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天线的性能外文文献翻译译文doc
英文论文翻译
天线性能
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.
2.1ANTENNARADIATION
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.
2.2GAIN
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.Precisiongainmeasurementsrequireatunerbetweentheantennaandreceivertoconjugate-matchthetwo.Alternatively,themismatchlo