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    Quarter-wave patch antenna with 35 % bandwidth

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    Quarter-wavepatchantennawith35%bandwidth
    VeliVoipio*,JaniOllikainenandPerttiVainikainen

    HelsinkiUniversityofTechnology
    IRC/RadioLaboratory
    P.O.Box3000,FIN-02015HUT,FinlandTel:+35894512242,Fax:+35894512152
    e-mail:vvo@radio.hut.fi
    Abstract
    Asmallquarterwavepatchantennawiththeimpedancebandwidthof35%wasconstructedforthefrequencyofabout2GHz.Theantennahasaprobefeedwithanintegratedcapacitortocompensatefortheprobeinductanceandproduceadual-resonantstructure.Theradiationpatternoftheproposedantennaisfairlyomnidirectionalandthepolarizationseparationislow,soitissuitableforcertainapplicationsinmobilecommunications.
    1Antennastructure
    Theproposedantennaisaquarterwaverectangularmicrostrippatchantennawithathickairsubstrate(Figure1.Theantennahasacapacitorinthemiddleoftheprobe.Thecapacitorisamatchingcapacitortocompensatefortheinductanceofthelongfeedprobe.Thecapacitanceandprobecombinationisinresonancenearthepatchresonance.
    Twopossibleconfigurationshavebeenstudied.Inthefirstversionthecapacitorwasmadeofapieceofcommonlow-permittivitysubstratehavingmetalonbothsides,thereforethestructureoftheantennaresemblesastackedpatchantenna.Inthesecondversionthecapacitorconsistedoftwochipcapacitorsontopoftheprobeandalmostsimilarbehaviourwasobtainedaswiththelargecapacitor.
    110
    24
    35
    10110
    Figure1.Structureanddimensionsoftheprototypeantenna(inmm.

    Theantennahastworesonances:onepatchresonance(parallelresonanceandoneloopresonance(seriesresonance.Theresultingcombinationofresonancesisthe



    sourceofthewidebandwidth.Comparableimpedancebandwidthsfrom20%to35%havebeenobtainedwithaperturecoupledantennas[1],butsometimestheadditionalPCBlayeroftheaperturefeedisnotdesirable.Therearealsosimilaritiesbetweentheproposedstructureandthatin[2]and[3].Themaindifferenciesaretheshort-circuitedquarterwavestructureandthattheplanarstacked-typestructureisonlyoneimplementationoptionofthedual-resonantprinciple.
    Becauseoftheposition,sizeandmaterialofthelargecapacitor,thecapacitormayhavealsosomeelectricalcouplingeffectwiththepatch,butthisneedsfurtherstudy.
    2Measuredproperties
    ThereflectioncoefficientfortheantennaversionwiththelargecapacitorisgiveninFigure2andtheinputimpedanceinFigure3.
    proto16b.asc
    proto16b.asc
    0
    -5
    -10
    S11[dB]
    -15
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    1600
    1800
    2000
    22002400Frequency[Hz]
    2600
    2800
    3000


    Figure2.Measuredreflectioncoefficient.Figure3.Measuredinputimpedance.
    Theimpedancebandwidthforthe10dBreturnlossisabout35%.FromFigure3itcanbenoticedthatthematchingwasnottotallyoptimalandsomewhatlargerbandwidthcanbeobtainedbytuningthematching.
    Fromameasurementwithoutthecapacitortheinductanceoftheprobewasestimatedtobeabout7nH.Thusatthe2400MHzresonancefrequencytherespectivecapacitanceis0.55pF.Whena0.67pF(+/-20%chipcapacitorwasusedtoreplacethelargecapacitora10dBbandwidthof31%wasobtained.
    Themeasuredradiationpattern(Figures4and5showfairlyomnidirectionalpattern.Gainis3±1dBi.Thecross-polarvalueisalmostashighasthecopolarvalueexceptintheboresightdirection.



    Proto16b,quarter-wavepatchantenna,E-plane,2154MHz,1997-5-19
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    30
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    90
    0
    CrosspolarCopolar
    Degrees


    Figure4.MeasuredradiationpatterninE-plane,0dBiat–37dB.

    Proto16b,quarter-waveantenna,H-plane,2154MHz,1997-05-19
    -30-35-40-45-50
    dB
    -55-60-65-70-75-80
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    CrosspolarCopolar
    Degrees


    Figure5.MeasuredradiationpatterninH-plane,0dBiat–37dB.

    3Computersimulations
    InFDTDsimulations,28.8%10dBbandwidthwasreached,seeFigures7and8.Heretheinputimpedancewasclosertooptimumthanfortheprototype.



    pa286imp.asc
    pa286imp.asc
    0
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    S11[dB]
    -15
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    -301.5
    2
    Frequency[Hz]
    2.5
    x10
    3
    9


    Figure6.Simulatedreflectioncoefficient(frequencyinGHz.

    Figure7.Simulatedinputimpedance
    4Conclusions
    Inthispaperwepresentedourfirstmeasuredandsimulatedresultsforaresonant-fedthickquarterwavepatch.Thebandwidthobtainedinthemeasurementswas35%.Furtherimprovementoftheperformancecanbeexpectedbyoptimisingthedual-resonantinputimpedance.Thesimulatedbandwidthwasclearlylowerthanthemeasured.
    Theantennacouldbeusedincellularbasestationswhenwidebandwidthisrequiredandpolarizationseparationisnotnecessary.Theantennacouldbemodifiedformobilehandsetse.g.byusingasubstratematerialofhigherpermittivityorbyreducingtheheight.Thesemodificationswouldshrinkthesizeoftheantennabutwouldalsoreducethebandwidth.Wideningoftheantennacouldbeusedtoreclaimsomeofthebandwidth.
    References
    [1]
    D.M.Pozar,S.M.Duffy“Adual-bandcircularlypolarizedaperture-coupledstackedmicrostripantennaforglobalpositioningsatellite,”IEEETransactionsonAntennasandPropagation,vol.45,no.11,November1997,pp.1619-1625
    [2]G.A.E.Vandenbosch,“Capacitivematchingofmicrostripantennas,”
    Electronicsletters,vol.31,no.18,31stAugust1995,pp.1535-1536[3]G.A.E.Vandenbosch,A.R.VandeCapelle“Studyofthecapacitivelyfed
    microstripantennaelement,”IEEETransactionsonAntennasandPropagation,vol.42,no.12,December1994,pp.1648-1652


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