Lec05 IE419 Antennas 13 Stds

7/23/2019 Lec05 IE419 Antennas 13 Stds

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IE 419/519

Wireless Networks

Lecture Notes #5

Antennas and Propagation


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Introduction

An antenna is a transducer thatconverts radio frequency electriccurrent to electromagnetic waves

that are radiated into space In two-way communication the

same antenna can !e used for

transmission and reception


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undamental Antenna$oncepts %eciprocity

%adiation Patterns Isotropic %adiator

&ain

Polari'ation


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%eciprocity

In general the various propertiesof an antenna apply equallyregardless of whether it is used for

transmitting or receiving)ransmission*reception e+ciency

&ain

$urrent and voltage distri!ution Impedance


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%adiation Patterns

%adiation pattern &raphical representation of radiation properties of

an antenna

,epicted as a two-dimensional cross section

%eception pattern %eceiving antennas equivalent to radiation pattern


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%adiation Patterns /cont01

eam width /or half-power !eam width1 3easure of directivity of antenna


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Antenna &ain

Antenna gain Power output in a particular direction

compared to that produced in any

direction !y an isotropic antenna 6ective area

%elated to physical si'e and shape of

the antenna


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Antenna &ain /cont01

%elationship !etween antenna gainand e6ective area

G antenna gain

Ae e6ective area f carrier frequency c speed of light /" 8 9:7m*s1 carrier wavelength


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Antenna &ain /cont01

An antenna with a G < "d improvesover the isotropic antenna in thatdirection !y "d or a factor of 2


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Polari'ation

,e=ned as the orientation of theelectric =eld /-plane1 of anelectromagnetic wave

)ypes of polari'ation Linear

>ori'ontal

?ertical $ircular


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Polari'ation

?ertically Polari'ed Antenna lectric =eld is perpendicular to the arths surface

e0g0 roadcast tower for A3 radio @whip antennaon an automo!ile

>ori'ontally Polari'ed Antenna lectric =eld is parallel to the arths surface e0g0 )elevision transmission /B0C01

$ircular Polari'ed Antenna Dave radiates energy in !oth the hori'ontal and

vertical planes and all planes in !etween


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Polari'ation
http://www.phy.ntnu.edu.tw/java/emWave/emWave.htmlhttp://www.phy.ntnu.edu.tw/java/emWave/emWave.html


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)ypes of Antennas

Isotropic antenna Ideali'ed %adiates power equally in all directions

Omnidirectional

,ipole antennas >alf-wave dipole antenna

>ert' antenna

Euarter-wave vertical antenna 3arconi antenna

Para!olic %eFective Antenna


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,ipole Antenna

http://www.rfcafe.com/references/electrical/antenna_patterns.htm

Powerradiated

A'imuth


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Propagation 3odes

&round-wave propagation

CGy-wave propagation

Line-of-sight propagation


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&round Dave Propagation

ollows contour of the earth

$an propagate considera!le distances

requencies up to 2 3>'

8ample A3 radio


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CGy Dave Propagation

Cignal reFected from ioni'ed layer ofatmosphere !acG down to earth

Cignal can travel a num!er of hops !acG andforth !etween ionosphere and earths surface

%eFection e6ect caused !y refraction

8amples

Amateur radio

$ radio


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Line-of-Cight Propagation

)ransmitting and receiving antennas must !ewithin line of sight

%efraction ending of microwaves !y the atmosphere ?elocity of electromagnetic wave is a

function of the density of the medium Dhen wave changes medium speed

changes Dave !ends at the !oundary !etween

mediums


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Line-of-Cight quations

Hptical line of sight

6ective /or radio1 line of sight

d< distance !etween antenna and

hori'on /Gm1 h< antenna height /m1 < adJustment factor to account for

refraction rule of thum! < (*"

hd 57.3=

hd = 57.3


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2:

Line-of-Cight quations

3a8imum distance !etween twoantennas for LHC propagationK

h9< height of antenna one

h2< height of antenna two

( )21max 57.3 hhd +=


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LHC Direless )ransmissionImpairments

Attenuation and attenuation distortion

ree space loss

Noise

Atmospheric a!sorption

3ultipath

%efraction

)hermal noise


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Attenuation

Ctrength of signal falls o6 with distanceover transmission medium

Attenuation factors for unguided mediaK %eceived signal must have su+cient strength

so that circuitry in the receiver can interpretthe signal

Cignal must maintain a level su+cientlyhigher than noise to !e received without error

Attenuation is greater at higher frequenciescausing distortion


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ree Cpace Loss

ree space loss Ideal isotropic antenna

Pt< signal power at transmitting antenna Pr< signal power at receiving antenna < carrier wavelength d< propagation distance !etween antennas c< speed of light /" 8 9:7m*s1

where d and are in the same units (e.g.,meters)


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ree Cpace Loss


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ree Cpace Loss

ree space loss accounting for gain ofother antennas

Gt< gain of transmitting antenna

Gr< gain of receiving antenna

At< e6ective area of transmitting antenna

Ar< e6ective area of receiving antenna


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$ategories of Noise

)hermal Noise

Intermodulation noise

$rosstalG Impulse Noise


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)hermal Noise

)hermal noise due to agitation ofelectrons

Present in all electronic devices

and transmission media $annot !e eliminated unction of temperature

Particularly signi=cant for satellitecommunication


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)hermal Noise

Amount of thermal noise to !e found in a!andwidth of 9>' in any device orconductor isK

N:< noise power density in watts per 9 >' of!andwidth

G < olt'manns constant < 90"7:" 9:-2"M*o T < temperature in Gelvins /a!solute

temperature1

( )W/Hzk0 TN =


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)hermal Noise

Noise is assumed to !e independent offrequency

)hermal noise present in a !andwidth of

B>ert' /in watts1K

or in deci!el-watts

TBN k=

BTN log10log10klog10 ++=

BT log10log10dBW6.228 ++=


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Noise )erminology

Intermodulation noise Hccurs if signals with di6erent frequencies

share the same medium

$rosstalG Bnwanted coupling !etween signal paths

http://www.cabletet!"g.comhttp://www.cabletet!"g.com


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Noise )erminology

Impulse noise Irregular pulses or noise spiGes

Chort duration and of relatively highamplitude

$aused !y e8ternal electromagneticdistur!ances or faults and Faws in thecommunications system


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Hther Impairments

Atmospheric a!sorption Dater vapor and o8ygen contri!ute to

attenuation

3ultipath H!stacles reFect signals so that

multiple copies with varying delays are

received %efraction

ending of radio waves as theypropagate through the atmosphere


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ading in 3o!ilenvironment ading

)ime variation of received signal powercaused !y changes in transmissionmedium or path/s1


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3ultipath Propagation /3P1

%eFection Hccurs when signal encounters a surface that

is large relative to the wavelength of the signal

,i6raction Hccurs at the edge of an impenetra!le !ody

that is large compared to wavelength of radiowave

Ccattering Hccurs when incoming signal hits an o!Ject

whose si'e is in the order of the wavelength ofthe signal or less


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)he 6ects of 3PPropagation 3ultiple copies of a signal may arrive at

di6erent phases If phases add destructively the signal level

relative to noise declines maGing detectionmore di+cult

nown as Intersym!ol Interference /ICI1


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)ypes of ading

ast fading

Clow fading

lat fading

Celective fading

%ayleigh fading

%ician fading


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ading

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Lec05 IE419 Antennas 13 Stds