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ID:56023033
大小:8.59 MB
页数:154页
时间:2020-06-03
《无线电波传播 第九讲 天线概要.ppt》由会员上传分享,免费在线阅读,更多相关内容在PPT专区-天天文库。
1、RadioWavePropagation无线电波传播第九讲天线概要纲要天线的定义与作用天线的种类有关术语通信距离雷达方程Whatisanantenna?天线的定义Metallicdevice(asarodorwire)forradiatingorreceivingradiowaves.–Webster’sdictionary.Ameansforradiatingorreceivingradiowaves.–IEEEAnantennaisadevicethatprovidesameansforradiatingorreceivingradiowaves
2、.Inotherwords,itprovidesatransitionfromaguidedwaveonatransmissionlinetoa“free-space”wave,andviceversainthereceivingcase.TransmissionlineFreespaceAntennaUnintendedAntennasAnyconductor/installationcarryingelectricalcurrent(e.g.electricalinstallationofvehicles)Anyconductingstructu
3、re/installationirradiatedbyEMwavesPermanent(e.g.Antennamasts,orpowernetwork)Time-varying(e.g.Windmills,orhelicopterpropellers)Transient(e.g.Re-radiatingaeroplane)AShortHistoryofAntennas1873,Maxwellunifiedthetheoriesofelectricityandmagnetism.Representedtherelationshipbetweenelec
4、tricityandmagnetismthroughMaxwell’sEquations.Predictedtheexistenceofelectromagneticwaves.1886,HeinrichHertz,aprofessorattheTechnicalInstituteinKarlsruheGermany,builtthefirstradioantennas.(end-loadeddipoleasthetransmittingantennas,square-loopantennaasreceiver.Alsoexperiencedwith
5、parabolicreflectorantennas.Meterwavelength).1901,GuglielmoMarconitransatlantictransmission.(Tuningcircuits,bigantennaandgroundsystems).Startofthewirelesscommunication.WorldwarII,radar,centimeterwavelengthsbecamepopularandtheentireradiospectrumopeneduptowideusage.Satellite,Globa
6、lPositionSatellite(GPS).1960,numericalanalysis.FromCutandTrytoengineeringart.Antennas:importantelementsofanyradiolink天线的作用AntennaTransmitterAntennaReceiverRadiowave天线的作用与地位无线电发射机输出的射频信号功率,通过馈线(电缆)输送到天线,由天线以电磁波形式辐射出去。电磁波到达接收地点后,由天线接下来(仅仅接收很小很小一部分功率),并通过馈线送到无线电接收机。可见,天线是发射和接收电磁波的
7、一个重要的无线电设备,没有天线也就没有无线电通信。天线品种繁多,以供不同频率、不同用途、不同场合、不同要求等不同情况下使用。按用途,可分为通信天线、电视天线、雷达天线等;按工作频段,可分为短波天线、超短波天线、微波天线等;按方向性,可分为全向天线、定向天线等;按外形,可分为线状天线、面状天线等;导线上有交变电流流动时,就可以发生电磁波的辐射,辐射的能力与导线的长度和形状有关。如下图所示,若两导线的距离很近,电场被束缚在两导线之间,因而辐射很微弱;将两导线张开,电场就散播在周围空间,因而辐射增强。必须指出,当导线的长度L远小于波长λ时,辐射很微弱;导线
8、的长度L增大到可与波长相比拟时,导线上的电流将大大增加,因而就能形成较强的辐射。传输线天线T-Antenn
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