Introduction_to_antennas

IntroductiontoantennasMichelAnciaux/APEXNovember2004Whatisanantenna?•Regionoftransitionbetweenguidedandfreespacepropagation•Concentratesincomingwaveontoasensor(receivingcase)•Launcheswavesfromaguidingstructureintospaceorair(transmittingcase)•Oftenpartofasignaltransmittingsystemoversomedistance•Notlimitedtoelectromagneticwaves(e.g.acousticwaves)FreespaceelectromagneticwaveMagneticfieldElectricfieldDirectionofpropagationMagneticField[A/m]ElectricField[V/m]Time[s]Time[s]•DisturbanceofEMfield•Velocityoflight(~300000000m/s)•EandHfieldsareorthogonal•EandHfieldsareinphase•Impedance,Z0:377ohmsxyzEMwaveinfreespace)(0ztjxeEE2200221zEtExx2200221zHtHyy)(0ztjyeHH2f2f001wavelengthPhaseconstantfrequency000Z000HEZMagneticfieldElectricfieldDirectionofpropagationxyzWaveinlossymediumtjzjztjzxeeeEeeEE00jAttenuationconstantPhaseconstantPropagationconstantAttenuationincreaseswithzPhasevarieswithzPeriodictimevariationPowerflowHES020221121ZHZESyxavPoyntingvectorAveragepowerdensityPolarisationofEMwaveElectricalfield,EverticalhorizontalcircularReflection,refractionir)sin()sin(21it)sin()sin(2211itReflectionRefractionifbothmediaarelosslessirEEReflectioncoefficient:Dependsonmedia,polarisationofincidentwaveandangleofincidence.ReflectionandrefractionaffectpolarisationGuidedelectromagneticwave•Cables–Usedatfrequenciesbelow35GHz•Waveguides–Usedbetween0.4GHzto350GHz•Quasi-opticalsystem–Usedabove30GHzGuidedelectromagneticwave(2)•TEMwaveincablesandquasi-opticalsystems(sameasfreespace)•TH,TEandcombinationsinwaveguides–EorHfieldcomponentinthedirectionofpropagation–Wavebouncesontheinnerwallsoftheguide–Lowerandupperfrequencylimits–CrosssectiondimensionsproportionaltowavelengthRectangularwaveguideLaunchingofEMwaveOpenupthecableandseparatewiresDipoleantennaOpenandflareupwaveguideHornantennaTransitionfromguidedwavetofreespacewaveReciprocity•Transmissionandreceptionantennascanbeusedinterchangeably•Mediummustbelinear,passiveandisotropic•Caveat:Antennasareusuallyoptimisedforreceptionortransmissionnotboth!Basicantennaparameters•Radiationpattern•Beamareaandbeamefficiency•Effectiveapertureandapertureefficiency•Directivityandgain•RadiationresistanceRadiationpattern•Farfieldpatterns•Fieldintensitydecreaseswithincreasingdistance,as1/r•Radiatedpowerdensitydecreasesas1/r2•Pattern(shape)independentondistance•Usuallyshownonlyinprincipalplanes2D2r:fieldFarD:largestdimensionoftheantennae.g.r220kmforAPEXat1.3mm!Radiationpattern(2)),(E),(E2022),(),(),(rZEEPFieldpatternsmax),(),(),(PPPn+phasepatterns),(),(HPBW:halfpowerbeamwidthBeamareaandbeamefficiency4200),()sin(),(dPddPnnAMainbeamareaMinorlobesareadPbeamMainnM),(dPlobesornmmin),(mMABeamareaAMMMainbeamefficiencyEffectiveapertureandapertureefficiencyReceivingantennaextractspowerfromincidentwaveeinrecASPForsomeantennas,thereisaclearphysicalapertureandanapertureefficiencycanbedefinedpeapAAAeA2Apertureandbeamareaarelinked:DirectivityandgainaveragePPD),(),(maxAndPD4),(44Isotropicantenna:4A1D24eADFrompatternFromapertureonlylossesohmictoduelowerthanis)1(0factorefficiencyGainDGkkDkGgggDirectivityRadiationresistance•AntennapresentsanimpedanceatitsterminalsAAAjXRZ•ResistivepartisradiationresistancepluslossresistanceLRARRRTheradiationresistancedoesnotcorrespondtoarealresistorpresentintheantennabuttotheresistanceofspacecoupledviathebeamtotheantennaterminals.TypesofAntenna•Wire•Aperture•ArraysWireantenna•Dipole•Loop•Foldeddipoles•Helicalantenna•Yagi(arrayofdipoles)•Cornerreflector•ManymoretypesHorizontaldipoleWireantenna-resonance•Manywireantennas(butnotall)areusedatornearresonance•Sometimesitisnotpracticaltobuiltthewholeresonantlength•Thephysicallengthcanbeshortenedusingloadingtechniques–Inductiveload:e.g.center,baseortopcoil(usuallyadjustable)–Capacitiveload:e.g.capacitance“hats”(flattopatoneorbothends)Yagi-UdaElementsGaindBiGaindBd37.55.548.56.55108611.59.5712.510.5813.511.5Apertureantenna•Collectpoweroverawelldefinedaperture•Largecomparedtowavelength•Varioustypes:–Reflectorantenna–Hornantenna–LensReflectorantenna•Shapedreflector:parabolicdish,cylindricalantenna…–Reflectoractsasalargecollectingareaandconcentratespowerontoafocalregionwherethefeedislocated•Combinedopticalsystems:Cassegrain,Nasmyth…–Two(Cassegrain)orthree(Nasmyth)mirrorsareusedtobringthefocustoalocationwherethefeedincludingthetransmitter/receivercanbeinstalledmoreeasily.Cassegrainantenna•Lesspronetobackscatterthansimpleparabolicantenna•Greaterbeamsteeringpossibility:secondarymirrormotionamplifiedbyopticalsystem•Muchmorecompactforagivenf/DratioCassegrainantenna(2)•Gaindependsondiameter,wavelength,illumination•Effectiveapertureislimitedbysurfaceaccuracy,blockage•Scaleplatedependsonequivalentfocallength•Lossinapertureefficiencydueto:–Taperedillumination–Spillover(illuminationdoesnotstopattheedgeofthedish)–Blockageofsecondarymirror,supportlegs–Surfaceirregularities(effectde