Antenna Intro

8/3/2019 Antenna Intro

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IntroductionIntroduction

to Antennasto Antennas

Dr. Sandra CruzDr. Sandra Cruz--PolPolElectrical and Computer EngineeringElectrical and Computer Engineering

University of Puerto Rico at MayaguezUniversity of Puerto Rico at Mayaguez


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What is an antenna?What is an antenna?

An antenna is aAn antenna is a passive structurepassive structure thatthatserves as transition between aserves as transition between atransmission linetransmission line andand airair used toused to

transmit and/or receive electromagnetictransmit and/or receive electromagneticwaves.waves.

Source

Tx

ReceiverCircuit

Rx


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AntennaAntenna

Ulaby, 1999


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Types of antennasTypes of antennas

Can be divided intoCan be divided into two groupstwo groups

WireWire antennas:antennas: dipoles, loops, Yagidipoles, loops, Yagi--UdaUda

ApertureAperture antennas:antennas: parabolic, horns, microstrip antennasparabolic, horns, microstrip antennas

http://www.kyes.com/antenna/antennatypes/antennatypes.html

http://en.wikipedia.org/wiki/Antenna_(electronics)#Overview


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Wire antennasWire antennas

YagiLog periodic

Yagi


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Wire antennasWire antennas

Log periodicLog periodic

YagiYagi--Uda with reflectorUda with reflector


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Aperture antennasAperture antennas

Spherical (main reflector)with Gregorian feed

Dipole with

parabolic andcorner reflector


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Reflector and Pyramidal hornReflector and Pyramidal horn

antennasantennas


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OutlineOutline

Antenna parametersAntenna parameters Solid angle,Solid angle, ;;

%%and Radiation intensity,and Radiation intensity, UU

Radiation pattern,Radiation pattern, PPnn,, sidelobessidelobes,, HPBWHPBW Far field zone,Far field zone, rrffff Directivity,Directivity, DD or Gain,or Gain, GG Antenna radiation impedance,Antenna radiation impedance, RRradrad Effective Area,Effective Area, AAee

All of these parameters are expressed in terms of aAll of these parameters are expressed in terms of atransmissiontransmission antenna, but are identicallyantenna, but are identicallyapplicable to aapplicable to a receivingreceiving antenna. Well alsoantenna. Well alsostudy:study:

FriisFriis Transmission EquationTransmission Equation Radar EquationRadar Equation


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Spherical coordinatesSpherical coordinates

z (zenith)

x

y

U

J

J= azimuth

U= elevationU=90

J=0

U=0

U=90

J=90


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Solid AngleSolid Angle

s1 = r dU s2 = rsin Uds = Ur= arco dA = s1 s2

dA = r2

sin U d dU=r2 d

U = ngulo plano d = elemento de ngulo slidoEl arco total en un crculo: El rea total en una esfera:

= 2Tr = 4Tr2

Angulo total: = 2T [radianes] Angulo slido total: =4T [rad2]

=4T [sr]1 steradian (sr) = (1 radian)2


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Radiation IntensityRadiation Intensity

Is theIs the powerdensitypowerdensity persolidanglepersolidangle::

vector.Poyntingasknown

alsodensitypowertheis

][W/mRe 2

r2

rH*}{E

where

rU

rv!

!

P

P [W/sr]


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Total radiated power by antennaTotal radiated power by antenna

Can be calculated as;Can be calculated as;

[W]

[W]

!

;!

dSP

or

dUP

rrad

rad

P


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Radiation PatternRadiation Pattern

Radiation pattern isRadiation pattern isthe 3D plot of thethe 3D plot of thegain, but usually thegain, but usually the2D horizontal and2D horizontal and

verticalvertical cross sectionscross sectionsof the radiationof the radiationpattern arepattern areconsidered.considered.

Refers to the variationRefers to the variationof the relativeof the relativeamplitude of theamplitude of theradiation as a functionradiation as a function

ofofdirectiondirection..

),(

),(

),(

),(),(

maxmax JU

JU

JU

JUJU

U

UFn !!

P

P

Field pattern:

Where U is the radiationintensity to be defined later.

),(

),(),(

max JU

JUJU

E

EEn !

Power pattern:


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TotalTotal Solid AngleSolid Angle of an antennaof an antenna

z

y

x

A

Patrn

|P |n

[sr]),(4

A ;!; dFnT

JU;A

Is as if you changed theradiationpatternbeam of an antenna

into a pencil beamshape and find outwhats the equivalentsolid angle occupied bythis pattern.


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Isotropic antennaIsotropic antenna

Its anIts an hypothetic antennahypothetic antenna,,i.e., it does not exist in reali.e., it does not exist in reallife, yet its used as alife, yet its used as ameasuring bar for realmeasuring bar for real

antenna characteristics.antenna characteristics.

Its a point source thatIts a point source thatoccupies a negligible space.occupies a negligible space.Has no directionalHas no directionalpreference.preference.

Its pattern is simply aIts pattern is simply a spheresphereso it hasso it has ;;AA==;;isotropicisotropic= 4= 4TT

??steradianssteradiansAA..

TJUUT

U

T

J

T

4sin)1(

)1(

0

2

0

4

isotropic

!

;!;

! !

dd

d


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Radiation PatternRadiation Pattern

Whenever weWhenever wespeak ofspeak ofradiationradiation

patterns, wepatterns, wenormally meannormally meanwe are at awe are at adistance fardistance far

enough fromenough fromthe antennathe antennaknown as theknown as thefarfieldfarfield..

_ 1

HPBW

-.25

-.7

|En|

- 0dB

-3dB

-10dB

| | | |HPBW

Patrn de campo o de potencia(Escalalogartmica)

Patrn de Campo(Escalalineal)

COORDENADAS RECTANGULARES

Patrnnormalizado

Note that when plotted indecibels, the power andfield patterns look exactly

the same.


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PatternPattern polarplotpolarplot

Lbulo

principal

.5

1

HPBW

Lbulosmenores

NNBW

("Mainlobe")

|Pn|

}

PATRON TIPICO(Coordenadas polares esfricas, 2 dimensiones)


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Dipole antenna patternDipole antenna pattern

Note the radiation pattern isdonut shaped.


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SidelobesSidelobes

Antennas sometimes showAntennas sometimes show sidelobessidelobes ininthe radiation pattern.the radiation pattern.

Side lobes are peaks in gain other thanSide lobes are peaks in gain other thanthe main lobe (the "beam").the main lobe (the "beam").

Side lobes have bad impact to theSide lobes have bad impact to the

antenna quality whenever the system isantenna quality whenever the system isbeing used to determine thebeing used to determine the directiondirection of aof asignal, for example insignal, for example in RADARRADAR systems.systems.


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Sidelobes of dipole arraysSidelobes of dipole arrays

sidelobe


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Antenna Pattern with sidelobesAntenna Pattern with sidelobes

Many applications require sidelobe levels

(SLL) to be below -20dB.


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Gain orDirectivityGain orDirectivity

An isotropic antenna and a practical antennafed with the same power. Their patterns

would compare as in the figure on the right.


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Directivity and GainDirectivity and Gain

All practical antennas radiate more than theAll practical antennas radiate more than theisotropic antenna in some directions and less inisotropic antenna in some directions and less inothers.others.

Gain is inherently directional; the gain of anGain is inherently directional; the gain of anantenna isantenna is usuallymeasuredinthedirectionusuallymeasuredinthedirectionwhichitradiates bestwhichitradiates best..

aveave /UUDD maxmaxmax /),( !!! PPJU

If lossless antenna, G=D


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Gain orDirectivityGain orDirectivity

Gain is measured by comparing anGain is measured by comparing anantenna to a model antenna,antenna to a model antenna,typically thetypically the isotropic antennaisotropic antenna whichwhichradiates equally in all directions.radiates equally in all directions.

rad

AVEP

r

dAA

D

),(4

1/),(

2 JUTJU

JU

P

P

),P(

PP !!!

//44

AisotropicAmax

;;!;!! TT

rad

o

P

UD


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DirectivityDirectivity

For an antenna with a single main lobeFor an antenna with a single main lobepointing in the zpointing in the z--direction ,direction , ;;AA can becan beapproximated to the product of the HPBWapproximated to the product of the HPBW

yzxz

yzxzA

D

then

FF

TT

FF

4/4 A $;!

$;

The Directivity:


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Far fieldFar field

The distance at which the fieldsThe distance at which the fieldstransmitted by an antenna (spherical)transmitted by an antenna (spherical)can be approximated to plane waves.can be approximated to plane waves.

Its defined asIts defined as

/2 2Drff !D = is the largest physical dimension of theantenna= wavelength of operation

rff= distance from the antenna to the observation

point


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Beamwidth,Beamwidth,HPBWHPBW

Is the distance in radians o degreesIs the distance in radians o degreesbetween the direction of thebetween the direction of theradiation pattern where the radiatedradiation pattern where the radiated

power is half of the maximum.power is half of the maximum. Can be found by solvingCan be found by solving FFnn((UJUJ)=.)=.55

DHPBM

dB-.

dB-.

o70

shape;beam"pencil"for

37070log20

350log10

}

!

!


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Antenna ImpedanceAntenna Impedance An antenna is seen" by the generator as a load withAn antenna is seen" by the generator as a load with

impedanceimpedance ZZAA,, connected to the line.connected to the line.

TherealpartistheradiationresistanceplustheTherealpartistheradiationresistanceplustheohmicresistanceohmicresistance.. Minimizing impedance differencesMinimizing impedance differences at each interface willat each interface will

reduce SWRreduce SWR andand maximize power transfermaximize power transfer through each partthrough each partof the antenna system.of the antenna system.

ComplexComplex impedance,impedance, ZZAA,, of an antenna is related to theof an antenna is related to the

electrical length of the antenna at the wavelength in use.electrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed lineThe impedance of an antenna can be matched to the feed line

and radio by adjusting the impedance of the feed line, using theand radio by adjusting the impedance of the feed line, using thefeed line as an impedancefeed line as an impedance transformertransformer..

More commonly, the impedance is adjusted at the load (seeMore commonly, the impedance is adjusted at the load (seebelow) with anbelow) with an antenna tunerantenna tuner, a, a balunbalun, a matching transformer,, a matching transformer,matching networks composed ofmatching networks composed ofinductorsinductors andand capacitorscapacitors, or, ormatching sections such as the gamma match.matching sections such as the gamma match.

ALradA jXRRZ !ZA


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Antenna efficiency,Antenna efficiency,LL

EfficiencyEfficiency is the ratiois the ratioof power put into theof power put into theantenna terminals toantenna terminals tothe power actuallythe power actually

radiatedradiated Radiation in anRadiation in an

antenna is caused byantenna is caused byradiation resistanceradiation resistancewhich can only bewhich can only be

measured as part ofmeasured as part oftotaltotal resistanceresistanceincluding lossincluding lossresistance.resistance.

inrad P P!

DL!G


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Radiation ResistanceRadiation Resistance

The antenna is connected to a T.L., andThe antenna is connected to a T.L., andit sees it as an impedance.it sees it as an impedance.

The power radiated isThe power radiated is

The loss power isThe loss power is

lossradrad

lossradrad

RR

R

PP

P

!!L

radorad RIP 221!

Loloss RIP2

2

1!


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Radar equationRadar equation What is a radar?What is a radar?

Received power by a radar isReceived power by a radar is

WhereWhere WW is the backscattering coefficient of theis the backscattering coefficient of thetarget [mtarget [m22]]

XW

T

P 243

22

4

! e

R

GPP ootr


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APPLICATIONSAPPLICATIONS

Application to several researchApplication to several researchprojects: CASA, NASAprojects: CASA, NASA--FAR,FAR,

NASANASA--TCESSTCESS Show results from undergradsShow results from undergrads

working in NASA and NSFworking in NASA and NSF

projectsprojects Relation to Grad studentsRelation to Grad students


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Antenna polarizationAntenna polarization

TheThe polarizationpolarization of an antenna is theof an antenna is thepolarization of the signals it emits.polarization of the signals it emits. The ionosphere changes the polarization of signalsThe ionosphere changes the polarization of signals

unpredictably, so for signals which will beunpredictably, so for signals which will bereflected by the ionosphere, polarization is notreflected by the ionosphere, polarization is notcrucial.crucial.

However, for lineHowever, for line--ofof--sight communications, it cansight communications, it canmake a tremendous difference in signal quality tomake a tremendous difference in signal quality to

have the transmitter and receiver using the samehave the transmitter and receiver using the samepolarization.polarization. Polarizations commonly considered arePolarizations commonly considered are verticalvertical,,

horizontalhorizontal, and, and circularcircular..


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Antenna BandwidthAntenna Bandwidth

TheThe bandwidthbandwidth of an antenna is the range ofof an antenna is the range offrequencies over which it is effective,frequencies over which it is effective,usually centered around the operating orusually centered around the operating or

resonant frequency.resonant frequency.

The bandwidth of an antenna may be increasedThe bandwidth of an antenna may be increasedby several techniques, including using thickerby several techniques, including using thickerwires, replacing wires withwires, replacing wires with cagescages to simulate ato simulate a

thicker wire, tapering antenna components (like inthicker wire, tapering antenna components (like inaa feed hornfeed horn), and combining multiple antennas), and combining multiple antennasinto a single assembly and allowing the naturalinto a single assembly and allowing the naturalimpedance to select the correct antenna.impedance to select the correct antenna.


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Effective AreaEffective Area

How a Rx antenna extracts energyHow a Rx antenna extracts energyfrom incident wave and delivers it tofrom incident wave and delivers it toa load?a load?

Above is valid for any antenna underAbove is valid for any antenna undermatchedmatched--load conditionsload conditions

T

P

4

2DP

Ainc

rece !!

P


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Friis Transmission Eq.Friis Transmission Eq.

In any communication link, thereIn any communication link, thereis a transmitting antenna and ais a transmitting antenna and areceiver with a receiver antenna.receiver with a receiver antenna.

2

t

R4

P

T

!isotr

P

22

tt

2

tt

R

PA

R4

PG

PT!!! isotrttx PGP

22 R

PAA

APtrt

trrec !!P

2

trt

R4

PGG 2

T

P!recP

TX

RX


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ExampleExample

Radar and FriisRadar and Friis


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Antenna ArraysAntenna Arrays

Uses many antennas synchronizedUses many antennas synchronizedwith each other to increasewith each other to increase

Pattern multiplicationPattern multiplication


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ExampleExample

Determine the direction of maximumDetermine the direction of maximumradiation , pattern solid angle,radiation , pattern solid angle,directivity and HPBW in thedirectivity and HPBW in the yy--zz planeplane

for an antenna with normalizedfor an antenna with normalizedradiation intensity given byradiation intensity given by

elsewhere0

20and

2

0forcos

),(

2

eeee!

TJT

UU

JUF


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Antenna Intro