Antenna and Propagation毕业论文.docx

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1、AntennaandPropagation毕业论文1.1ANTENNASAnantennacanbedefinedasanelectricalconductororsystemofconductorsusedeitherforradiatingelectromagneticenergyorforcollectingelectromagneticenergy.Fortransmissionofasignal,radio-frequencyelectricalenergyfromthetransmitterisconverte

2、dintoelectromagneticenergybytheantennaandradiatedintothesurroundingenvironment(atmosphere,space,water).Forreceptionofasignal,electromagneticenergyimpingingontheantennaisconvertedintoradiofrequencyelectricalenergyandfedintothereceiver.RadiationPatternsAnantennawill

3、radiatepowerinalldirectionsbut,typically,doesnotperformequallywellinalldirections.Acommonwaytocharacterizetheperformanceofanantennaistheradiationpattern,whichisagraphicalrepresentationoftheradiationpropertiesofanantennaasafunctionofspacecoordinates.Thesimplestpatt

4、ernisproducedbyanidealizedantennaknownastheisotropicantenna.Anisotropicantennaisapointinspacethatradiatespowerinalldirectionsequally.Theactualradiationpatternfortheisotropicantennaisaspherewiththeantennaatthecenter.However,radiationpatternsarealmostalwaysdepicteda

5、satwo-dimensionalcrosssectionofthethree-dimensionalpattern.ThepatternfortheisotropicantennaisshowninFigure1.1a.Thedistancefromtheantennatoeachpointontheradiationpatternisproportionaltothepowerradiatedfromtheantennainthatdirection.Fogure1.1bshowstheradiationpattern

6、ofanotheridealizedantenna.Thisisadirectionalantennainwhichthepreferreddirectionofradiationisalongoneaxis.Theactualsizeofaradiationpatternisarbitrary.WhatisImportantistherelativedistancefromtheantennapositionineachdirection.Therelativedistancedeterminestherelativep

7、ower.TodeterminetherelativepowerInagivendirection,alineisdrawnfromtheantennapositionattheappropriateangle,andthepointofinterceptwiththeradiationpatternisdetermined.Figure1.1showsacomparisonoftwotransmissionangles,AandB,drawnonthetworadiationpatterns.heisotropicant

8、ennaproducesanomnidirectionalradiationpatternofequalstrengthinalldirections,sotheAandBvectorsareofequallength.FortheantennapatternofFigure1.1b,theBvecto