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1、AntennaandPropagation畢業(yè)論文1.1ANTENNASAnantennacanbedefinedasanelectricalconductororsystemofconductorsusedeitherforradiatingelectromagneticenergyorforcollectingelectromagneticenergy.Fortransmissionofasignal,radio-frequencyelectricalenergyfromthetransmitterisconvertedintoelectromagneticen
2、ergybytheantennaandradiatedintothesurroundingenvironment(atmosphere,space,water).Forreceptionofasignal,electromagneticenergyimpingingontheantennaisconvertedintoradiofrequencyelectricalenergyandfedintothereceiver.RadiationPatternsAnantennawillradiatepowerinalldirectionsbut,typically,doe
3、snotperformequallywellinalldirections.Acommonwaytocharacterizetheperformanceofanantennaistheradiationpattern,whichisagraphicalrepresentationoftheradiationpropertiesofanantennaasafunctionofspacecoordinates.Thesimplestpatternisproducedbyanidealizedantennaknownastheisotropicantenna.Anisot
4、ropicantennaisapointinspacethatradiatespowerinalldirectionsequally.Theactualradiationpatternfortheisotropicantennaisaspherewiththeantennaatthecenter.However,radiationpatternsarealmostalwaysdepictedasatwo-dimensionalcrosssectionofthethree-dimensionalpattern.Thepatternfortheisotropicante
5、nnaisshowninFigure1.1a.Thedistancefromtheantennatoeachpointontheradiationpatternisproportionaltothepowerradiatedfromtheantennainthatdirection.Fogure1.1bshowstheradiationpatternofanotheridealizedantenna.Thisisadirectionalantennainwhichthepreferreddirectionofradiationisalongoneaxis.Theac
6、tualsizeofaradiationpatternisarbitrary.WhatisImportantistherelativedistancefromtheantennapositionineachdirection.Therelativedistancedeterminestherelativepower.TodeterminetherelativepowerInagivendirection,alineisdrawnfromtheantennapositionattheappropriateangle,andthepointofinterceptwith
7、theradiationpatternisdetermined.Figure1.1showsacomparisonoftwotransmissionangles,AandB,drawnonthetworadiationpatterns.heisotropicantennaproducesanomnidirectionalradiationpatternofequalstrengthinalldirections,sotheAandBvectorsareofequallength.FortheantennapatternofFigure1.1b,theBvecto
