antenna and propagation论文

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1、AntennaandPropagation毕业论文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