Basic Antennas: Understanding Practical Antennas and Design

183 Pages·2009·38.43 MB·English

by  Joel R. Hallas

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Chapter I IntroductiontoAntennas
Chapter 2 The Half-Wave Dipole Antenna in Free Space
Chapter 3 The Field From a Dipole Near the Earth
Chapter 4 TheImpedance of anAntenna
Chapter 5 TransmissionLines
Chapter 6 Making Real Dipole Antennas
Chapter7 The Field From Two Horizontal Dipoles
Chapter 8 The Field From Two Vertical Dipoles
Chapter 9 TransmissionLines asTransformers
Chapter 10 PracticalTwo Element AntennaArrays
Chapter 11 Wideband Dipole Antennas
Chapter 12 Multiband Dipole Antennas
Chapter 13 Vertical Monopole Antennas
Chapter 14 Arrays ofVertical Monopole Antennas
Chapter 15 Practical Multielement Driven Arrays
Chapter 16 Surface ReflectorAntennas
Chapter 17 Surface Reflector Antennas You Can Build
Chapter 18 Antenna Arrays With Parasitically Coupled Elements
Chapter 19 The Yagi-Uda or Yagi,Parasitically Coupled Antenna
Chapter 20 Practical Yagis for HI' and VHF
Chapter 21 Log Periodic Dipole Arrays
Chapter 22 LoopAntennas
Chapter 23 Loop Antennas YouCan Build
Chapter 24 Antennasfor Microwave Applications
Chapter 25 Vehicle Antennas
Chapter 26 AntennaMeasurements
Appendix A Getting Started in Antenna Modeling with E7flEC
Appendix B UsingDecibels in Antenna Calculations
Index
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Introduction to Antennas
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This Impressivecollection01'antennasIs part
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Contents
SoWhatisanAntenna? 1-2
ConceptsYou'll Need to be FamiliarWrth 1-5
So Whot is 1\nten
011 110 :
lorsomething sosimpletocheck
out,andoften so simple tomake, an larger
E~ctricalSignal
antennaisremarkablydifficultfor
manypeopleto understand.That's
unfortunate,because for many radio
systemsthe antennaisone ofthe
mostimportantelements, one that can
make the differencebetweenasuc
cessfulandanunsuccessfulsystem.
rransdu"",
InpoJl
How CanYou Picture (Mkrophoo..)
Anlennes?
Ag 1-1-Illustration 01'transducerschanging oneform ofenergy
Perhaps an analogyfrom The Intoanother.
ARRLAnrennaBookwillhelp.You
Me familiar withsoundsystems.
Whetherrepresentedbyyourhome
stereo orby an airportpublicad
dress, soundsystemshave onething
incommon.Thesystem'slaststopon
itswaytoyourearsisatransducer,
adevicethat transformsenergy from
oneform toanother,inthiscase
- a loudspeaker.The loudspeaker
transformsanelectricalsignalthat
theamplifierdeliversintoenergyin
anacousticwavethat canpropagate
throughthe airtoyour ears. Modulator DemodUlator
A radiotransmitteracts litesame
way,exceptthat itsamplifierpro
ducesenergy atahigher frequency
than the sound you can hear,andthe
transducerisanantenna that trans
forms the high-frequency electrical
energy intoanelectromagneticwave.
This wave canpropagate throughair
(or space) for long distances.
Forsomereason,perhapsbecause
ofourfamiliarity with audiosystems Fig1-2- At(A),simpliffedsystem diagramofasonarsystem.At(8),
or becauseyoucan actually hear the almplltled systemdlltQrarnofaredar,
results inyourears.it seemseasierto
grasptheconceptofthe generation electromagneticwaves andtrans parabolic eavesdropping microphone,
and propagationofacousticwaves formsthem intoelectricalsignals orthat sonarand radaroperateinthe
thanitis tounderstand thegeneration that canbeprocessed inareceiver. same fashion.Sonarreliesonacuus
andtransmissionofradio waves. Fig 1-1showsa system orblockdia tic wavespropagatingthrough water
The audio transmitteranalogy can gramofasound system with trans to findand reflect backfromunder
becontinuedin thereceivingdirec ducers(loudspeakerarulmicrophone) waterobjects,suchassubmarinesor
tion.A microphone isjust another ateach end. schoolsoffish. Similarly, radarsends
transducer that transforms acoustic Many ofthe phenomenathat act outelectromagnetic wavesthrough
wavescontainingspeech ormusic upon acoustic wave",alsn occur with space,listening for signalsreflected
into weak electrical signalsthat can electromagnetic waves. Itis notan hack from objectssuch asaircraft.
he amplified and processed.Similarly, accidentthat 1Iparabolic reflector space vehiclesor weatherfronts.
areceiving antennacaptures weak radarantennalooks verymuchlike a Fi~ 1-2shows simplifiedsystem
1·2 Chapter1
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,.,
Y'h"e"'"Curren!eon.ngo..t01PIl9ll
~F.-cIWnIpoiCouI"Mr-Ciod<wse
1--- -'
,- .'
Ag '·3- AwireImmersedInthe magneUcfleld ofapermanentmagnet.
AtA.Ifeverything is staticthere ls nocurrenlflowInthewire.At B,if C...,.eolFlow
you move thewireup and downwnhlnthe-f~d,thewireexperiencesa inCiredlOro
ofTt\umb
changing magneticfield and currentIsInducedtoflow.
diagramsofradarandsonarsystems. your hair stand up.That lam until
Whilewe're mentally picturing youdischargetheaccumulationby
antennas.perhapsanevenbetter touching3 groundedobject,such as a
analogythanacousticwavesisto screwon a light switchplate,oreven
compareelectromagneticwavesof Ibclong-sufferingdog''Snose.Then
lighttothoseofradio signals.Light thecharge isdissjpated sometimes
wavespropagatethroughspaceus quite dramatically-withabigspark.
me
iog samemechanismsandatthe Similarly,astatic magnetic field
samespeedasradiowa...es.Similar exists around apermanentmagnet.
parabolic shapescanreflect and focus Youcan observethe effectofmagne
both lightandradiowaves.Light tismonacompas..<; needleore..-enon
I.}
needsapolishedmirrorasareflec a screwdriver, "RQI'll-HandR"·
tor.suchasthemirroredrefteciorin Staticelectricand magnetic fields c;l'lICtioroofw..lilroelicnekl
AblgCII"'ltClfi'lliJ8fS
yourflashlightorcarheadlight Iwill donOI resultinelecrromagreric
sometimes drawon your appreciation waves.It is only whenamagneticor Fig1-4- Directionofmagnetic
oflightreflectionasIdiscusssome demit fieldis changing that youcan flefd around.wire.
typesofantennas. haveelectromagnetic radiation, You
are familiar.when you change the
canvisualizethe effectbyconsid
What's an Electromagnetic ering some other things thatmake currentin thetransformer winding,
Wave? uscofelectric and magnetic fields. youcanmovebetweenelectricand
Anelectromagneticwave,asthe Fig 1-3 showsa wire immersed inthe magneticfields.
nameimplies, consistsofacombina magneticfieldofapermanentmag Direction of Fields
tion ofthe properties ofbothelectric net.Ifeverythingisstatic (Fig 1-3A)
and magnetic fields. there is nocurrent flow in(hewire.If Both magneticand electric fields
you changethe magnetic fieldatthe actin particulardirections. A magnet
Static Electric and Magnetic wire bymovingthewire uplinddown icfieldactsaround ihe conductorcar
Fields withinthe field (Fig 1-38),thechang ryingthecurrent.Theconvention for
ing magneticfiejdcausesa currentto itsdirection isthatifpositivecurrent
You are nodoubt familiar with
flow,Thisishowanelectricpower is flowingina particulardirection.
magnetism and electricityfrom
generatortransformsmechanical the magnetic field willgoaround
everydayexperience.One kindof
energy intoelectrical energy. the wire.assbownin Fig I~A.This
magnetism andelectricityisthesuuic
You couldcause thesameeffectby can berememberedbycallingon the
form.Static electricity isacollection
keepingthewire inone positionand right-Iwfldnil" Thisrulesaysthat
ofpositiveornegativechargesthat
changingthe magnetic fieldaround ifyouhold acurrentcarrying wire
are atrestOilabodyuntildischarged
it, You coulddothisbymovingthe in yourright hand, ....ith thecurrent
byacurrentflow,Thishappens to
magnet. but moreinterestingly you goinginthedirection ofyourthumb.
yourbodywhen ),OUwalkacrossa couldreplace the permanentmagnet the magnetic field willbeiothe direc
rug00 adryday.Inclimateswhere with anelectromagnetandchangethe tion ofyourcurled fingers,as shown
the humidity is very low,particularly
currentin it.lhisisjust atrans inFig1-4B.
during the winter, yourbodycan ac
former,somethingwith whichyou Electric fields actbetweenareas
cumulateacharge,perhapsmaking
Introduction toAntennas 1-3
withcharges ofdifferent sense.That althougharguably itcouldbeused to ........,
is.anareawithexcesselectrons measure the drynessofthe carpet.or -~ R.,.,nOelcient.i"nE.l.ec.l,r1ln:!Iis
iwshsialeidatnoahraevaehaavnienggataivdeecfihcaiergnecy, of bpuetrhbaoptshtahreelaocbaittioofnaosftyreotucrh.fiTngheer.US .-4• • ••••+ +++.~
electronshas anetpositivecharge. FederalCommunicationsCommis
Theelectric field goes from the posi sion(R.'C)classifiesthis kindofra o..--
clihvaerlygecdhaarregae,daasreshaotwonthiennFeiggat1i-v5e.ly dgieanteiorantoasroanfaunniunnuinntdetnddeedmemmeirtt.eTrhise """"RoO ~-_-:_- --- ----:1/
/ -
required not: tointerfere withlicensed ••
Changing Fialds in Space usersoftbe radiospectrum. ~...oMtIl:>::-f~
""9'"'"'""ItCt-g1'lll
Thekeytoelectromagnetic waves Electromagnetic wavesof this son
isto understandthat achanging CUf are generated every lime you tum on Fig 1005 - Direction ofelectric
rentinspacewill causea changing alightswitchorwhen the span: plugs field between two charged
electric field.Thechangingelectric in yourcar fire as the engine turns. In areas.
field willin tuminduce achanging thesecases,ratherthanbyingto make
magnetic field inspace and the lWO antennas,theresponsibleuserattempts
will propagateoutwardfromthe toavoidthepropagationofelectromag or giveitup to areceiver?
source,continuingtothe end ofthe netic waves- byshieldingtheengine • Howdoyoucause the radiation
universe. compartmentorbykeepinghishandsin from atransmitantennato goin the
Thissurprising factwas predicted hispocketas he walkson dry TU!JS. directionyOllwant.orequivalently,
howdoyou getareceiveantenna
bya number offorward-looking
SoWhat ifYou DoWant to toaccept radiation(only) from a
scientists. notablyJamesClerke
Maxwell in hisTreatise0" f1utriciry Send OutaWave? particulardirection?
andMagnLtism,pabtisbedin 1873. lfyoureallywanttotransmita Therearemanyperipheralbu(per.
written withoetthebenefitofMax radio signal,youlikelywould like hapsequally important issues, such
wenactuallybeingabletoexperience tosenditonaparticularfrequency" as."Howdo)'OUkeepthe antenna
the phenomenon.Maxwell'sfamous Note thatthisisdifferentthanthe structurefromfallingdown'! Inthis
equationsdefined therelariomhip" waveresultingfrom asperk, as used book, I will focuson theelectrical
between currents and fields inacon in \'ef)'early transmitters.Sparks.or iSMJe5andleavethe structural issues
cise way that remainsthebasis for all otherfastchangingpulses.contain to mechanicalandcivilengineers,
.....orkinthefield. frequencycomponents acrossawide whohave the:toolstodeal withsuch
Inthe followingdecade, Hein- radiospectrum. matters.
rich Hertz,aprofessorofphysicsat A modemtransmitterstartswithan
SoWhy Don't AllWiresWith
Germany's KarlsruhePolytechnic electrical currentmatis changing at
University,usedanelectricspark to a particularfrequency,Causing that Changing CUlTent Radiate?
generate electromagnetic waves.He currentto flowthrough aconductor Well,in fact. they alldo.Inmost
was ableto measure their.....avclcngtb willresultin anelectromagnetic wave cases. however,circuits have arerum
andvelocity,reinforcingMaxwell's thatvariesatthat frequency"The wiretocomplete thecircuit dose to
theoretical work. frequency willbemaintained asthe the first wire,Since both these wires
wave propagates from the conductor. carry equalcurrent,they will both
How DoYou Make an Ifthesignal isstrongenoughas it radiate equally,butinopposite phase.
ElectromagneticWave? reachesareceiving antenna, are Thenet radiationisthusthe sumof
Asthe previousd iscussionde ceiver tuned(0that frequencywill be the radiation from thetwo wires.But
scribes.it isconceptually easyto able (0 receivethe signaland decode since:they an: equaland opposite. the
generateanelectromagneticwave. anyinformation that wasembedded netfield..arealmost 7.eJO- except.in
Allyouneed todoistocausea in the signal- Youhave radio! the region between(hem.Thusakey
changing current ina conductorand ThisSounds Simple requirementfOf aconductortobe an
awave will propagateoutward to the efficientradiatoris thatitshould not
What's the Big Deal?
end oftheuniverse.Thusifyouwere haveareturn pathincloseproximity.
to listencarefullytoaradio when As with manythings,aconcep In thiscontext.dlJS~meansasmall
youdischargeyourfinger againsttht' tually simple conceptonlygets fraction ofII wavelength atthe fn:
wallswitchplate,you would havea complicatedwhen you try to makeit queocyofthecurrent.A wavelength
suddenincrease in currentandarela happenin areally usefulway.There i:oothe distancea "avetravels.during
tively short-durationelectromagnetic are reallyonly two basicpractical the limeofonecycle.
wavepropagatingfromyOW"finger. issues that arise when dealingwith
Theresultingwave would not be antennas:
verystrong,nor would itbe very • Howdoyou get the antenna to ac
useful.Itcarries little information, ceptthe pmlo'er fromthe transmitter.
1~4 Chapter1
Some Concepts 'II Need to be Familiar
YOll
With
Thisbook isintendedto provide Thusa signal.....-itha frequency of cycle. Inthe above example.dur-
abasicunderstandingofantennas,It 3,600,000Hz isthe same as oneof ing the0.278 useethat thesignal
wouldbeniceifyoucouldjuststart 3600 kHz or 3.6MHz. takes to completeacycle. itwill
here.butyou doneed tohavejusta travel OJ)OO()()()278seconds, x
bitofbackgroundin some concepts Period 300,())(),(X)()meterspersecond or
fromelectronicsandradiotomake it Period isdefinedas the time it 83.3 meters. That isthe wavelengthof
allcometogether.Thesewill be in takes for a wavcfonn tomakea asignal at3.6 MH7" Jwill generally
troducedfrom time totime. with the completecycle.Thebasic units are follow industry practiceandusethe
hopethatiftheyarenotfamiliar,you seconds,Ifa waveformmalesF Greekletterlambda(i..)[I)indicatea
willgobacktosomeofyourearlier cycles inone second.the lime it takes wavelength.Thus, ifsomethingwere
referencebookstoreviewthetopic. forasingle cycle isjust llF.Thus, in two wavelengthslong. you wouldsay
Tostartwith, youshouldclearly theexampleabove, a3.6MHL signal 2;1... Forhalfawavelength.you would
understandthefollowing terms. wouldhave aperiodof1/3.600,000, say iv'2.
which is0,o00ooo278 seconds.I A signalcanbedescribedequally
Frequency willuse themultiplierofmicrosec well by eitheritsfrequency orits
Thisisthe rate at whichan alter OIIdstodescribeevents inmillionths wavelength.It may beofinterestto
natingcurrentwaveformorsignal ofseconds, sothe periodwould be note that before the then-new FCC
goes tbmugha fullcycle.from zero described as0.278microseconds. startedassigningthe spectrumby
voltage tomaximum,back to zero. to often writtenas0.278usee,where the frequency around 1930.the useof
voltage minimumand then back to Greek letter Jlisshort for"micro." wavelengthwas morecommon.
zero.Thebasic unit ishertz(Hz). A Manyradios ofthe periodhadtheir
signalwith a frequencyof1Hzcom Wavelength dials labeledinboth frequency and
pletesone cycle every second.You Radiosignals travel atafinite wavelength.since it wasn't clear
will generallyhe dealingwith signals speed.In free space, OJ inthe Earth's whichleonwouldbecomethe more
athigher frequencies and the number atmosphere, they travel atthe same commonsystemofsignaldefini-
(IfHz willgenerally beprecededwith speed aslight. Light travelsat tion.Commonusage now istodefine
a modifierindicatingthat the value is about 186,000 milesper second, or particularsignals byfrequency, while
some multiple ofHz.The usual ones 300,000,000metersper second in usingtheapproximate wavelength
you willsec are: toindicateabandoffrequencies as
soece.I will generallywork inunits
signedtoa particularservice.Thusan
• kilohertz (kHz), thousands ofHi: ofmetersper second toconformto
83.3-metersignalisone ofanumber
• Megahertz(MHz).millionsofHz, the practiceofmostpeople working
offrequencies inthe "SOmeter"
or thousandsofkHz inthe radio field.
• Gigahertz(GHz),billionsofHz.or Wavelengthisdefinedas the amateurband.
thousands ofMHz. distance awave travels duringone
IntroductiontoAntennas 1-5
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The Half- Wave Dipole Antenna in
Free Space
R8dl8UonpatternofadipoleInfree space.
Contents
The HaW·Wave Dipole 2-2
So How Doesa Dipole Work? 2-3
Where Does the Power Go? 2-4
Antenna AnalysisTools 2-5
Polar Plots 2-5
WhichW~ is Up? 2-6
ReviewQuestions 2·6
The Ha(f- Wave Dipole
It'stemptingto startoutlooking at
L1 L2
atinyradiating elementconsistingof
ashortconductorwithan alternating
currentflowing in it.Then youcould Half-wa"'l1Dlpc'e;L1+LZ=~IZ
consider all otherantenna structures
ascollections ofsuch mini-antennas. Fig 2-1-Heff-weve012)dipole.
eachpiecewith a slightly different lo
cation, currentmagnitudeandcurrent
phase. The radiating electromagnetic timeofone cycleofthe sine wave multipliedbythetime it takesto
field results fromthe summation of alternating current[ac) signalfrom a makeacompletecycle. The time for
allthefieldsradiated by all themini generatoratthe operating frequency onecycle is the waveform'speriodor
antennas.Infact,that'sthe way many fo'I willdiscuss the antenna first asa lifo.Thusa wavelength infree space
antenna-analysissoftware programs transmittingantennaandlater discuss is given bythe following formula:
analyzethe performanceofcompli receiving applications.Thillantenna t..==effa
catedantennas. isa goodplace to start because: Herecisthespeed of lightand
However, Iwon', start there • The 'iJ2 dipole isfrequentlyused fois the frequency ofthe signal that
because it'shardto imaginejusthow generatedthe wave.There are many
asan antenna in many real applica
youCHnmakethosemini-antennas unitsthatcan beused;however, itis
tions.
work withoutconsideringall (he importantthat they be consistent. liar
• Manyother antennasarc made up
interconnecting wires,and these example,if cis in metersper second,
ofcombinationsofj.J2dipoles in
interconnecting wires may well be fuis in Hertz(cyclespersecond), then
variousconfigurations.
more significantthan the antennas Awill bein meters.For example.for
• The IJ2 dipole isareference stan
themselves.Jwill thusstartwith a frequencyof 10MHz,you have
dardforcomparison oftheperfor
anexamination ofanantennathat }, '"300,000,000 metersper second
mance ofotherantennas.
will actually work and isfrequently + 10,000.000 Hertz==30 meters.
encountered- thehalf wave(i,J2) The layoutofa»2dipole isshown Note,for radio work,itisccnve-
dipole.I inFig2-1. The circlein the center nientto use avalueforcof300mil
f"
AIJ2dipoleantenna isformed representsageneratorthatgenerates lion metersper second,and in
by having aconductorthat iselectri asignalattheoperatingfrequency MHz, whichneatlycancelsoutthe
cally haIfa wavelength longatthe f~. The total lengthofthe conductor sixzerosinnumerator and denomma
frequency that will betransmitted fromend-to-endis approximately tor.
orreceived. Here,Arepresentsthe in. ).'"300million meterspcT second
distance the electricalsignalwould The distanceofonewavelength in + IO:MHz==30 meters.
travel down the wireduringthe free spa<.:e isjustthe speedoflight
2~2 Chapter 2