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Chapter21ElectromagneticWaves
21.1WaveequationsMaxwell’sequations:??
Forconductingmedia,Jf=sE,rf=0.dampingFornon-conductingmedia,wehave
Forfreespace,wehavethespeedoflightinfreespaceWhym0e0=constant?
Historicallyvacuumpermeabilityisassigned:
Inmediaindexofrefraction
21.2TravelingwaveswherekiswavevectorandIntroducingintermediatevariable
Maxwell’sdivergenceequation?·E=0validforarbitraryxsimilarly
Faraday’slaw
validforarbitraryx
?ThePoyntingvectoriskEcB
PolarizationLinearpolarizedwaveisEllipticalorcircularpolarizedwaveissizeandshapeofvirusparticlesstructureranginginsizefromagalaxy(~1020m)toanucleus(~10–14m)polarizerPolarizationstudies:偏振
*21.3RadiationForamovingchargeBuq
theradiationpowerisForanacceleratingcharge
Electromagneticwavemayberadiatedfromanantennadifferentialantennallhalfwavelengthantennal~l/2antennaarray……AnAntennacarryingacurrentisequivalenttoanoscillatingelectricdipole
Radiationsfromspace1931BellLabengineerKarlG.Jansky(radioastronmy)In1965BellLabengineersA.A.PensiasandR.W.Wilsonfoundcosmicmicrowavebackgroundradiation.(1979NobelPrize)
Microwavebackgroundradiation?itswavelengthisl=7.35cm,?itisisotropic:withaprecisionofbetterthanonepartinathousandonallscalefrom1arcminto360o,?microwaveradiationspectrumisPlanckianat3K(bestvalue2.7K).PenziasAA,WilsonRW.AMeasurementofExcessAntennaTemperatureat4080Mc/s.Ap.J.,1965,142:420Fromchapter32tinyinhomogeneityandtemperaturefluctuationandpolarization
Fromchapter32
Fromchapter32
Thelargehorn-shapedantennaattheBellLaboratories’spacecommunicationcenterinHolmdel,NewJersey.Itwasthe3Kbackgroundradiation.Aftertheyremovedallpossiblesourcesofnoise(byfixingfaultyconnectionsandlooseantennajoints,andbyremoving“stickywhitecontributions”fromnest
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