave.

Consideratwo-wiretransmissionlinewithfieldsboundtoit.Thecurrentsonasinglewirewillradiate,butaslongasthegroundreturnpathisnear,itsradiationwillnearlycanceltheotherline’sradiationbecausethetwoare180°outofphaseandthewavestravelaboutthesamedistance.Asthelinesbecomefartherandfartherapart,intermsofwavelengths,thefieldsproducedbythetwocurrentswillnolongercancelinalldirections.Insomedirectionsthephasedelayisdifferentforradiationfromthecurrentoneachline,andpowerescapesfromtheline.Wekeepcircuitsfromradiatingbyprovidingclosegroundreturns.Hence,high-speedlogicrequiresgroundplanestoreduceradiationanditsunwantedcrosstalk.

1.1ANTENNARADIATION

Antennasradiatesphericalwavesthatpropagateintheradialdirectionforacoordinatesystemcenteredontheantenna.Atlargedistances,sphericalwavescanbeapproximatedbyplanewaves.Planewavesareusefulbecausetheysimplifytheproblem.Theyarenotphysical,however,becausetheyrequireinfinitepower.

ThePoyntingvectordescribesboththedirectionofpropagationandthepowerdensityoftheelectromagneticwave.ItisfoundfromthevectorcrossproductoftheelectricandmagneticfieldsandisdenotedS:

S=E×H*W/

Rootmeansquare（RMS）valuesareusedtoexpressthemagnitudeofthefields.H*isthecomplexconjugateofthemagneticfieldphasor.Themagneticfieldisproportionaltotheelectricfieldinthefarfield.Theconstantofproportionisη,theimpedanceoffreespace（η=376.73Ω）:

W/

（1.1）

BecausethePoyntingvectoristhevectorproductofthetwofields,itisorthogonaltobothfieldsandthetripletdefinesaright-handedcoordinatesystem:

（E,H,S）.

Considerapairofconcentricspherescenteredontheantenna.Thefieldsaroundtheantennadecreaseas1/R,1/

1/

andsoon.Constant-ordertermswouldrequirethatthepowerradiatedgrowwithdistanceandpowerwouldnotbeconserved.Forfieldtermsproportionalto1/

1/

andhigher,thepowerdensitydecreaseswithdistancefasterthantheareaincreases.Theenergyontheinnersphereislargerthanthatontheoutersphere.Theenergiesarenotradiatedbutareinsteadconcentratedaroundtheantenna;theyarenear-fieldterms.Onlythe1/

termofthePoyntingvector（1/Rfieldterms）representsradiatedpowerbecausethesphereareagrowsas

andgivesaconstantproduct.Alltheradiatedpowerflowingthroughtheinnerspherewillpropagatetotheoutersphere.Thesignoftheinputreactancedependsonthenear-fieldpredominanceoffieldtype:

electric（capacitive）ormagnetic（inductive）.Atresonance（zeroreactance）thestoredenergiesduetothenearfieldsareequal.IncreasingthestoredfieldsincreasesthecircuitQandnarrowstheimpedancebandwidth.

Farfromtheantennaweconsideronlytheradiatedfieldsandpowerdensity.Thepowerflowisthesamethroughconcentricspheres:

Theaveragepowerdensityisproportionalto1/

.Considerdifferentialareasonthetwospheresatthesamecoordinateangles.Theantennaradiatesonlyintheradialdirection;therefore,nopowermaytravelintheθorφdirection.Powertravelsinfluxtubesbetweenareas,anditfollowsthatnotonlytheaveragePoyntingvectorbutalsoeverypartofthepowerdensityisproportionalto1/

:

SinceinaradiatedwaveSisproportionalto1/

Eisproportionalto1/R.Itisconvenienttodefineradiationintensitytoremovethe1/

dependence:

U（θ,φ）=S（R,θ,φ）

W/solidangle

Radiationintensitydependsonlyonthedirectionofradiationandremainsthesameatalldistances.Aprobeantennameasurestherelativeradiationintensity（pattern）bymovinginacircle（constantR）aroundtheantenna.Often,ofcourse,theantennarotatesandtheprobeisstationary.

Somepatternshaveestablishednames.Patternsalongconstantanglesofthesphericalcoordinatesarecalledeitherconical（constantθ）orgreatcircle（constantφ）.Thegreatcirclecutswhenφ=0°orφ=90°aretheprincipalplanepatterns.Othernamedcutsarealsoused,buttheirnamesdependontheparticularmeasurementpositioner,anditisnecessarytoannotatethesepatternscarefullytoavoidconfusionbetweenpeoplemeasuringpatternsondifferentpositioners.Patternsaremeasuredbyusingthreescales:

（1）linear（power）,

（2）squareroot（fieldintensity）,and（3）decibels（dB）.ThedBscaleisusedthemostbecauseitrevealsmoreofthelow-levelresponses（sidelobes）.

Figure1.1demonstratesmanycharacteristicsofpatterns.Thehalf-powerbeamwidthissometimescalledjustthebeamwidth.Thetenth-powerandnullbeamwidthsareusedinsomeapplications.Thispatterncomesfromaparabolicreflectorwhosefeedismovedofftheaxis.Thevestigiallobeoccurswhenthefirstsidelobebecomesjoinedtothemainbeamandformsashoulder.Forafeedlocatedontheaxisoftheparabola,thefirstsidelobesareequal.

1.2GAIN

Gainisameasureoftheabilityoftheantennatodirecttheinputpowerintoradiationinaparticulardirectionandismeasuredatthepeakradiationintensity.ConsiderthepowerdensityradiatedbyanisotropicantennawithinputpowerPoatadistanceR:

S=Po/4π

.Anisotropicantennaradiatesequallyinalldirections,anditsradiatedpowerdensitySisfoundbydividingtheradiatedpowerbytheareaofthesphere4π

.Theisotropicradiatorisconsideredtobe100%efficient.Thegainofanactualantennaincreasesthepowerdensityinthedirectionofthepeakradiation:

or

（1.2）

Gainisachievedbydirectingtheradiationawayfromotherpartsoftheradiationsphere.Ingeneral,gainisdefinedasthegain-biasedpatternoftheantenna:

powerdensity

radiationintensity（1.3）

FIGURE1.1Antennapatterncharacteristics.

ThesurfaceintegraloftheradiationintensityovertheradiationspheredividedbytheinputpowerPoisameasureoftherelativepowerradiatedbytheantenna,ortheantennaefficiency:

efficiency

wherePristheradiatedpower.Materiallossesintheantennaorreflectedpowerduetopoorimpedancematchreducetheradiatedpower.Inthisbook,integralsintheequationaboveandthosethatfollowexpressconceptsmorethanoperationsweperformduringdesign.Onlyfortheoreticalsimplificationsoftherealworldcanwefindclosed-formsolutionsthatwouldcallforactualintegration.Wesolvemostintegralsbyusingnumericalmethodsthatinvolvebreakingtheintegrandintosmallsegmentsandperformingaweightedsum.However,itishelpfulthatintegralsusingmeasuredvaluesreducetherandomerrorsbyaveraging,whichimprovestheresult.

Inasystemthetransmitteroutputimpedanceorthereceiverinputimpedancemaynotmatchtheantennainputimpedance.Peakgainoccursforareceiverimpedanceconjugatematchedtotheantenna,whichmeansthattheresistivepartsarethesameandthereactivepartsarethesamemagnitudebuthaveoppositesigns.Precisiongainmeasurementsre