ec1352 antenna wave propagation(1)

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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ec1352 antenna wave propagation(1)

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