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FIGURE6Sleevedipolewithadditionaldecouplingsleeve
FIGURE7Enddrivenhalfwavedipolewithquarterwavedecoupling
sleeve
AchievingomnidirectionalgainThemostpracticalwaytoobtainomnidirectionalgaininaverticalantennasystemistostackanumberof
separateantennastoformaverticallyorientedcolineararray.Itisrelativelyeasytoobtain3dBofonmidirectionalgainwithrespecttoaverticalhalfwavedipole.Highergainsrequiremuchmorecomplexstructures,multiplefeedpoints,powerdividers,phasingsections,impedancematchingnetworks,etc.While3dBofomnidirectionalgaincanbeobtainedinantennascostinglittlemorethansimpledecoupledhalf
wavewhips,highergainantennasaregenerallymuchmoreexpensive.
Oneofthemostpopularwaystoobtain3dBofomnidirectionalgainoveraverticalhalfwavedipoleistoemployacenterdrivendipolewhosetotallengthis11/4wavelengths,thatis,5/8wavelengthoneachleg.
Thepropertiesofsuchadipolehavebeenknownformanyyears.Whenorientedverticallyandwelldecoupledfromthecoaxialfeedcable,thisantennaproducesalmostexactly3dBofomnidirectionalgainwithrespecttoahalfwavedipole,yetrequiresbutonefeedpoint.The11/4wavelength(or"twin5/8")designproducesthemaximumpossiblegainforasingledipole.Iftheantennaiseithershorterorlonger,
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FIGURE8Centerdriventwin5/8dipolewithtwin
decouplingsleeves
thegaindropsoff.
Figure8showshowthisdipolecanbeemployedinaverticalorientation.Noticethatthecoaxiallineisbroughtuptothecenterofthedipole,wherethecenterconductorexcitesthe
upper5/8wavelengthelement.Thelower5/8elementconsistsoftheoutsideoftheouterconductor,downforadistanceof3/8wavelengthfromthefeedpoint,andthentheoutsideofa1/4wavelengthdecouplingsleeve,The"spillover"current,whichmustbeequaltothecurrentatthebaseoftheupperelement,
constitutesthecurrententeringthelowerelementofthedipole.Thelower5/8elementterminatesattheopenendofaresonantquarterwavelengthsleeve.Testshaveshownthepresenceofa
smallamountofspillovercurrentbelowthissleeve.Accordingly,asecondresonantsleeveisnecessarytoproduceahighdegreeofdecoupling.Thecurrentdistributionalongthe
antennaissketchedinFigure8.Notethestandingwavedistribution,andthefactthattheupperandlowercurrent"loops"areinphasewitheachother.Thisisthedesiredactionwhich
producesthegain.Notealsothatthereisaphasereversalofthecurrentsnearthefeedpoint.Theeffectproducessmall"side
lobes"athighandlowanglesacharacteristicofmostgaintypeantennaarrays.
ThedecouplingsleevesarefilledwithStyrofoamcoreswithacentralholetoaccommodatethecoaxialline.AnLCnetworkis
usedatthefeedpointtomatchtheantennaimpedanceto50ohms.Theentireantennaisfittedsnuglyinsideastiff,tapered
fiberglasstube,andweathersealed.
GainThestatedgainofanantennamustuseareferencetohaveanymeaning.Commercialantennamanufacturersuseafreespacehalfwavedipoleasthegainreferenceandthegainwillbe
shownwiththesubscript"d"(fordipole).Sothenumber3dBdindicates3dBmorepowerinthedirectionofmaximum
radiationthanadipole.
Anothercommonreferenceisgainoverthemythicalisotropicradiator.Thisisanonexisting,butmathematicallyconvenientantennathatradiateswiththesamepowerintensityinalldirections.Thegainofahalfwavedipoleis2.15dBiandour3dBdantennais5.15dBi.Inordertomakethegainnumberslarger,somemanufacturersusegainnumbersthatcouldtheoreticallybeachievediftheantennawereplacedovera
perfect,infinitelylargegroundplanesothatallthepowerdeliveredtotheantennawouldbeabovethegroundplane.Thistheoreticallywoulddoublethepowerabovethegroundplaneandaddanother3dB.Ourdipolecouldhavean"advertisedgain"of5.15dBandour3dBdantennacouldbeadvertisedtohaveagain
of8.15dB.Notethatthereisnoreferenceinthesenumbers.Theprospectiveantennapurchasershouldinsistonthegainreference.
Verticalbasestationantennasshouldhavetheirdirectionofmaximumradiationonthehorizon.SomecommercialantennasintendedforamateurusecannotcontroltheirpatternandmaybeseveraldBdownonthehorizon.Gainis,unfortunately,measuredinthedirectionofmaximumradiationnotonthehorizon.
