线天线详细讲解.pdf

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1、1ÙUWireAntennasWireantennasaretheoldestandstillthemostprevalentofallantennaforms.Wireantennascanbemadefromeithersolidwireortubularconduc-tors.Theyarerelativelysimpleinconcept,areeasytoconstruct,andareveryinexpensive.§1.1>ó4f£>6¤UTheIdealDipoleAntennaInthissectiontheprinciplespre

2、sentedintheprevioussectionareusedtofindthefieldsofaninfinitesimalelementofcurrent.Weshallusethetermidealdipoleforapieceofuniformamplitudecurrentwhichisofinfinitesimallengthorofverysmallfinitelength,,Itdoesnotexitbyitself,butitmaybeconsideredtobeapieceofalargercurrentonanactualantenna.

3、Theidealdipoleconceptisalsousefulbecauseitsfieldsapproximatethoseofelectricallysmalldipolestobediscussednext.Consideranelementofcurrentoflengthalongthez-axiscenteredonthecoordinateorigin.Now,webegintosolvetheelectromagneticfieldssimulatedbythesource.FollowthestepsrepresentedaboveZ∆

4、z−jkR−jβr2eIe0A=ˆzIdz=∆zzˆ−∆z4πR4πr2I∆z1−jkrH=∇×A=jk1+esinθφˆ4πrjkr121ÙUWireAntennas1kI∆z11−jkrE=∇×H=jη1++esinθθˆjωε4πrjkr2(jkr)kI∆z11−jkr+jη+ecosθrˆ2πrjkr2(jkr)Ifkr1orrλ,I∆zH=sinθφˆ4πr2I∆zI∆zE=−jηsinθθˆ−jηcosθrˆ334πkr2πkrWecanfindthatinthisarea,thecomplexPoyntingvectorisa

5、pureimagi-nary.Thatistosay,thepowerinthisareacan’tbespreadout.Wecallitnear-fieldregion.Thephasedifferencebetweenelectricfieldandmagneticfieldis90◦,whichformsastandingwavenottravellingwave.it’scomplexPoyntingvector"#2221E×H∗=1j−ηI∆zsinθrˆ+η(I∆z)sinθcosθθˆ22k4πr5k8π2r5hasonlyimaginar

6、ypart.—Ifkr1orrλ,I∆z−jkrH=jkesinθφˆ4πrkI∆z−jkrE=jηesinθθˆ4πrThecomplexpoyntingvector21∗1kI∆zE×H=rηˆsinθ(1.1)224πrhasrealpart,thismeansthatpowerisradiatedoutward.ThetotalpowerradiatedbyantennaisfoundbyintegratingthePoyntingvectormagnitude§1.2áfElectricallySmallDipoles3overasp

7、hereI1∗Pr=E×H·ds2Z2πZπ21kI∆z2=sinθrsinθdθdφ2004πr2122∆z=I80π2λWecallthisarearadiationfieldsorfar-fieldregion.§1.2áfElectricallySmallDipolesDefinition:Anantennawhosedimensionsaremuchsmallerthanawavelengthisreferredtoasanelectricallysmallantenna.Firstconsideranactualshortdipole.I

8、nthisrealantenna,thecur-ã1.1:Theshortdip