IEEE Transactions on Antennas and Propagation (August)

286 Pages·2004·19.87 MB·English

AUGUST2004 VOLUME52 NUMBER 8 IETPAK (ISSN0018-926X)
EDITORIAL
ANoteFromtheOutgoingEditor-in-Chief ... ... ... ... ... .... ... ...... ... ... ... ... ... ... ... A.W.Glisson 1926
PAPERS
BandwidthEnhancementandFurtherSizeReductionofaClassofMiniaturizedSlotAntennas... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... . N.BehdadandK.Sarabandi 1928
MiniatureBuilt-InMultibandAntennasforMobileHandsets .. ..... ..... ... . Y.-X.Guo,M.Y.W.Chia,andZ.N.Chen 1936
MiniatureReconfigurableThree-DimensionalFractalTreeAntennas. ... ... ...... ... ... . J.S.PetkoandD.H.Werner 1945
InvestigationsonMiniaturizedEndfireVerticallyPolarizedQuasi-FractalLog-PeriodicZigzagAntenna .. ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... .. S.K.SharmaandL.Shafai 1957
CompactWide-BandMultimodeAntennasforMIMOandDiversity. ... .... ..... ... .C.WaldschmidtandW.Wiesbeck 1963
GroundInfluenceontheInputImpedanceofTransientDipoleandBow-TieAntennas ... ... ... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... A.A.Lestari,A.G.Yarovoy,andL.P.Ligthart 1970
AdaptiveCrossedDipoleAntennasUsingaGeneticAlgorithm. .... ... ... ... ..... .... ... ... ... ... ..R.L.Haupt 1976
ModelingandInvestigationofaGeometricallyComplexUWBGPRAntennaUsingFDTD.. ... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ..K.-H.Lee,C.-C.Chen,F.L.Teixeira,andR.Lee 1983
RadiationPropertiesofanArbitrarilyFlangedParallel-PlateWaveguide . ..... ... D.N.Chien,K.Tanaka,andM.Tanaka 1992
ScanBlindnessFreePhasedArrayDesignUsingPBGMaterials. ...... .L.Zhang,J.A.Castaneda,andN.G.Alexopoulos 2000
FractileArrays:ANewClassofTiledArraysWithFractalBoundaries ... .... D.H.Werner,W.Kuhirun,andP.L.Werner 2008
A New Millimeter-Wave Printed Dipole Phased Array Antenna Using Microstrip-Fed Coplanar Stripline Tee Junctions
.. ... ... .... ... ... ... ... ... ... ... ...... ... ... ... .... ... ... ... ... ... ... ..Y.-H.SuhandK.Chang 2019
PhysicalLimitationsofAntennasinaLossyMedium.. ... ... .... ... ... ..... .... ... ... ... ... ... . A.Karlsson 2027
MinimumNormMutualCouplingCompensationWithApplicationsinDirectionofArrivalEstimation... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... .C.K.E.Lau,R.S.Adve,andT.K.Sarkar 2034
APhase-SpaceBeamSummationFormulationforUltrawide-bandRadiation.... ... ... ... ... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... . A.Shlivinski,E.Heyman,A.Boag,andC.Letrou 2042
(ContentsContinuedonPage1925)
(ContentsContinuedfromFrontCover)
TheoreticalConsiderationsintheOptimizationofSurfaceWavesonaPlanarStructure .. ... ... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... .. S.F.Mahmoud,Y.M.M.Antar,H.F.Hammad,andA.P.Freundorfer 2057
GeneralizedSystemFunctionAnalysisofExteriorandInteriorResonancesofAntennaandScatteringProblems.. ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... ... ... L.LiandC.-H.Liang 2064
MIMOWirelessCommunicationChannelPhenomenology. ... ...... .... ... .D.W.Bliss,A.M.Chan,andN.B.Chang 2073
ServiceOrientedStatisticsofInterruptionTimeDuetoRainfallinEarth-SpaceCommunicationSystems . ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... ... ... ... .. E.Matricciani 2083
Full-WaveAnalysisofDielectricFrequency-SelectiveSurfacesUsingaVectorialModalMethod . ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... .. A.Coves,B.Gimeno,J.Gil,M.V.Andrés,A.A.SanBlas,andV.E.Boria 2091
OntheInteractionBetweenElectricandMagneticCurrentsinStratifiedMedia ... .....D.LlorensdelRíoandJ.R.Mosig 2100
ScatteringbyArbitrarily-ShapedSlotsinThickConductingScreens:AnApproximateSolution .. ..... .... ..J.R.Mosig 2109
Double HigherOrderMethodofMomentsforSurfaceIntegralEquationModeling ofMetallicandDielectric Antennas
andScatterers . ... ... ... ... ... ... ... .... ..... ... ... .... ... ... ... ... . M.Djordjevic´ andB.M.Notarosˇ 2118
Loop-Tree Implementation of the Adaptive Integral Method (AIM) for Numerically-Stable, Broadband, Fast
ElectromagneticModeling . ... ... ... .... ..... ... ... ... . V.I.Okhmatovski,J.D.Morsey,andA.C.Cangellaris 2130
ASingle-LevelLowRankIE-QRAlgorithmforPECScatteringProblemsUsingEFIEFormulation.. ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... ... .. S.M.SeoandJ.-F.Lee 2141
AcceleratedGradientBasedOptimizationUsingAdjointSensitivities... ... ... ... ... ... ... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... . N.K.Nikolova,R.Safian,E.A.Soliman,M.H.Bakr,andJ.W.Bandler 2147
ATheoreticalStudyoftheStabilityCriteriaforHybridizedFDTDAlgorithmsforMultiscaleAnalysis... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... ...M.MarroneandR.Mittra 2158
COMMUNICATIONS
Full-WaveAnalysisofaWaveguidePrintedSlot .. ... ... ... ...... .... ... ... ... .. G.MontisciandG.Mazzarella 2168
DualPolarizedWide-BandApertureStackedPatchAntennas.. .... ..... .... ... ...K.GhorbaniandR.B.Waterhouse 2171
ResonantFrequencyofEquilateralTriangularMicrostripAntennaWithandWithoutAirGap.... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... ...D.GuhaandJ.Y.Siddiqui 2174
EffectofaCavityEnclosureontheResonantFrequencyofInvertedMicrostripCircularPatchAntenna... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... ...D.GuhaandJ.Y.Siddiqui 2177
DesignandDevelopmentofMultibandCoaxialContinuousTransverseStub(CTS)AntennaArrays.. ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... . R.Isom,M.F.Iskander,Z.Yun,andZ.Zhang 2180
Near-Field,Spherical-ScanningAntennaMeasurementsWithNonidealProbeLocations . ... ... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... .. R.C.Wittmann,B.K.Alpert,andM.H.Francis 2184
ResonanceSeriesRepresentationoftheEarly-TimeFieldScatteredbyaCoatedCylinder. ... ... ... ... ... ... ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... ... ... H.VollmerandE.J.Rothwell 2186
HighOrderSymplecticIntegrationMethodsforFiniteElementSolutionstoTimeDependentMaxwellEquations . ....
.. ... ... .... ... ... ... ... ... ... ... ... ... ... ... .... ... ... ... .R.Rieben,D.White,andG.Rodrigue 2190
CORRECTIONS
Correctionsto“PhasedArraysBasedonOscillatorsCoupledonTriangularandHexagonalLattices” .... .. R.J.Pogorzelski 2196
CALLSFORPAPERS
SpecialIssueonMultifunctionAntennasandAntennaSystems..... ... ... ... ... ... ...... ... ... ... ... ... .... 2197
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1926 IEEETRANSACTIONSONANTENNASANDPROPAGATION,VOL.52,NO.8,AUGUST2004
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ALLENW.GLISSON,OutgoingEditor-in-Chief
TheUniversityofMississippi
DepartmentofElectricalEngineering
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0018-926X/04$20.00©2004IEEE
IEEETRANSACTIONSONANTENNASANDPROPAGATION,VOL.52,NO.8,AUGUST2004 1927
AllenW.Glisson(S’71–M’78–SM’88–F’02)receivedtheB.S.,M.S.,andPh.D.degreesinelec-
tricalengineeringfromtheUniversityofMississippi,in1973,1975,and1978,respectively.
In 1978, he joined the faculty of the University of Mississippi, where he is currently a
ProfessorandChairoftheDepartmentofElectricalEngineering.Hiscurrentresearchinterests
include the development and application of numerical techniques for treating electromagnetic
radiationandscatteringproblems,andmodelingofdielectricresonatorsanddielectricresonator
antennas. He has been actively involved in the areas of numerical modeling of arbitrarily
shaped bodies and bodies of revolution with surface integral equation formulations. He has
alsoservedasa consultanttoseveraldifferentindustrialorganizationsintheareaofnumerical
modeling in electromagnetics.
Dr.GlissonisaMemberofSigmaXiResearchSocietyandtheTauBetaPi,PhiKappaPhi,and
EtaKappaNuHonorSocieties.HeisaMemberofseveralprofessionalsocietieswithintheIEEE,
CommissionBoftheInternationalUnionofRadioScience(URSI),andtheAppliedComputa-
tionalElectromagneticsSociety.HewasaU.S.delegatetothe22nd,23rd,and24thGeneralAssembliesofURSI.Hewasselected
astheOutstandingEngineeringFacultyMemberin1986,1996,and2004.HereceivedaRalphR.TeetorEducationalAwardin
1989andtheFacultyServiceAward inthe SchoolofEngineeringin2002.HereceivedaBestPaperAwardfromthe SUMMA
FoundationandtwicereceivedacitationforexcellenceinrefereeingfromtheAmericanGeophysicalUnion.Heistherecipient
ofthe2004MicrowavePrizeawardedbytheMicrowaveTheoryandTechniquesSociety.HehasservedasamemberoftheIEEE
AntennasandPropagationSocietyAdministrativeCommitteeandiscurrentlyamemberoftheIEEEPressLiaisonCommittee.
HecurrentlyservesontheBoardofDirectorsoftheAppliedComputationalElectromagneticsSocietyandhasrecentlyservedas
Co-Editor-in-ChiefoftheAppliedComputationalElectromagneticsSocietyJournal.HehasalsoservedasanAssociateEditorfor
RadioScienceandastheSecretaryofCommissionBoftheU.S.NationalCommitteeofURSI.FromAugust2001toJuly2004
hewastheEditor-in-ChiefoftheIEEETRANSACTIONSONANTENNASANDPROPAGATION.
1928 IEEETRANSACTIONSONANTENNASANDPROPAGATION,VOL.52,NO.8,AUGUST2004
Bandwidth Enhancement and Further Size Reduction
of a Class of Miniaturized Slot Antennas
NaderBehdad,StudentMember,IEEE,andKamalSarabandi,Fellow, IEEE
Abstract—In this paper, new methods for further reducing allowforimpedancematchingarehighlydesirable.Thefunda-
the size and/or increasing the bandwidth (BW) of a class of mentallimitationintroducedbyChu[1]andlaterre-examined
miniaturized slot antennas are presented. This paper examines
by McLean [4] relates the radiation Q of a single resonance
techniques such as parasitic coupling and inductive loading to
antenna with its BW. However, whether such limitation can
achieve higher BW and further size reduction for this class
of miniaturized slot antennas. The overall BW of a proposed be directly extended to multiresonance antenna structures or
double resonant antenna is shown to be increased by more than notisunclear.Infact,throughacomparisonwithfiltertheory,
94% compared with a single resonant antenna occupying the designingarelativelywidebandantennamaybepossibleusing
same area. The behavior of miniaturized slot antennas, loaded
multipole(multiresonance)highQstructures.Inthispaperwe
with series inductive elements along the radiating section is also
examinetheapplicabilityofmultiresonanceantennastructures
examined. The inductive loads are constructed by two balanced
shortcircuitedslotlinesplacedonoppositesidesoftheradiating toenhancetheBWofminiaturizedslotantennas.
slot. These inductive loads can considerably reduce the antenna Differenttechniqueshavebeenusedforantennaminiaturiza-
sizeatitsresonance.Prototypes ofa doubleresonantantennaat tionsuchas:miniaturizationusingoptimalantennatopologies
850MHzandinductivelyloadedminiaturizedantennasataround
[5]–[7]andminiaturizationusingmagneto-dielectricmaterials
1 GHz are designed and tested. Finally the application of both
[8], [9]. In pursuit of antenna miniaturization while main-
methodsinadualbandminiaturizedantennaispresented.Inall
cases measured and simulated results show excellent agreement. taining ease of impedance matching and attaining relatively
highefficiency,anovelminiaturizedslotantennawasrecently
IndexTerms—Slotantennas,electricallysmallantennas,para-
siticantennas,multifrequencyantennas. presented [6]. Afterwards, a similar architecture in the form
of a folded antenna geometry was presented in order to in-
crease the BW of the previously mentioned miniaturized slot
I. INTRODUCTION antenna[7].Herewere-examinethistopology[6]andpropose
CURRENT advancements in communication technology modifications that can result in further size reduction or BW
and significant growth in the wireless communication enhancement without imposing any significant constraint on
market and consumer demands demonstrate the need for impedance matching or cross polarization level. In Section II,
smaller, more reliable and power efficient, integrated wireless a dual-resonant antenna topology is examined for BW en-
systems. Integrating entire transceivers on a single chip is hancement. This miniaturized antenna shows a BW which is
the vision for future wireless systems. This has the benefit of 94%largerthanthatofasingle-resonantminiaturizedantenna
cost reduction and improving system reliability. Antennas are with the same size.
consideredtobethelargestcomponentsofintegratedwireless Usingseriesinductiveelementsdistributedalongtheantenna
systems; therefore antenna miniaturization is a necessary task apertureresultsintheincreaseofinductanceperunitlengthof
inachievinganoptimaldesignforintegratedwirelesssystems. the line. Therefore the guided wavelength of the resonant slot
Thesubjectofantennaminiaturizationisnotnewandhasbeen lineisshortened.Thus,theoveralllengthoftheantennaisde-
extensively studied by various authors [1]–[4]. Early studies creased.InSectionIII,thistechniqueisfirstdemonstratedusing
have shown that for a resonant antenna, as size decreases, a standard resonant slot antenna and then incorporated in the
bandwidth(BW)andefficiencywillalsodecrease[1].Thisisa miniaturizedantennatopologyof[6]tofurtherreducetheres-
fundamental limitation which, in general, holds true indepen- onant frequency without increasing the area occupied by the
dent of antenna architecture. However, research on the design antenna.
of antenna topologies and architectures must be carried out The aforementioned techniques for BW enhancement and
to achieve maximum possible BW and efficiency for a given further size reduction can be used individually or in combina-
antenna size. Impedance matching for small antennas is also tion.ThecombinedapplicationofthetechniquesofSectionsII
challenging and often requires external matching networks; andIIIispresentedinSectionIVbydemonstratingthedesign
Therefore antenna topologies and structures which inherently ofadualbandminiaturizedslotantenna.
ManuscriptreceivedMay22,2003;revisedSeptember30,2003.Thiswork II. MINIATURIZEDSLOTANTENNAWITHENHANCEDBW
wassupportedinpartbytheEngineeringResearchCentersprogramoftheNa-
tionalScienceFoundation(NSF)underAwardEEC-9986866andbytheU.S. A. DesignProcedure
ArmyResearchOfficeunderContractDAA-99-1-01971.
TheauthorsarewithTheRadiationLaboratory,DepartmentofElectricalEn- In this section the design of coupled miniaturized slot an-
gineeringandComputerScience,TheUniversityofMichigan,AnnArbor,MI
tennas for BW enhancement is studied. The configuration of
48109-2122USA(e-mail:[email protected]).
DigitalObjectIdentifier10.1109/TAP.2004.832330 theproposedcoupledslotantennaisshowninFig.1(b)where
0018-926X/04$20.00©2004IEEE
BEHDADANDSARABANDI:BWENHANCEMENTANDFURTHERSIZEREDUCTION 1929
Fig.1. Geometryofsingle-anddouble-elementminiaturizedslotantennas.(a)Single-elementminiaturizedslotantenna.(b)Double-elementminiaturizedslot
antenna.
two miniaturized slot antennas are arranged so that they are entirefrequencyband.Heretheresonantfrequenciesofbothan-
parasitically coupled. Each antenna occupies an area of about tennasarefixedat MHzand isusedasthe
[Fig.1(a)]andachievesminiaturizationbythe tuningparameter.However,itisalsopossibletochange and
virtueofaspecialtopologydescribedindetailin[6].However, slightly, in order to achieve a higher degree of control for
thisantennademonstratesasmallBW(lessthan1%).Aclose tuningtheresponse.
examination of the antenna topology reveals that the slot-line The input impedance of a microstrip-fed slot antenna, for a
trace of the antenna only covers about half of the rectangular givenslotwidth,dependsonthelocationofthemicrostripfeed
printed-circuit board (PCB) area. Therefore another antenna, relativetooneendoftheslotandvariesfromzeroattheshort
with the same geometry, can be placed in the remaining area circuited end to a high resistance at the center. Therefore an
without significantly increasing the overall PCB size. Placing off-centermicrostripfeedcanbeusedtoeasilymatchaslotan-
two antennas in close proximity of each other creates strong tennatoawiderangeofdesiredinputimpedances.Theoptimum
coupling between the antennas which, if properly controlled, locationofthefeedlinecanbedeterminedfromthefull-wave
canbeemployedtoincreasethetotalantennaBW. simulation.Inthedoubleantennaexamplethefeedlineconsists
AsseeninFig.1(b),onlyoneofthetwoantennasisfedby ofa50 transmissionlineconnectedtoanopen-circuited75
amicrostripline.Theotherantennaisparasiticallyfedthrough linecrossingtheslot[Fig.1(b)].The75 lineisextendedby
capacitive coupling mostly at the elbow section. The coupling beyondthestrip-slotcrossingtocouplethemaximum
isamixtureofelectricandmagneticcouplingsthatcounteract energytotheslotandalsotocompensatefortheimaginarypart
eachother.Attheelbowsection,wheretheelectricfieldislarge, oftheinputimpedance.Usingthis75 lineasthefeed,allows
theslotsareveryclosetoeachother;therefore,itisexpectedthat for compact and localized feed of the antenna and tuning the
theelectricfieldcouplingisthedominantcouplingmechanism location of the transition from 50 to 75 provides another
andtheelectricfields(magneticcurrents)inbothantennaswill tuningparameterforobtainingagoodmatch.
beinphaseandthustheradiatedfarfieldisenhanced.
The two coupled antennas are designed to resonate at the B. FabricationandMeasurement
same frequency, , where is the center fre-
A double-element antenna (DEA) and two different single-
quencyand and aretheresonantfrequenciesofthetwo
element antennas (SEAs) (SEA 1 and SEA 2) were designed,
antennas.Inthiscasethe spectralresponseofthecoupled
fabricated,andmeasured.SEA1istheconstitutiveelementof
antennawillshowtwonulls,theseparationofwhichisafunc-
DEA and SEA 2 is an SEA with the same topology as SEA 1
tion of the separation between the two antennas, , and their
[seeFig.1(a)]butwiththesameareaastheDEA.SEA2isused
overlap distance . In order to quantify this null separation a
tocomparetheBWofthedoubleresonantminiaturizedantenna
couplingcoefficientisdefinedas
with that of the single-resonant miniaturized antenna with the
samesize.AllantennasweresimulatedusingIE3D[12]which
(1)
isafullwavesimulationsoftwarebasedonmethodofmoments
(MoM) and fabricated on a Rogers RO4350B substrate with
where and arethefrequenciesoftheupperandlowernulls thickness of 500 m, a dielectric constant of , and a
in . Hence can easily be adjusted by varying and losstangentof withacoppergroundplaneof
[Fig.1(b)],anddecreasesas isincreasedand isdecreased. 33.5 23 cm . The return losses of the SEAs as well as the
Afull-waveelectromagneticsimulationtoolcanbeusedtoex- DEAarepresentedinFig.2.SEA1showsaBWof8MHzor
tract asafunctionof and inthedesignprocess.BWmaxi- nearly 0.9% and SEA 2 shows a BW of 11.7 MHz or 1.31%
mizationisaccomplishedbychoosingacouplingcoefficient(by whereas the BW of the DEA is 21.6 MHz (2.54%) which in-
choosing and )suchthat remainsbelow dBoverthe dicates a factor of 1.94 increase over a SEA (SEA) with the
1930 IEEETRANSACTIONSONANTENNASANDPROPAGATION,VOL.52,NO.8,AUGUST2004
Fig.2. ReturnlossesofDEAandSEA2:SEAwiththesamesizeasDEA.(a)ReturnlossesoftheDEAandSEAofthesamesize(SEA2).(b)Returnlossof
theSEAthatconstitutestheDEA(SEA1).
same area. Choosing a different substrate with different thick- TABLE I
nessanddielectricconstant,canincreasetheoverallBWofboth COMPARISONBETWEENTHEDEAANDITSCONSTITUTIVESEAS
SHOWNINFIG.1
antennas.However,itisalsoexpectedthattheBWratioofthe
DEAtotheSEAremainsthesame.TheoverallsizeoftheDEA
is whichshowsa25%increaseinareawhen
comparedtothesizeoftheSEA1 .TheQ
ofeachantennahasalsobeencalculatedusingthemethodpre-
sentedin[10]andcomparedwiththefundamentallimitonthe
Q of small antennas [4] in Table II. Demonstrably the quality
factors of both SEAs are well above the minimum theoretical
TABLE II
limit.SinceQisonlydefinedforsingleresonantstructures,no COMPARISONBETWEENMEASUREDQANDTHEMINIMUMATTAINABLEQ.(cid:3)
valueforQisreportedfortheDEAinTableII.Incalculatingthe CALCULATEDUSINGTHEFOSTERREACTANCETHEOREM[10].(cid:3)(cid:3)CALCULATED
minimumQfortheslotantennasusingtheChulimit,itisnec- USINGTHECHU-MCLEANFORMULAFORASINGLE-RESONANTANTENNA[4]
essarytofindtheradiusofthesmallestspherethanenclosesthe
antenna.Atfirst,itmaynotbeclearwhetherthissphereshould
onlycovertheaperture or,inadditiontothat,someportionof
thegroundplanetoo(becauseoftheelectriccurrentsthatexist
inthegroundplane).Thisbecomesclearbyapplyingtheequiv-
alencetheoremtothisproblem,whichshowsthatthemagnetic
currentsresponsibleforradiation,existonlyontheapertureand
accordingthethederivationoftheChulimit,thesmallestsphere H-Plane pattern. Table I shows the radiation characteristics of
thatenclosestheseradiatingmagneticcurrentsshouldbeused. theDEAsandSEAs.Itisseenthatthegain-BWproductofthe
The gain of the double resonant antenna was mea- proposeddouble-antennaissignificantlyhigherthanthatofthe
sured at three different frequencies and is presented in singleantenna.
Table I. Radiation patterns of the antenna were measured
at MHz and found to be similar to each III. IMPROVED ANTENNA MINIATURIZATION USING
other.Fig.3showstheco-andcross-polarizedE-andH-plane DISTRIBUTEDINDUCTIVELOADING
radiation patterns at MHz. The E- and H-plane radi-
A. DesignProcedure
ation patterns of this antenna are expected to be dual of those
Amicrostrip-fedslotantennahasthelengthof ,where
ofashortelectricdipole.Fig.3(a)showstheH-Planeradiation
isthewavelengthintheslot,atitsfirstresonance.Theelectric
pattern which is similar to the E-Plane radiation pattern of
an electric dipole with deeps instead of nulls at . currentdistributioncanbemodeledbythevoltagedistribution
This can be attributed to the finiteness of the ground plane over a transmission line short circuited at both ends. The
where some radiation comes from the electric currents on the resonantlengthofatransmissionline canbemadesmaller
antenna ground plane at the edges of the substrate. Fig. 3(b), iftheinductanceperunitlengthofthelineisincreased.Thiscan
however, does not show a uniform radiation pattern like the beaccomplishedbyinsertinganumberofseriesinductorsinthe
H-Planeradiationofashortelectricdipole.Thisisbecauseof transmission line. For slot-lines, insertion ofseries lumped el-
the180 differenceinphasebetweenthenormalcomponentof ementsisnotpossible.Besides,serieslumpedelementshavea
the electric fields at the top and bottom of the antenna ground lowQwhichadverselyaffectsantennaefficiency(gain).Tore-
plane. The H-Plane pattern is expected to have deep nulls at alizeaslotlinewithhigherinductanceperunitlength,anarray
these angles; therefore, this does not significantly affect the of distributed, short circuited, narrow slot-lines can be placed
BEHDADANDSARABANDI:BWENHANCEMENTANDFURTHERSIZEREDUCTION 1931
Fig.3. Farfieldradiationpatternsofthedouble-elementminiaturizedslotantennaat852MHz.(a)H-planeand(b)E-plane.
Fig.4. Loadedandunloadedstraightslotantennas.(a)Geometryofamicrostrip-fedstraightslotantenna.(b)Geometryofamicrostrip-fedstraightslotantenna
loadedwithanarrayofseriesinductiveelements.
along the radiating segment of the slot antenna as shown in
Fig. 4(b). The impedance of a short circuited slot line is ob-
tained by
(2)
where is the propagation constant, is the characteristic
impedance, and is the length of the short circuited slot-line.
Thecharacteristicimpedanceofaslot-lineisinverselypropor-
tional to its width [11] therefore by using wider series slots,
moreinductancecanbeobtainedforafixedlengthofshortcir-
cuitedtransmissionline.Thebestlocationtoputseriesinduc-
tors in a slot is near its end where the amplitude of magnetic
currentissmall.Puttingthematthecenteroftheslotwherethe
magneticcurrentisatitsmaximum,stronglydegradesradiation
efficiency. It can easily be seen that by increasing the number
and value of inductors, the length of transmission line neces- Fig.5. Geometryofaminiaturizedslotantennaloadedwithseriesdistributed
inductors(slits).
sarytosatisfytheboundaryconditionsatbothendsoftheslot
decreases.
Thesizereductionmayalsobeexplainedbyconsideringthe perpendicular to it. The latter is described by the continuity
electric current distribution in the conductor around the slot. of the electric current and displacement current at the slot
Therearetwocomponentsofelectriccurrentinthegroundplane discontinuity. Putting a discontinuity (a slit) normal to the
of the slot, one that circulates around the slot and one that is circulating current path forces the current to circle around
1932 IEEETRANSACTIONSONANTENNASANDPROPAGATION,VOL.52,NO.8,AUGUST2004
Fig.6. Simulatedandmeasuredreturnlossesofthestraightslotantennasandminiaturizedslotantennaswithandwithoutinductiveloading.(seeFigs.4and5).
(a)Returnlossesofstraightloadedandunloadedslotantennas.(b)Returnlossesofordinaryandloadedminiaturizedslotantennas.
Fig.7. FarfieldradiationpatternsofloadedstraightslotantennashowninFig.4(b).(a)H-planeand(b)E-plane.
the discontinuity. Hence the electric current traverses a longer of the microstrip line are found by trial and error, using full
path length than the radiating slot length which in turn lowers wave simulations. For both straight slots (with and without
the resonant frequency. Fig. 4(b) shows a slot antenna loaded series inductors) the lengths of the extended microstrip lines
with a number of narrow slits which act as an array of series are found to be , where is the wavelength in the
inductors. These slits are designed to have a length smaller microstrip lines at their respective resonance frequencies.
than andcarryamagneticcurrentwithadirectionnormal Fig. 5 shows a miniaturized slot antenna (similar to the
to that of the main radiator. Placing them only on one side of topology in [6]) loaded with series inductive slits to further
theradiatingslotresultsinasymmetryinphaseandamplitude reduce its resonant frequency. The antenna without the series
of the current along the slot which could create problems in inductorsisalreadysmall,andaddingseriesinductiveelements
matchingandworsencrosspolarization.Inordertocircumvent further reduces the resonant frequency or equivalently the
this problem, two series slits are placed on the opposite sides electricaldimensionsoftheantenna.Insteadofusingidentical
ofthemainslot.Theseslitscarrymagneticcurrentswithequal inductive elements along the radiating slot, differently sized
amplitudes and opposite directions. Since the lengths of these inductive slits are used to cover most of the available area on
narrow slits are small compared to the wavelength and since thePCBinordertomaintaintheareaoccupiedbytheantenna.
they are closely spaced, the radiated fields from the opposite The antenna is matched to a microstrip transmission line in
slitscanceleachotherandtheydonotcontributetotheradiated a manner similar to the straight slots described earlier. The
farfield.Matchingisperformedbyusinganoff-centeredopen feedlineiscomposedofa75 open-circuitedmicrostripline
circuited microstrip feed. The optimum location and length connected to a 50 feed line. In this case, the open circuited