无线通信网络第5章-天线与传播.ppt

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1、IntroductionAnantennaisanelectricalconductororsystemofconductorsTransmission-radiateselectromagneticenergyintospaceReception-collectselectromagneticenergyfromspaceIntwo-waycommunication,thesameantennacanbeusedfortransmissionandreceptionRadiationPatternsRad

2、iationpatternGraphicalrepresentationofradiationpropertiesofanantennaDepictedastwo-dimensionalcrosssectionBeamwidth(orhalf-powerbeamwidth)MeasureofdirectivityofantennaReceptionpatternReceivingantenna’sequivalenttoradiationpatternTypesofAntennasIsotropicante

3、nna(idealized)RadiatespowerequallyinalldirectionsDipoleantennasHalf-wavedipoleantenna(orHertzantenna)Quarter-waveverticalantenna(orMarconiantenna)ParabolicReflectiveAntennaAntennaGainAntennagainPoweroutput,inaparticulardirection,comparedtothatproducedinany

4、directionbyaperfectomnidirectionalantenna(isotropicantenna)EffectiveareaRelatedtophysicalsizeandshapeofantennaAntennaGainRelationshipbetweenantennagainandeffectiveareaG=antennagainAe=effectiveareaf=carrierfrequencyc=speedoflight(»3´108m/s)=carrierwaveleng

5、thPropagationModesGround-wavepropagationSky-wavepropagationLine-of-sightpropagationGroundWavePropagationGroundWavePropagationFollowscontouroftheearthCanPropagateconsiderabledistancesFrequenciesupto2MHzExampleAMradioSkyWavePropagationSkyWavePropagationSigna

6、lreflectedfromionizedlayerofatmospherebackdowntoearthSignalcantravelanumberofhops,backandforthbetweenionosphereandearth’ssurfaceReflectioneffectcausedbyrefractionExamplesAmateurradioCBradioLine-of-SightPropagationLine-of-SightPropagationTransmittingandrece

7、ivingantennasmustbewithinlineofsightSatellitecommunication–signalabove30MHznotreflectedbyionosphereGroundcommunication–antennaswithineffectivelineofsiteduetorefractionRefraction–bendingofmicrowavesbytheatmosphereVelocityofelectromagneticwaveisafunctionofth

8、edensityofthemediumWhenwavechangesmedium,speedchangesWavebendsattheboundarybetweenmediumsLine-of-SightEquationsOpticallineofsightEffective,orradio,lineofsightd=distancebetweenantennaandhorizon(km)h=antennahei