Monopole Antenna Project

2007

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MonopoleAntenna Analysis,DesignandSimulationusingComputer          

Supervisor:

Dr.OmerAlSaraereh  Students:

AbdulKaremA.AlSbeeh AimanS.Resiq AhmadH.Zaid IbrahimM.Hruob MohammedHisham IsmailAbdelRazzaq YazeedSulaiman JaafarH.AbuRaad

DepartmentofElectricalandComputerEngineering HashemiteUniversity 2007 HashemiteUniversity|DepartmentofElectricalandComputerEngineering

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Introduction 

ChapterI

OverviewaboutmonopoleAntenna Nowdays,wirelesscommunicationsystemsarebecomingincreasinglypopular.However,thetechnologiesforwireless communicationstillneedtobeimprovedfurthertosatisfythehigherresolutionanddataraterequirements.Inthe communicationsystemthemorethingsislooktoisthecoastandlowpowerdeviceanditisthemonopolewhich previousthingisusedandstillbeimproveforthecommunicationsystem.  Monopoleisatypeoftheradioantennaformedbyreplacingonehalfofadipoleantennawithagroundplaneat rightanglestotheremaininghalf.Ifthegroundplaneislargeenough,themonopolebehavesexactlylikeadipole,  asifitsreflectioninthegroundplaneformedthemissinghalfofthedipole.   This study is restricted to a monopole geometry consisting of a vertical cylindrical element at the center of a perfectly conducting, infinitely thin, circular ground plane in free space. This geometry is of interest because its radiation pattern is uniform in the azimuth direction and because its electrical characteristics are primarily a function of only three parameters, namely, the element length, the element radius, and the groundplane radius, when each is normalized to the excitation wavelength. Therefore, this geometry is conducive to analysis, experimentalverification,andstandardization.  A typical feed for the monopole antenna is a coaxial line with its inner conductor connected through a hole in the ground plane to the vertical monopole element and its outer conductor connected by means of a flange to the ground plane. Typically, the inner conductor’sdiameterisequaltothemonopole element’s diameter and the outer conductor’s diameter is equal to the ground plane hole diameter.Unlessstatedotherwise,suchafeed will be assumed in this study. The ratio of the coaxial line’s outertoinner conductor diameters affects the antenna’s input impedance,butonlysignificantlyforarelatively thickmonopoleelementonaverysmallground plane.  Fig.1:MonopoleAntenna For the idealized case of a ground plane of infinite extent and infinite conductivity, the monopole antenna may be modeled by the method of images as a dipolewithonehalftheinputimpedanceand  double the peak directivity of the dipole. The infinite ground plane prevents monopole radiation into the hemisphere below the ground plane, but allows a radiation pattern identical to that of the dipole in the upper HashemiteUniversity|DepartmentofElectricalandComputerEngineering

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 hemisphere.However,foramonopoleelementmountedonagroundplaneoffiniteextent,theouteredgeofthe ground plane diffracts incident radiation in all directions, and consequently modifies the currents on the ground planeandtheverticalelementfromthoseofaninfinitegroundplane.Attheouteredgeofthegroundplane,the currentsonitstopandbottomfacesareequalinmagnitudebutoppositeindirectionbecausethenetcurrentmust bezeroattheedge.Outeredgediffractionbecomesincreasinglysignificantwithdecreasingsizeofthegroundplane becauseoftheincreasingmagnitudeofthecurrentsonthegroundplanefacesattheouteredge.Edgediffraction canaltertheinputimpedancebymorethan3dBanddirectivityintheplaneofthegroundplanebymorethan6dB fromthevaluesforagroundplaneofinfiniteextent.  Theoreticalmodelsexistforpredictingtheeffectsofdiffractionbytheouteredgeofthegroundplane.Theexisting models may be classified into two categories, distinguished by whether the current distribution on the monopole elementisinitiallyknownorunknown.  When the monopole element is very thin and not too long, its current distribution is approximately sinusoidal and independent of the radius of the ground plane. Consequently, the element’s current distribution can be initially specified and wended only determine the ground plane’s current distribution. For this category of monopoles, the theoretical models reported in the literature essentially consist of Bardeen’s integral equation method for a groundplane radius that is small compared to a wavelength[1],Richmond’s method of moments(ground plane only)foragroundplaneradiusthatisnottoolargecomparedtoa wavelength [2], Leitner and Spence’s method of oblate spherical wavefunctionsforagroundplaneradiusthatiscomparabletoa wave length [3–5], Tang’s scalar theory of diffraction and the geometric theory of diffraction (GTD) for a groundplane radius that is large compared to a wavelength, and Storer’s variation methodforagroundplaneradiusthatisverylargecomparedtoa wavelength[3].  Whenthemonopoleelementisrelativelythick,itscurrent distributionisnolongersinusoidal,andthecurrentdistributionon boththemonopoleelementandthegroundplaneconsequently  Fig.2:MonopoleBroadcasting needtobedeterminedasafunctionofthegroundplaneradius. Forthiscategoryofmonopoles,thetheoreticalmodelsreportedin the literatureessentiallyconsistofRichmond’smethodofmomentsforgroundplaneradiusthatisnottoolarge comparedtoawavelength[4]andAwadallaMaclean’smethodofmoments(monopoleelementonly)combined withthegeometrictheoryofdiffractionforgroundplaneradiusthatislargeorcomparabletoawavelength[9,10]. ThieleandNewhousehavealsoreportedamodelthatcombinesthemethodofmomentswiththegeometrictheory ofdiffraction,buttheircomputerprogramisunavailable 

Applicationsofmonopoleantenna Monopoleantennasarecommonlyemployedinairborneandgroundbasedcommunication systems at a wide range of frequencies. The electrical properties of such antennas are dependent upon the geometry of both the monopole element and the ground plane. Typically, the monopole element may be electrically short (length is much less than a quarterwavelength) or nearresonant (length approximately a quarterwavelength), and it maybethin(lengthtoradiusratioismuchgreaterthan104)orrelativelythick(lengthto radius ratio of 101 to 104). In addition, the groundplane dimensions may vary from a

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 fraction of a wavelength to many wavelengths. Therefore, it is desirable to know how the input impedance and radiationpatternoftheantennachangeasthedimensionsofthemonopoleelementandthegroundplanevary.The directivity on or near the radio horizon (the ground plane is assumed to be horizontal) is of particular interest becausethemaximumoperationalrangeofacommunicationsystemoftendependsonthedirectivityontheradio horizon.  Thebroadbandmonopoleantennaisverticallypolarized,hasalow angleradiationpatternandisusedforshortrangecircuitsbyground waveandmediumtolongrangecircuitsbyskywave.Theantennais broadband,Omnidirectionalandconical.Thebroadbandfrequency capability does not require tuning. The radiation pattern in the azimuth plane is essentially unidirectional, while the elevation patternvarieswithfrequency.Thesupportstructureisaguyedmast supported on a base insulator. The radiator comprises a biconical cageofwire.Monopoleantennasrequirearadialgroundscreenfor specified performance. The ground screen consists of wire conductorslaidoutradiallyfromthetowerbaseandterminatedat theouterend.  ThemonopoleAntennaiscommonlyusedinradiobroadcasting,the Fig.3:AzimuthallyPlan radio frequency power from the broadcasting transmitter is fed across the base insulator between the tower and a ground system. The ground system normally comprises 120 buried copper or phosphor bronze radial wires at least onequarter wavelength long and a groundscreen in the immediatevicinityofthetower.Allthegroundsystemcomponentsarebondedtogether,usuallybybrazingorusing coinsilversoldertohelpreducecorrosion.Monopoleantennasthatuseguywiresforsupportarecalledmastsin some countries. In the United States, the term “mast” is not generally used to relate to antennas, so both self supportingandguyedradioantennasaresimplycalledmonopolesiftheystandalone.Ifusedinnumberstocontrol thedirectionofradiofrequencypropagation,theyarecalleddirectionalantennaarrays.  Examplesofmonopoleantennasare: 1.

Thewhipantenna.

2.

Theradiomastwhenisolatedfromthegroundandbottomfed. 



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Monopoleantenna 

ChapterII

ElectricandMagneticfieldinnearandfarfiledregions 2.1.1BOUNDARYCONDITIONS Thecurrentamplitudes,I(z)andI(r),onthemonopoleelementandgroundplane,respectively,aregenerally complexandinitiallyunknownquantities.Considernowthecasewherethecurrentdistributiononthemonopole elementisassumedtobesinusoidal.

………………………………………………………………..(Eq.1) ………………………………………………………………..(Eq.2)

        Althoughasinusoidaldistributionofcurrentisnotpossible,evenforaninfinitelythinantenna,ismostlikelyafair approximationtothecurrentifthemonopoleelementissufficientlythinelectricallyandnottoolong[13].Fora centerfeddipoleofradiusbandtotallength2,Elliot[5]givesexampleswherethecurrentdistributionis approximatelysinusoidalandofapproximatelyconstantargumentfor Elliotdemonstratesthatthecurrentdistributionisno longersinusoidalnearthecenterofthedipole,noris(argz)approximatelyconstant.Balanis[6]shows ThecurrentdistributionisnotsinusoidalnearthecenteroftheDipole. Thecurrentdistributionisneithersinusoidal,norofconstantphase,andthatthedeviationsfromEq.(1)and(2) increasewithincreaseinvaluesofh/andb/.Onthebasisoftheaboveresults,itappearsthatEq.(1)and(2)are approximatelyvalidfortheconditions:

………………………………………………………………..(Eq.3) InadditiontotheconstraintonI(z)givenbyEq.(1),assumethatthereturncurrentIr(z)ontheoutsideofthe coaxialline’souterconductorisgivenby:

………………………………………………………………..(Eq.4)

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 TheconstraintofEq.(4)correspondstotheidealizedconditionthattheferritetoroidshavea1/ecurrent attenuationdistanceh1,givenby:  Thecurrentconstraintsonthemonopoleantennaaregivenby: 

  Whereisthecurrentonthebottomofthegroundplaneataradiusequaltothatoftheouterconductor

………………………………………………………………..(Eq.5)

 ForthecurrentconstraintsofaboveequationsthetotalmagneticandelectricfieldintensitiesH(total),E(total)atan arbitraryfieldpointP(X,Y,Z)externaltotheelementexcitationsourcepointsaresimplythevectorsumofthefields resultingfromtheelementcurrentandthecurrentinducedonthegroundplanebythefieldsincidentbythe element. 

………………………………………………………………..(Eq.6) Where:

2.1.2CONCEPTOFAGROUNDPLANEOFZEROEXTENT  Consideramonopoleantennaexcitedbyacoaxiallinewithanouterconductorofradiusb1thatisterminatedby freespace,ratherthanbyagroundplane.Thegroundplaneforsuchanantennaisdenotedasbeingofzeroextent.

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 Thecoaxiallineexcitationmaybereplacedbyanequivalentmagneticcurrent(frill)Msittingontopofathick groundplaneofradiusP=b1.Forsufficientlylossyferritetoroidsalongtheoutsideofthecoaxialline,thecurrenton theexteriorofthecoaxialline’souterconductormaybeneglected.Themagneticfrillmayberemovedfromthe circuitwithoutappreciablyaffectingtheresultsbecausekb1