Antenna Intro Extend

8/7/2019 Antenna Intro Extend

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Introduction to

Antennaprinciples

Dr. San dra Cruz-Pol

INEL 5305Electri cal and Computer En gi n eeri n g

Uni versi ty of Puert o Ri co at Mayaguez

What is an antenna?

An antenna is a passive structure thatserves as transition between atransmission line and air used totransmit and/or receive electromagneticwaves.

Source

Tx

ReceiverCircuit

Rx

Antenna

Ulaby, 1999

Types of antennas

Can be divided into two groups

Wire antennas: dipoles, loops, Yagi-Uda

Aperture antennas: parabolic, horns, microstrip antennas

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

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

Wire antennas

YagiLog periodic

Yagi

Wire antennas

Log periodic

Yagi-Uda with reflector


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Aperture antennas

Spherical (main reflector)with Gregorian feed

Dipole withparabolic andcorner reflector

Reflector and Pyramidal hornantennas

Related parameters

Solid angle, and Radiation intensity, U Radiation pattern, Pn, sidelobes, HPBW Far field zone, rff Directivity,D or Gain, G Antenna radiation impedance, Rrad Effective Area, Ae

All of these parameters are expressed in terms of atransmission antenna, but are identicallyapplicable to a receiving antenna. Well alsostudy:

Friis Transmission Equation Radar Equation

Spherical coordinates

z (zenith)

x

y

= azimuth

= elevation=90=0

=0

=90=90

Solid Angle

s1 = r d s2 = rsin ds = r= arco dA = s1 s2

dA = r2 sin d d= r2 d

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

= 2r = 4r2Angulo total: = 2 [radianes] Angulo slido total: =4 [rad2]

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

Radiation Intensity

Is the power density per solid angle:

vector.Poyntingasknown

alsodensitypowertheis

][W/mRe2

r

2

rH*}{E

where

rU

r =

=

P

P [W/sr]


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Total radiated power by antenna

Can be calculated as;

[W]

[W]

=

=

dSP

or

dUP

rrad

rad

P

Radiation Pattern

Radiation pattern isthe 3D plot of thegain, but usually thetwo dimensional

horizontal andvertical crosssections of theradiation pattern areconsidered.

Refers to thevariation of therelative amplitude ofthe radiation as afunction of direction.

),(

),(

),(

),(),(

maxmax

U

UFn ==

P

P

Field pattern:

Where U is the radiationintensity to be defined later.

),(

),(),(

max

E

EEn =

Power pattern:

Total Solid Angle of an antenna

z

y

x

A

Patrn|P |n

[sr]),(4

A = dFn

A

Is as if you changed theradiation patternbeam of an antennainto a pencil beamshape and find outwhats the equivalent

solid angle occupied bythis pattern.

Isotropic antenna

Its an hypothetic antenna,i.e., it does not exist in reallife, yet its used as ameasuring bar for realantenna characteristics.

Its a point source thatoccupies a negligible space.Has no directionalpreference.

Its pattern is simply a sphereso it has A= isotropic= 4[steradians].

4sin)1(

)1(

0

2

0

4

isotropic

=

=

= =

dd

d

Radiation Pattern

Whenever wespeak of

radiationpatterns, wenormally meanwe are at adistance farenough fromthe antennaknown as thefar field.

_ 1

HPBW

-.25

-.7

|En|

- 0 dB

-3dB

-10dB

| | | |HPBW

Patrn de campo o de potencia(Escala logartmica)

Patrn de Campo(Escala lineal)

COORDENADAS RECTANGULARES

Patrnnormalizado

Note that when plotted indecibels, the power andfield patterns look exactlythe same.

Pattern polar plot

Lbuloprincipal

.5

1

HPBW

Lbulos menores

NNBW

("Mainlobe")

|Pn|

}

PATRON TIPICO(Coordenadas polares esfricas, 2 dimensiones)


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Dipole antenna pattern

Note the radiation pattern isdonut shaped.

Sidelobes

Antennas sometimes show side lobes inthe radiation pattern.

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

Side lobes have bad impact to theantenna quality whenever the system isbeing used to determine the direction of asignal, for example in RADAR systems.

Sidelobes of dipole arrays

sidelobe

Antenna Pattern with sidelobes

Many applications require sidelobe levels(SLL) to be below -20dB.

Gain or Directivity

An isotropic antenna and a practical antennafed with the same power. Their patterswould compare as in the figure on the right.

Directivity and Gain

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

Gain is inherently directional; the gain of anantenna is usually measured in the directionwhich it radiates best.

aveave /UUDD maxmaxmax /),( === PP

If lossless antenna, G=D


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Gain or Directivity

Gain is measured by comparing anantenna to a model antenna,

typically the isotropic antenna whichradiates equally in all directions.

( ) pn dF

rD

=

=

4

,

4),(

4

2

/ Aisotropic =oD

Directivity

For an antenna with a single main lobepointing in the z-direction , A can beapproximated to the product of the HPBW

e

yzxz

yzxzA

AD

then

2A

44/4

==

The Directivity:

Far field

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

Its defined as

/2 2Drff =D = is the largest physical dimension of theantenna = wavelength of operationrff = distance from the antenna to the observationpoint

Beamwidth, HPBW

Is the distance in radians o degreesbetween the direction of theradiation pattern where the radiatedpower is half of the maximum.

DHPBM

dB-.

dB-.

o 70

shape;beam"pencil"for

37070log20

350log10

=

=

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

impedance ZA, connected to the line.

The real part is the radiation resistance plus theohmic resistance. Minimizing impedance differences at each interface will

reduce SWR and maximize power transfer through each partof the antenna system.

Complex impedance, ZA, of an antenna is related to theelectrical length of the antenna at the wavelength in use. The impedance of an antenna can be matched to the feed line

and radio by adjusting the impedance of the feed line, using thefeed line as an impedance transformer.

More commonly, the impedance is adjusted at the load (seebelow) with an antenna tuner, a balun, a matching transformer,matching networks composed of inductors and capacitors, ormatching sections such as the gamma match.

( ) ALradA jXRRZ ++=ZA

Antenna efficiency,

Efficiency is the ratioof power put into theantenna terminals to

the power actuallyradiated

Radiation in anantenna is caused byradiation resistancewhich can only bemeasured as part oftotal resistanceincluding lossresistance.

inrad PP =

DG =


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Radiation Resistance

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

The power radiated is

The loss power is

lossrad

rad

lossrad

rad

R

R

PP

P

R++==

rad

2

orad RI2

1P =

L

2

oloss RI2

1P =

Radar equation What is a radar?

Received power by a radar from a singletarget is

Where is the backscattering coefficient ofthe target [m2]

( )

243

22

4

= eR

GPP ootr

Antenna polarization

The polarization of an antenna is thepolarization of the signals it emits. The ionosphere changes the polarization of

signals unpredictably, so for signals whichwill be reflected by the ionosphere,polarization is not crucial.

However, for line-of-sight communications, itcan make a tremendous difference in signalquality to have the transmitter and receiverusing the same polarization.

Polarizations commonly considered are

vertical, horizontal, and circular.

Antenna Bandwidth

The bandwidth of an antenna is the range offrequencies over which it is effective, usuallycentered around the operating or resonantfrequency.

The bandwidth of an antenna may be increased byseveral techniques, including using thicker wires,replacing wires with cages to simulate a thicker wire,tapering antenna components (like in a feed horn), andcombining multiple antennas into a single assembly andallowing the natural impedance to select the correctantenna.

Effective Area

How a Rx antenna extracts energyfrom incident wave and delivers it to

a load?

Above is valid for any antenna undermatched-load conditions

4

P 2

inc

rec DAe ==P

Example

Determine the direction of maximumradiation , pattern solid angle,

directivity and HPBW in the y-z planefor an antenna with normalizedradiation intensity given by

elsewhere0

20and2

0forcos

),(

2

=

F

oAnswers 90,6,3

2),0,0(:


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Friis Transmission Eq.

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

2

t

R4

P

=isotrP

22

tt

2

tt

R

PA

R4

PG

=== isotrtt G PP

22

trt

R

PAA

== trrec A PP

( )2trt

R4

PGG 2

=recP

TX

RX

Antenna Arrays Uses many antennas synchronized

with each other to increase

Pattern multiplication, AF

=

Arreglo

del

Factor

individual

antena

Patrn

)(rE

=

2sin

2sin

N

N

AFN

1

-1 1x

|T (x)|4

-R

x0

Uniform illumination Tschebyscheff Illumination

Planar Arrays

( )

=

2sin

2sin

1

2sin

2sin

1,

y

y

x

x

n

N

N

M

MAF

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Antenna Intro Extend