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8/2/2019 Ec1352 Antenna Wave Propagation(1)
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NOORULISLAMCOLLEGEOFENGG,KUMARACOIL.
DEPARTMENTOFELECTRONICSANDCOMMUNICATIONENGG.
SUBJECTCODE:EC1352SUBJECTNAME:Antennawavepropagation
Preparedby,T.Gopalakrishnan(Lect/ECE)S.BeemaBeevi(Lect/ECE)
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NOORULISLAMCOLLEGEOFENGINEERING,KUMARACOIL
ELECTRONICSANDCOMMUNICATIONENGINEERING
SUBJECTCODE:EC1352SUBJECT:ANTENNASANDWAVEPROPAGATION
SEMESTER:VI
TWOMARKQUESTIONS
1.Defineanantenna.Antennaisatransitiondeviceoratransducerbetweenaguidedwaveandafreespacewaveorviceversa.Antennaisalsosaidtobeanimpedancetransformingdevice.
2.Whatismeantbyradiationpattern?Radiationpatternistherelativedistributionofradiatedpowerasafunctionofdistanceinspace.Itisa
graphwhichshowsthevariationinactualfieldstrengthoftheEMwaveatallpointswhichareatequaldistancefromtheantenna.TheenergyradiatedinaparticulardirectionbyanantennaismeasuredintermsofFIELDSTRENGTH.(EVolts/m)
3.DefineRadiationintensity?
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ThepowerradiatedfromanantennaperunitsolidangleiscalledtheradiationintensityU(wattspersteradianorpersquaredegree).Theradiationintensityisindependentofdistance.
4.DefineBeamefficiency?Thetotalbeamarea(WA)consistsofthemainbeamarea(WM)plustheminorlobearea(Wm).ThusWA=WM+Wm.
Theratioofthemainbeamareatothetotalbeamareaiscalledbeamefficiency.Beamefficiency=SM=WM/WA.
5.DefineDirectivity?
ThedirectivityofanantennaisequaltotheratioofthemaximumpowerdensityP(q,f)maxtoitsaveragevalueoverasphereasobservedinthefarfieldofanantenna.
D=P(q,f)max/P(q,f)av.DirectivityfromPattern.
D=4p/WA..Directivityfrombeamarea(WA).
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6.Whatarethedifferenttypesofaperture.?
i)Effectiveaperture.ii).Scatteringaperture.iii)Lossaperture.iv)collectingaperture.v).Physicalaperture.
7.Definedifferenttypesofaperture.?
Effectiveaperture(Ae).
Itistheareaoverwhichthepowerisextratedfromtheincidentwaveanddeliveredtotheloadiscalledeffectiveaperture.
Scatteringaperture(As.)Itistheratioofthereradiatedpowertothepowerdensityoftheincidentwave.Lossaperture.(Ae).Itistheareaoftheantennawhichdissipatespowerasheat.
Collectingaperture.(Ae).Itistheadditionofabovethreeapertures.
Physicalaperture.(Ap).Thisapertureisameasureofthephysicalsizeoftheantenna.
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8.DefineApertureefficiency?Theratiooftheeffectiveaperturetothephysicalapertureistheapertureefficiency.i.e
Apertureefficiency=hap=Ae/Ap(dimensionless).
9.Whatismeantbyeffectiveheight?Theeffectiveheighthofanantennaistheparameterrelatedtotheaperture.Itmaybedefinedastheratiooftheinducedvoltagetotheincidentfield.i.e
H=V/E.
10.Whatarethefieldzone?Thefieldsaroundanantennaaybedividedintotwoprincipalregions.
i.Nearfieldzone(Fresnelzone)ii.Farfieldzone(Fraunhoferzone)11.WhatismeantbyPolarization.?
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ThepolarizationoftheradiowavecanbedefinedbydirectioninwhichtheelectricvectorEisalignedduringthepassageofatleastonefullcycle.Alsopolarizationcanalsobedefinedthephysicalorientationoftheradiatedelectromagneticwavesinspace.
Thepolarizationarethreetypes.Theyare
Ellipticalpolarization,circularpolarizationandlinearpolarization.
12.Whatismeantbyfronttobackratio.?Itisdefinedastheratioofthepowerradiatedindesireddirectiontothepowerradiatedintheoppositedirection.i.e
FBR=Powerradiatedindesireddirection/powerradiatedintheoppositedirection.
13.Defineantennaefficiency.?Theefficiencyofanantennaisdefinedastheratioofpowerradiatedtothetotalinputpowersuppliedtotheantenna.
Antennaefficiency=Powerradiated/Totalinputpower
14.Whatisradiationresistance?
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Theantennaisaradiatingdeviceinwhichpowerisradiatedintospaceintheformofelectromagneticwave.
W=I2RRr=W/I2WhereRrisafictitiousresistancecalledcalledasradiationresistance.
15Whatismeantbyantennabeamwidth?
Antennabeamwidthisameasureofdirectivityofanantenna.Antennabeamwidthisanangularwidthindegrees,measuredontheradiationpattern(majorlobe)betweenpointswheretheradiatedpowerhasfallentohalfitsmaximumvalue.Thisiscalledasbeamwidthbetweenhalfpowerpointsorhalfpowerbeamwidth.(HPBW).
16.WhatismeantbyreciprocityTheorem.?Ifane.m.fisappliedtotheterminalsofanantennano.1andthecurrentmeasuredattheterminalsoftheanotherantennano.2,thenanequalcurrentbothin
amplitudeandphasewillbeobtainedattheterminaloftheantennano.1ifthesameemfisappliedtotheterminalsofantennano.2.
17.Whatismeantbyisotropicradiator?
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Aisotropicradiatorisafictitiousradiatorandisdefinedasaradiatorwhichradiatesfieldsuniformlyinalldirections.Itisalsocalledasisotropicsourceoromnidirectionalradiatororsimplyunipole.
18.DefinegainTheratioofmaximumradiationintensityingivendirectiontothemaximumradiationintensityfromareferenceantennaproducedinthesamedirectionwithsameinputpower.i.e
Maximumradiationintensityfromtestantenna
Gain(G)=-------------------------------------------------------
Maximumradiationintensityfromthereferenceantennawithsameinputpower
19.DefineselfimpedanceSelfimpedanceofanantennaisdefinedasitsinputimpedancewithallotherantennasarecompletely
removedi.eawayfromit.
20.Definemutualimpedance
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Thepresenceofnearbyantennano.2inducesacurrentintheantennano.1indicatesthatpresenceofantennano.2changestheimpedanceoftheantennano.1.Thiseffectiscalledmutualcouplingandresultsinmutualimpedance.
21.Whatismeantbycrossfield.?NormallytheelectricfieldEisperpendiculartothedirectionofwavepropagation.InsomesituationtheelectricfieldEisparalleltothewavepropagationthatconditioniscalledCrossfield.
22.Defineaxialratio
TheratioofthemajortotheminoraxesofthepolarizationellipseiscalledtheAxialRatio.(AR).
23.WhatismeantbyBeamArea.?ThebeamareaorbeamsolidangleorWAofanantennaisgivenbythenormalizedpowerpatternoverasphere.
WA=4pPn(q,f)dW
WheredW=Sinqdq.df
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24.Whatisdualityofantenna.?Itisdefinedasanantennaisacircuitdevicewitharesistanceandtemperatureontheonehandandthespacedeviceontheotherwithradiationpatterns,beamangle,directivitygainandaperture.
25.StatePoyntingtheorem.
ItstatesthatthevectorproductofelectricfieldintensityvectorEandthemagneticfiledintensityvectorHatanypointisameasureoftherateofenergyflowperunitareaatthatpoint.Thedirectionofpowerflowisperpendiculartoboththeelectricfieldandmagneticfieldcomponents.
26.Whatispointsource?Itisthewavesoriginateatafictitiousvolumeless
emittersourceatthecenterOoftheobservationcircle.
27.Whatismeantbyarray.?Anantennaisasystemofsimilarantennasorientedsimilarlytogetgreaterdirectivityinadesireddirection.
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28.Whatismeantbyuniformlineararray.?
Anarrayislinearwhentheelementsofthearrayarespacedequallyalongthestraightline.Iftheelementsarefedwithcurrentsofequalmagnitudeandhavingauniformprogressivephaseshiftalongtheline,thenitiscalleduniformlineararray.
29.Whatarethetypesofarray.?
a.Broadsidearray.b.Endfirearrayc.Collineararray.d.Parasiticarray.30.WhatisBroadsidearray.?
Broadsidearrayisdefinedasanarrangementinwhichtheprincipaldirectionofradiationisperpendiculartothearrayaxisandalsotheplanecontainingthearrayelement.ForBroadsidearraythephasedifferenceEHWZHHQDGMDFHQWHOHPHQWLV.
31.DefineEndfirearray.?
Endfirearrayisdefinedasanarrangementinwhichtheprincipaldirectionofradiationiscoincideswiththearrayaxis.)RUHQG)LUHDUUD\.
:KHUH...$QGGGLVWDQFHEHWZHHQWKHHOHPHQWV.32.Whatiscollineararray.?
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Inthisarraytheantennaelementsarearrangedcoaxiallybymountingtheelementsendtoendinstraightlineorstackingthemoneovertheotherwithradiationpatterncircularsymmetry.Eg.Omnidirectionalantenna.
33.WhatisParasiticarray.?Inthisarraytheelementsarefedparasiticallytoreducetheproblemoffeedline.Thepowerisgiventooneelementfromthatotherelementsgetbyelectromagneticcoupling.Eg.Yagiudaantenna.
34.Whatistheconditiononphasefortheendfirearraywithincreaseddirectivity.?:KHQ.
GSURGXFHVPD[LPXPILHOGLQWKHGLUHFWLRQ.=0butdoesnotgivethemaximumdirectivity.IthasbeenshownbyHansenandwoodyardthatalargedirectivityisobtainedbyincreasingthephasechange
EHWZHHQWKHVRXUFHVVRWKDW.
-.
Q
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.Thisconditionwillbereferredtoastheconditionforincreaseddirectivity.
35.Definearrayfactor.Thenormalizedvalueofthetotalfieldisgivenby,
(Q
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VLQQ.
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VLQ.
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.ThefieldisgivenbytheexpressionEwillbereferredtoasarrayfactor.
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36.36.Itisdefinedtheanglebetweenthefirstnulls(or)itisdefinedastwicetheanglebetweenthefirstnullandthemajorlobemaximumdirection.
37.Listouttheexpressionofbeamwidthforbroadsidearrayandendfirearray.Forbroadsidearraytheexpressionforbeamwidthbetweenthefirstnullsisgivenby,
BWFN=((+/-
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.QG.ForEndfirearraytheexpressionforbeamwidthbetweenthefirstnullsisgivenby,
1/2
BWFN=((+/-
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.QG
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.
38.DifferentiatebroadsideandEndfirearray.?BroadsidearrayEndfirearray1.AntennasfedinPhase.2.MaximumRadiationisperpendiculartothedirectionofarrayaxis.3.BeamwidthofmajorlobeistwicetheAntennaelementsarefedofoutofPhase.-.MaximumRadiationisdirectedalongthearrayaxis.Beamwidthisgreaterthanthatforabroadsidearrayofsamelength.BW=((+/-
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.QG
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1/
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reciprocalofthearraylength.BW=((+/
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.QG.2.
39.WhatistheneedfortheBinomialarray.?
Theneedforabinomialarrayis
i).Inuniformlineararrayasthearraylengthisincreasedtoincreasethedirectivity,thesecondarylobesalsooccurs.
ii)Forcertainapplications,itishighlydesirablethatsecondarylobesshouldbeeliminatedcompletelyorreducedtominimumdesirablelevelcomparedtomainlobes.
40.Definepowerpattern.?GraphicalrepresentationoftheradialcomponentofthepointingvectorSrataconstantradiusasafunctionofangleiscalledpowerdensitypatternorpowerpattern.
41.WhatismeantbysimilarPointsources.?Wheneverthevariationoftheamplitudeandthephaseofthefieldwithrespecttotheabsoluteangleforanytwosourcesaresamethentheyarecalledsimilarpointsources.
Themaximumamplitudesoftheindividualsourcesmaybeunequal.
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42.42.Similarpointsourceswithequalmaximumamplitudesarecalledidenticalpointsources.
43.Whatistheprincipleofthepatternmultiplication?Thetotalfieldpatternofanarrayofnonisotropicbutsimilarsourcesistheproductofthe
i)individualsourcepatternand
ii)Thearraypatternofisotropicpointsourceseach
locatedatthephasecenteroftheindividualsourcehavingthesameamplitudeandphase.
Whilethetotalphasepatternisthesumofthephasepatternsoftheindividualsourcepatternandarraypattern.044.Whatistheadvantageofpatternmultiplication?
Usefultoolindesigningantenna
Itapproximatesthepatternofacomplicatedarray
withoutmakinglengthycomputations
45.Whatistaperingofarrays?
Taperingofarrayisatechniqueusedforreductionofunwantedsidelobes.Theamplitudeofcurrentsinthelineararraysourceisnon-uniform;hencethecentralsourceradiatesmoreenergythantheends.Taperingisdonefromcentertoend.
46.Whatisabinomialarray?
Itisanarrayinwhichtheamplitudesofthe
antennaelementsinthearrayarearrangedaccordingtothecoefficientsofthebinomialseries.
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47.Whataretheadvantagesofbinomialarray?Advantage:Nominorlobes
Disadvantages:Increasedbeamwidth.thelargeratioofcurrent
Maintainingamplitudeinlargearraysisdifficult
48.Whatisthedifferencebetweenisotropicandnon-isotropicsource.sourceenergyinall
Isotropicradiatesdirectionsbutnon-isotropicsourceradiatesenergyonlyinsomedesireddirections.Isotropicsourceisnotphysicallyrealizablebutnon-isotropicsourceis
physicallyrealizable.
49.DefineSideLobeRatio
SideLobeRatioisdefinedastheratioofpowerdensityintheprincipalormainlobetothepowerdensityofthelongestminorlobe.
50.Listthearraysusedforarraytapering.Array:TaperingtheBinomialfollowscoefficientofbinomialseries
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DolphTchebycheffArray:TaperingfollowsthecoefficientofTchebycheffpolynomial
51.WhatisaShortDipole?
Ans:Ashortdipoleisoneinwhichthefieldisoscillatingbecauseoftheoscillatingvoltageandcurrent.Itiscalledso,becausethelengthofthedipoleisshortandthecurrentisalmostconstantthroughtouttheentirelengthofthedipole.ItisalsocalledasHertzianDipolewhichisahypotheticalantennaandisdefinedasashortisolatedconductorcarryinguniformalternatingcurrent.
52.HowradiationsarecreatedfromashortDipole?
Ans:Thedipolehastwoequalchargesofoppositesignoscillatingupanddowninaharmonicmotion.Thechargeswillmovetowardseachotherandelectricfiledlineswerecreated.Whenthechargesmeetatthemidpoint,thefieldlinescuteachotherandnewfieldarecreated.Thisprocessisspontaneousandsomorefieldsarecreatedaroundtheantenna.Thisishowradiationsareobtainedfromashortdipole.(SeeFigurefromJohn.D
.KrausBook)
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53.Whyashortdipoleisalsocalledanelementaldipole?
Ashortdipolethatdoeshaveauniformcurrentwillbeknownastheelementaldipole.Suchadipolewillgenerallybeconsiderablyshorterthanthetenthwavelengthmaximumspecifiedforashortdipole.Elementaldipoleisalsocalledaselementarydipole,elementarydoubletandhertziandipole.
54.WhatisaInfinitesimalDipole?
Whenthelengthoftheshortdipoleisvanishinglysmall,thensuchadipoleiscalledainfinitesimaldipole.IfdlbetheinfinitesimallysmalllengthandIbethecurrent,thenIdliscalledasthecurrentelement.
55.Whyashortdipoleiscalledaoscillatingdipole?
Ashortdipoleisinitiallyinneutralconditionandthemomentacurrentstartstoflowinonedirection,onehalfofthedipolerequireanexcessofchargeandtheotheradeficitbecauseacurrentisaflowofelectrical
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charge.Then,therewillbeavoltagebetweenthetwohalvesofthedipole.Whenthecurrentchangesitsdirectionthischargeunbalancewillcauseoscillations.Henceanoscillatingcurrentwillresultinanoscillatingvoltage.Since,insuchdipole,electricchargeoscillates,itmaybecalledasOscilllatingelectricdipole.
56.Whatdoyouunderstandbyretardedcurrent?
Since,theshortelectricdipoleissoshort,thecurrentwhichisflowingthroughthedipoleisassumedtobeconstantthroughtoutitslength.Theeffectofthiscurrentisnotfeltinstantaneousatadistancepointonlyafteranintervalequaltothetimerequiredforthewavetopropagateoverthedistanceriscalledtheretardationtime.
Theretardedcurrent[I]=Ioexp(jW-r/c))WhereUFLVWKHSKDVHUHWDUGDWLR.
57.Defineinductionfield
Theinductionfieldwillpredominateatpoints
closetothecurrentelement,wherethedistancefromthecentreofthedipoletotheparticularpointisless.Thisfield
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ismoreeffectiveinthevicinityofthecurrentelementonly.Itrepresentstheenergystoredinthemagneticfieldsurroundingthecurrentelementorconductor.Thisfieldisalsoknownasnearfield.
58.DefineRadiationfield
Theradiationfieldwillbeproducedatalargerdistancefromthethecurrentelement,wherethedistancefromthecentreofthedipoletotheparticularpointisverylarge.Itisalsocalledasdistantfieldorfarfield.
59.Atwhatdistancefromthedipoleistheinductionfieldequaltotheradiationfield?
Asthedistancefromthecurrentelementortheshortdipoleincreases,bothinductionandradiationfieldsemergeandstartdecreasing.However,adistancereachesfromtheconductoratwhichboththeinductionandradiationfieldbecomesequalandtheparticulardistancedependsuponthewavelength.Thetwofieldswillthushaveequalamplitudeatthatparticulardistance.Thisdistanceisgivenby
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U.
60.DefineRadiationResistance
Itisdefinedasthefictitiousresistancewhichwheninsertedinserieswiththeantennawillconsumethesameamountofpowerasitisactuallyradiated.Theantennaappearstothetransmissionlineasaresistivecomponentandthisisknownastheradiationresistance.
61.Givetheexpressionfortheeffectiveapertureofashortdipole
Theeffectiveapertureofashortdipoleisgivenby$H.2
62.Whatisadipoleantenna?
Adipoleantennamaybedefinedasasymmetricalantennainwhichthetwoendsa
reatequalpotentialrelativetothemidpoint.
63.Whatisahalfwavedipole?
Ahalfwaveantennaisthefundamentalradioantennaofmetalrodortubingorthinwirewhichhasaphysicallengthofhalfwavelengthinfreespaceatthefrequencyofoperation
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64.GivetheexpressionfortheeffectiveapertureofaHalfwaveDipole
64.GivetheexpressionfortheeffectiveapertureofaHalfwaveDipole
Ae=0.2
65.WhatistheradiationresistanceofahalfwavedipoleTheradiationresistanceofahalfwavedipoleisgivenbyRr=73ohm
66.Whatisaloopantenna?
Aloopantennaisaradiatingcoilofanyconvenientcross-sectionofoneormoreturnscarryingradiofrequencycurrent.itmayassumeanyshape(e.g.rectangular,square,triangularandhexagonal)
67.GiveanexpressionofradiationresistanceofasmallloopRadiationresistanceofasmallloopisgivenbyRr=31,200(A/2)2
68.HowtoincreasetheradiationresistanceofaloopantennaTheradiationresistanceofaloopantennacanbeincreasedby:
1.increasingthenumberofturns2.insertingaferritecoreofveryhighpermeabilitywithloopantennascircumferencewhichwillrisethemagneticfieldintensitycalledferriteloop.69.Whatarethetypesofloopantennas?Loopantennasareclassifiedinto:
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1.(OHFWULFDOO\6PDOO&LUFXPIHUHQFH..2.(OHFWULFDOO\/DUJH'LPHQVLRQFRPSDUDEOHWR..70.WhatareElectricallySmallloopantennas?
ElectricallySmallloopantennasisoneinwhichtheoveralllengthoftheloopislessthanone-tenthofthewavelength.ElectricallySmallloopantennashavesmallradiationresistancesthatareusuallysmallerthantheirloopresistances.Theyareverypoorradiatorsandseldomemployedfortransmissioninradiocommunication.
71.WhatareElectricallylargeloopantennas?
ElectricallyLargeloopantennasisoneinwhichtheoveralllengthoftheloopapproachesthewavelength.
72.Listouttheusesofloopantenna
Varioususesofloopantennaare:
.Itisusedasreceivingantennainportableradioandpagers
.Itisusedasprobesforfieldmeasurementsandasdirectionalantennasforradio
wavenavigation
.Itisusedtoestimatethedirectionofradiowavepropagation
73.Whataretheparameterstobeconsideredforthedesignofanhelicalantenna?Theparameterstobeconsideredforthedesignofanhelicalantennaare:
1.Bandwidth2.Gain3.Impedance4.AxialRatio74.WhatarethetypesofradiationmodesofoperationforanhelicalantennaThetwotypesofradiationmodesofoperationpossibleforanhelicalantennaare:
1.Normalmodeofoperation
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2.2.75.Whichantennawillproducecircularlypolarizedwaves
Helicalantennaradiatescircularlypolarizedwave.
76.Listtheapplicationsofhelicalantenna
Theapplicationsofhelicalantennaare:
.Itbecamethwworkhouseofspacecommunicationsfortelephone,televisionand
data,beingemployedbothonsatellitesandatgroundstations
.Manysatellitesincludingweathersatellites,datarelaysatellitesallhavehelical
antennas
.Itisonmanyotheprobesofplanetsandcomets,includingmoonandmars,beingused
alone,inarraysorasfeedsforparabolicreflectors,itscircularpolarizationandhigh
gainandsimplicitymakingiteffectiveforspaceapplication
77.DefineSkywave.
Wavesthatarriveatthereceiverafterreflectionintheionosphereiscalledskywave.78.DefineTroposphericwave.
WavesthatarriveatthereceiverafterreflectionfromthetroposphereregioniscalledTroposphericwave.(ie10KmfromEarthsurface).
79.DefineGroundwave.Wavespropagatedoverotherpathsneartheearthsurfaceiscalledgroundwavepropagation.80.WhatarethetypeofGroundwave.
Groundwaveclassifiedintotwotypes.
i.Spacewave
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ii.Surfacewave.81WhatismeantbySpaceWave.?Itismadeupofdirectwaveandgroundreflectedwave.Alsoincludestheportionofenergyreceivedasaresultofdiffractionaroundtheearthsurfaceandthereflectionfromtheupperatmosphere.
82.WhatismeantbySurfaceWave.?WavethatisguidedalongtheearthssurfacelikeanEMwaveisguidedbyatransmissioniscalledsurfacewave.Attenuationofthiswaveisdirectlyaffectedbytheconstantofearthalongwhichittravels.
83.Whatismeantbyfading.?Variationofsignalstrengthoccuronlineofsightpathsasaresultoftheatmosphericconditionsanditiscalled.Itcannotbepredictedproperly.
84.Whatarethetypeoffading.?Twotypes.i.Inversebending.iii.Multipathfading.85.Whatisinverseandmultipathfading.?Inversebendingmaytransformlineofsight
pathintoanobstructedone.Multipathfadingiscausedbyinterferencebetweenthedirectandgroundreflectedwavesaswellasinterferencebetweentwoaremorepathsintheatmosphere.86.Whatismeantbydiversityreception.?
Tominimizethefadingandtoavoidthemultipathinterferencethetechniqueusedarediversityreception.Itisobtainedbytwoways.
i.Spacediversityreception.ii.Frequencydiversityreception.iii.Polarizationdiversity.
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87.DefineSpacediversityReception.Thismethodexploitsthefactthatsignalsreceivedatdifferentlocationsdonotfadetogether.ItUHTXLUHVDQWHQQDVVSDFHGDWOHDVW.DSDUWDUHSUHIHUUHG.andtheantennawhichhighsignalstrengthatthemomentdominates.88.DefinefrequencydiversityReception.
Thismethodtakesadvantageofthefactthatsignalsofslightlydifferentfrequenciesdonotfadesynchronously.Thisfactisutilizedtominimizefadinginradiotelegraphcircuits.
89.Definepolarizationdiversityreception.Itisusedinnormallyinmicrowavelinks,anditisfoundthatsignaltransmittedoverthesamepathintwopolarizationshaveindependentfadingpatterns.inbroadbanddishantennasystem,Polarizationdiversitycombinedwithfrequencydiversityreceptionachieveexcellentresults.
90.WhatismeantbyFaradaysrotation.?
Duetotheearthsmagneticfields,theionoshericmediumbecomesanisotropicandtheincidentplanewaveenteringtheionospherewillsplitintoordinaryandextraordinarywaves/modes.
Whenthesemodesre-emergefromtheionospheretheyrecombineintoasingleplanewaveagain.
Finallytheplaneofpolarizationwillusuallyhavechanged,thisphenomenonisknownasFaradaysrotation.
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91.Whatarethefactorsthataffectthepropagationofradiowaves.?i.Curvatureofearth.ii.Earthsmagneticfield.iii.Frequencyofthesignal.iv.Planeearthreflection.92.Definegyrofrequency.FrequencywhoseperiodisequaltotheperiodofanelectroninitsorbitundertheinfluenceoftheearthsmagneticfluxdensityB.
93.Definecriticalfrequency.Foranylayer,thehighestfrequencythatwillbereflectedbackforverticalincidenceisfcr=91max
94.DefineMagneto-IonsSplitting.Thephenomenonofsplittingthewaveinto
twodifferentcomponents(ordinaryandextra-ordinary)bytheearthsmagneticfieldiscalledMagneto-IonsSplitting.95.DefineLUHF.
ThelowestusefulHFforagivendistanceandtransmitterpowerisdefinedasthelowestfrequencythatwillgivesatisfactoryreceptionforthatdistanceandpower.
Itdependson
i.Theeffectiveradiatedpower
ii.Absorptioncharacterofionosphereforthepathsbetweentransmitterandreceiver.
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iii.Therequiredfieldstrengthwhichinturndependsupontheradionoiseatthereceivinglocationandtypeofserviceinvolved.96.DefineRefractiveindex.Itisdefinedasinvacuan=c/vpVelocityoflightn=themediumPhasevelocityinn=.r
97DefinemaximumUsableFrequency.
ThemaximumFrequencythatcanbereflectedbackforagivendistanceoftransmissioniscalledthemaximumusablefrequency(MUF)forthatdistance.
MUF=fcrVHF.
98.Defineskipdistance.Thedistancewithinwhichasignalofgivenfrequencyfailstobereflectedbackistheskipdistanceforthatfrequency.Thehigherthefrequencythegreatertheskipdistance.
99.DefineOptimumfrequency.?Otimumfrequencyfortransmittingbetweenanytwopointsisthereforeselectedassomefrequency
i
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lyingbetweenabout50and85percentofthepredictedmaximumusablefrequencybetweenthosepoints.
100.Whatiswaveimpedance.?.0/-(fc/f)LH.-(fc/f)101.DefinewavevelocityandGroupvelocity.?wavevelocityvp=c/-(fc/f)2Groupvelocity,vpvg=c2vg=c2/vp
16MARKQUESTIONS
1.Writethepotentialfunctionindifferentform.2.ExplainindetailabouttheapertureConceptAperturerepresentstheareaoftheantennaconfiningtheeffectiveradiationsThevarioustypesofantennaaperturesare
i)Effectiveaperture.
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ii).Scatteringaperture.
iii)Lossaperture.
iv)collectingaperture.
v).Physicalaperture.
Effectiveaperture(Ae).
Itistheareaoverwhichthepowerisextratedfromtheincidentwaveanddeliveredtotheloadiscalledeffectiveaperture.
Scatteringaperture(As.)
Itistheratioofthereradiatedpowertothepowerdensityoftheincidentwave.
Lossaperture.(Ae).Itistheareaoftheantennawhich
dissipatespowerasheat.
Collectingaperture.(Ae).Itistheadditionofabovethreeapertures.
Physicalaperture.(Ap).Thisapertureisameasureofthephysicalsizeoftheantenna.
Theratiooftheeffectiveaperturetothephysicalapertureistheapertureefficiency.i.e
Apertureefficiency=hap=Ae/Ap(dimensionless).
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Antennamatching:
Whentheantennaisreceivingwithaloadresistancematchedtotheantennaradiationresistance,maximumpoweristransferredtotheloadandthepowerisalsoreradiatedfromthedipole.Thisiscalledantennamatching(Givedetailedexplanation)
3.Brieflyexplaintheradiationfromashortdipole
Defn:Ashortdipoleisoneinwhichthefieldisoscillatingbecauseoftheoscillatingvoltageandcurrent.Itiscalledso,becausethelengthofthedipoleisshortandthecurrentisalmostconstantthroughtouttheentirelengthofthedipole.
FieldsfromOscillatingDipole:Thedipolehastwoequalchargesofoppositesignoscillatingupanddowninaharmonicmotion.Thechargeswillmovetowardseachotherandelectricfiledlineswerecreated.whenthechargesmeetatthemidpoint,thefieldlinescuteachotherandnewfieldarecreated.Thisprocessisspontaneousand
somorefiledarecreatedaroundtheantenna.Thisishowradiationsareobtainedfromashortdipole.(SeeFigurefromJohn.D.KrausBook)
AntennaFieldZones:TheregionscontainingtheradiationsthatarepresentaroundtheantennaarecalledZones.Thefieldsaroundanantennaaybedividedintotwoprincipalregions.
a)Nearfieldzone(Fresnelzone)
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b)Farfieldzone(Fraunhoferzone)
ElectricandMagneticfieldcomponentsofshortDipole:WritethederivationsbyreferringTheBook,K.D.Prasad.
4.Givesnotesontheantennaimpedances.FindtheeffectiveapertureandDirectivityofashortdipoleantenna.SelfImpedance:Defn:Selfimpedanceofanantennaisdefinedasitsinputimpedancewithallotherantennasarecompletelyremovedi.eawayfromit.Writetheformularequired
MutualImpedance:
Defn:Thepresenceofnearbyantennano.2inducesacurrentintheantennano.1indicatesthatpresenceofantennano.2changestheimpedanceoftheantennano.1.Thiseffectiscalledmutualcouplingandresultsinmutualimpedance.
StateReciprocitytheorem
Formularequired
EffectiveapertureandDirectivityofashortdipoleantenna.
Consideraplanewaveincidentonashortdipole.Thewaveisassumedtobelinearlypolarizedwithelectricfieldintheydirection.Thecurrentinthedipoleis
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assumedconstantandinthesamephaseoveritsentirelength,andtheterminatingresistanceisassumedequaltothedipoleradiationresistance.Theeffectiveapertureofthisdipoleisgivenby
$H.
Thedirectivityisfoundtobe
'.$HVT..
5.DefinePolarization?Explainthedifferenttypesofpolarizationindetail.
Polarizationisdefinedastheorientationofelectricfieldasafunctionofdirection.ThepolarizationoftheradiowavecanbedefinedbydirectioninwhichtheelectricvectorEisalignedduringthepassageofatleast
onefullcycle.Alsopolarizationcanalsobedefinedthephysicalorientationoftheradiatedelectromagneticwavesinspace.Thepolarizationareofthreetypes.Theyare:
Ellipticalpolarization
Circularpolarization
Linearpolarization.
LinearPolarisation:
Alinearlypolarizedwaveisoneinwhichthe
electricfieldremainsinonlyonedirection.Foralinearlypolarizedwave,theaxialratioisinfinity.Ellipticalpolarization
Theelectricfieldvectorrotatesandformaellipsecalledpolarizationellipse.Theratioofthemajorto
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theminoraxesofthepolarizationellipseiscalledtheAxialRatio.(AR).ARisgreaterthan1.
Circularpolarization
Theelectricfiledvectorrotatesandformacircleandthiswaveiscalledcircularlypolarizedwave.ARisunity.
6.Explainindetailthedifferentcasesofthearraycontainingtwoisotropicsources
Case1:ArraysoftwoisotropicsourcesfedwithcurrentsofequalamplitudeandinphaseCase2:ArraysoftwoisotropicsourcesfedwithcurrentsofequalamplitudeandoppositephaseCase3:ArraysoftwoisotropicsourcesfedwithcurrentsofunequalamplitudeandanyphaseCase1:Arraysoftwoisotropicsourcesfedwithcurrentsofequalamplitudeandinphasequadrature.
Writeaboutthefollowing:
Fieldpatternoftheindividualcases
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Findthemaxima,minimadirectionandhalfpowerpointdirectionDrawtheradiationpattern.7.Whatisbroadsidearray?Derivethemaxima,nulldirectionsandalsothebeamwidthofabroadsidearray.Broadsidearrayisdefinedasanarrangementinwhichtheprincipaldirectionofradiationisperpendiculartothearrayaxisandalsotheplanecontainingthearrayelement.ForBroadsidearraythephasedifferenceEHWZHHQDGMDFHQWHOHPHQWLV.
FieldpatternofalineararraywithnisotropicsourcesDeterminethemaxima,minimadirectionandhalfpowerpointdirectionDrawtheradiationpattern.8.WhatisEndFirearray?Derivethemaxima,nulldirectionsandalsothebeamwidthofaEndfirearray.
Endfirearrayisdefinedasanarrangementinwhichtheprincipaldirectionofradiationiscoincideswiththearrayaxis.)RUHQG)LUHDUUD\.
:KHUH...$QGGGLVWDQFHEHWZHHQWKHHOHPHQWV.Fieldpatternofalineararraywithnisotropicsources
Determinethemaxima,minimadirectionandhalfpowerpointdirection
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Drawtheradiationpattern.9.Explaintheprincipleofpatternmultiplicationwithsomeexamples.Principleofpatternmultiplication:Thetotalfieldpatternofanarrayofnon
isotropicbutsimilarsourcesistheproductoftheiii)individualsourcepatternandiv)Thearraypatternofisotropicpointsourceseach
locatedatthephasecenteroftheindividualsourcehavingthesameamplitudeandphase.
Whilethetotalphasepatternisthesumofthephasepatternsoftheindividualsourcepatternandarraypattern.0
Situation1:ArrayoftwopointsourcesfedinphasewiththeamplitudeoftheindividualsourcetobeEo=Eo1Sin(theta)Situation2:ArrayoftwopointsourcesfedinphasewiththeamplitudeoftheindividualsourcetobeEo=Eo1Cos(theta)Situation3:Arrayoffourpointsourcesfedin
phasewiththeamplitudeoftheindividualsourcetobeEo=Eo1Sin(theta)
10.Explainthedifferenttechniquesusedfortaperingofarrays
ArrayTapering:
Taperingofarrayisatechniqueusedforreductionofunwantedsidelobes.Theamplitudeofcurrentsinthelineararraysourceisnon-uniform;hencethe
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centralsourceradiatesmoreenergythantheends.Taperingisdonefromcentertoend.
Techniquesusedforarraytapering:
.Array:Taperingthe
BinomialfollowscoefficientofbinomialseriesDolphTchebycheffArray:TaperingfollowsthecoefficientofTchebycheffpolynomial.
1.BinomialArray:Itisanarrayinwhichtheamplitudesoftheantennaelementsinthearrayarearrangedaccordingtothecoefficientsofthebinomialseries.Theneedforabinomialarrayis
i).Inuniformlineararrayasthearraylengthisincreasedtoincreasethedirectivity,thesecondarylobesalsooccurs.
ii)Forcertainapplications,itishighlydesirable
thatsecondarylobesshouldbeeliminatedcompletelyorreducedtominimumdesirablelevelcomparedtomainlobes.Advantage:
Nominorlobes
Disadvantages:
Increasedbeamwidth.thelargeratioofcurrent
Maintainingamplitudeinlargearraysisdifficult
2.DolphTchebycheffArray:.TaperingfollowsthecoefficientofTchebycheffpolynomial.
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11.Derivethefieldsradiatedfromashortelectricdipole.Listthefarfieldcomponents.Determineitsradiationresistanceanddirectivity.
.Fieldsradiatedfromtheshortdipoleandradiationresistance(ReferAntennas&propagationByK.D.Prasad,PageNo.210to227).Directivityis1.5(ReferAntennas&propagationByK.D.Prasad,PageNo.251to252)
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12.Derivetheexpressionsforthefieldsandpowerradiatedfromahalfwavedipoleantenna.Finditsradiationresistanceanddirectivity.
.Fieldsradiatedfromtheshortdipoleandradiationresistance(ReferAntennas&propagationByK.D.Prasad,PageNo.229to234).Directivityis1.5(ReferAntennas&propagationByK.D.Prasad,PageNo.252to253)
13.Derivethefieldradiatedfromasmallloopantenna
.Smallloopradiatedfields(ReferAntennasByJohn.D.Kraus,PageNo.200to208)
14.ExplainindetailaboutthehelicalantennaDefinitionofhelicalantenna.HelicalGeometryRadiatedfieldsofhelicalantenna.TypesofhelixReferAntennasByJohn.D.Kraus
15.Explainthedifferentmodesofoperationofhelicalantenna.Normalmodeofoperation
.Axialmodeofoperation.
16.ExplainGroundwavePropagation.Skywave.
Wavesthatarriveatthereceiverafterreflectionintheionosphereiscalledskywave.Troposphericwave.
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WavesthatarriveatthereceiverafterreflectionfromthetroposphereregioniscalledTroposphericwave.(ie10KmfromEarthsurface).Groundwave.
Wavespropagatedoverotherpathsneartheearthsurfaceiscalledgroundwavepropagation.TypeofGroundwave.
Groundwaveclassifiedintotwotypes.
iv.Spacewavev.Surfacewave.SpaceWave.Itismadeupofdirectwaveandgroundreflectedwave.Alsoincludestheportionofenergyreceivedasaresultofdiffractionaroundtheearthsurfaceandthereflectionfromtheupperatmosphere.SurfaceWave.
WavethatisguidedalongtheearthssurfacelikeanEMwaveisguidedbyatransmissioniscalledsurface
wave.Attenuationofthiswaveisdirectlyaffectedbytheconstantofearthalongwhichittravels.
17.Explaindiversityreception.?
Tominimizethefadingandtoavoidthemultipathinterferencethetechniqueusedarediversityreception.Itisobtainedbytwoways.
1.Spacediversityreception2.Frequencydiversityreception.3.Polarizationdiversity.SpacediversityReception.
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Thismethodexploitsthefactthatsignalsreceivedatdifferentlocationsdonotfadetogether.It
UHTXLUHVDQWHQQDVVSDFHGDWOHDVW.DSDUWDUHSUHIHUUHG.
andtheantennawhichhighsignalstrengthatthemomentdominates.
FrequencydiversityReception.
Thismethodtakesadvantageofthefactthatsignalsofslightlydifferentfrequenciesdonotfadesynchronously.Thisfactisutilizedtominimizefadinginradiotelegraphcircuits.
Polarizationdiversityreception.
Itisusedinnormallyinmicrowavelinks,anditisfoundthatsignaltransmittedoverthesamepathintwopolarizationshaveindependentfadingpatterns.inbroadbanddishantennasystem,Polarizationdiversity
combinedwithfrequencydiversityreceptionachieveexcellentresults.
18.Explainindetailionosphericpropagation.Wavesthatarriveatthereceiverafterthepropagationthroughionosphereisionosphericpropagation..
Theionosphereisthatregionoftheearthsatmosphereinwhichtheconstituentgasesareionizedbyradiationfromtheouterspace.
Theregionis50Kmto400Km.
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EffectiveDielectricandconductivityofanionizedgas.Reflectionandrefractionwavesbytheionosphere.(ReferPageno667to681.ElectromagneticwavesandradiatingSystems.By.C.JORDANandG.BALMAIN)
19.Explaina.Effectoftheearthmagneticfield.b.FaradayrotationinSkywavePropagation.EffectoftheearthmagneticfieldElectronsandionsintheionosphereareinfluencednotonlybythefieldsofapassingelecromagneticwavebutalsobytheearthmagneticfield,whichcausesthechargedparticlestomoveincircularorspiralpaths.(ReferPageno687.ElectromagneticwavesandradiatingSystems.By.C.JORDANandG.BALMAIN)
FaradayrotationinSkywavePropagation
Duetotheearthsmagneticfields,theionoshericmediumbecomesanisotropicandtheincident
planewaveenteringtheionospherewillsplitintoordinaryandextraordinarywaves/modes.
Whenthesemodesre-emergefromtheionospheretheyrecombineintoasingleplanewaveagain.
Finallytheplaneofpolarizationwillusuallyhavechanged,thisphenomenonisknownasFaradaysrotation.(ReferPageno693.ElectromagneticwavesandradiatingSystems.By.C.JORDANandG.BALMAIN)
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20.DerivetheexpressionforPermitivityandconductivityofionizedgas.?Plasma:.Assemblyofchargedparticlesinwhichthetimeaveragechargedensityis
Zero.
PlasmaisformedWhenvertheatomsinagasareionizedtoproduceequalnumberofionsandelectrons.Egearthionosphere.
,
2
r=1-Ne2/(.v..2)2
>1H2P..2)]Note:Conductivityismaximummeansthewavewillpassthroughthatmedium.Conductivityissmall,wavesgetsreflected.(ReferPageno670fn.ElectromagneticwavesandradiatingSystems.By.C.JORDANandG.BALMAIN)
21.ExplaintheconceptofReflectionandrefractionwavesbytheionosphere.
Thereflectionandrefractionofradiowavesbytheionosphereisafunctionoffrequency.
Brieflydescribethefollowing.
i.Reflectionatlowfrequency.ii.Reflectionathighfrequency.iii.Maximumusablefrequency.
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iv.Optimumfrequency.v.Skipdistance.Importantformulaforproblems:
i
Criticalfrequency,fcr=91max
ii.RelativeDielectricconstantr=1-Ne2P.2+v2)iii.Phaseconstant,b=(2p/l)-(fc/f)iv.Waveimpedance..0/-(fc/f)v.Wavevelocity.vp=c/-(fc/f)2viGroupvelocity,
2
vpvg=cvg=c2/vp
vii.Incidentangle.sini=-(fc/f)
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