microwave and antenna theory

8/11/2019 Microwave and Antenna theory

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ObjectivesDiscuss the makeup of electromagnetic waves

Explain

ReectionRefractionDiraction

Describe the important aspects of satellite

communication


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Describe ground wave propagation

Describe space wave propagationCalculate the approximate radio horion

based on antenna height

Discuss the eects of the ionosphere on sk!

wave propagationDe"ne critical angle and skip one for sk!

wave


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Compliments of#$%

&C


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Electrical to 'agneticConversion

(he antennas are the transducers(he transmitting antenna changes the electrical

energ! into electromagnetic or waves

(he receiving antenna changes theelectromagnetic energ! back into electrical energ!

(hese electromagnetic waves propagate atrates ranging from )*+k, to -++.,


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Electromagnetic /aves

Consist of'agnetic waveElectrical wave

'ost of the energ! is returned to the circuit0#f it isn1t2 then some it must be 3set free4 or

radiated0 Radiated energ! is not desirable0

link


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Radio5fre6uenc!

#nterference#f the radiated energ! comes from anotherradio transmitter2 then it is considered radio5fre6uenc! interference 7R8#9

(he transmitting antenna should be

speci"call! designed to prevent the energ!from being returned to the circuit0

#t is desirable that the antenna 3free4 theenerg! in order that it might radiate intospace


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Electromagnetic#nterference#f the energ! comes from else where2 then it

is electromagnetic interference 7E'#9


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(ransverse /aves(ransverse waves are those whose direction ofpropagation is perpendicular to both the electrical"eld and the magnetic "eld (he electrical "eldand the magnetic "elds lie in planes that areperpendicular to each other0 7x and ! planes9

(hus the direction of propagation will be in the plane or third dimension


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$olariation of the Electrical 8ield(he polariation of the electrical "eld is

determined b! the direction of oscillations#f the oscillations are in the vertical direction

then the polariation is said to be vertical

#f the oscillations are in the horiontal directionthen the polariation is said to be horiontal


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:ertical ;ntenna

; vertical antenna is one that consists of avertical tower2 wire2 or rod2 usuall! a 6uarterwavelength in length that is fed at the ground

and uses the ground as a reecting surface0(hus a 3vertical4 antenna will result in a

verticall! polaried wave0


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/avefronts; wavefront is a plane joining all points

of e6ual phase in a wave

(ake a point in space0 #magine waves

radiating outward in all directions fromthis point0 (he result would resemble asphere0 (he point of radiation is calledthe isotropic point source


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%ince the power at an! point awa! fromthe isotropic point is inversel! proportionalto the s6uare of the distance from thepoint2 then the power decreases rapidl!the further awa! from the point !ou need0

;lthough the wavefront is curved in shape2

from a distance small sections appearplanar and can be thought of as planewavefronts


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Characteristic #mpedance of 8ree %pace#n free space? x )+5?,@m

= A0A* x )+5)>

8@m= -BB


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8our (!pes of /aves#ncident

Reected

RefractedDiracted


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ReectionReection is the abrupt reversal in directionCaused b! an! conductive medium such as'etal surfaces or

Earth1s surface(here will normall! be a shift in phase

Coecient of reection is less than )


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Complete ReectionComplete reection will occur onl! in perfect

conductors and when the electric "eld isperpendicular to the reecting element or

mediumCoecient of Reection will be )

Coecient of Reection is the ratio of thereected wave intensit! to the incident wave

intensit!


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Electrical 8ields $arallel to the

Conductor/hen the E "elds are parallel to the

conductor or conducting medium2 theelectrical energ! is absorbed b! the mediumthus acting like a short


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RefractionOccurs when the waves pass from one mediumto another whose densities are dierent

Coecient of reection is less than )

(he angle of incidence and the angle ofrefraction is related b! %nell1s &aw


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Diraction/aves traveling in straight lines bend around obstacles

ased on ,u!gen1s principle 7)?+9Each point on a wavefront can be thought of as an

isotropic point or a source of secondar! spherical energ!

Concepts explains wh! radio waves can be heard behindtall mountains or buildings that are normall! considered toblock line of sight transmissions


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.round /avesRadio wave that travels along the earth1s surface

7surface wave9:erticall! polariedChanges in terrain have strong eect;ttenuation directl! related to surface impedances'ore conductive the more attenuatedetter over water


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Gsed to communicate with submarines

E&8 7-+ to -++ ,9 propagation is utilied


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FIGURE 12-6 Direct and ground reected space waves0

Gary M. Miller, Jefrey S.Gary M. Miller, Jefrey S.BeasleyBeasley

Modern Electronic Communication,Modern Electronic Communication,

Cop!right H>++> b! $earson Education2Cop!right H>++> b! $earson Education2#nc0#nc0

Gpper %addle River2 Few Ierse! +BJ*AGpper %addle River2 Few Ierse! +BJ*A


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%pace /ave(wo t!pes

Direct

.round reected


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Direct&imited to 3line5of sight4 transmission distances;ntenna height and curvature of earth are

limiting factors

Radio horion is about A+K greater than line ofsight because of diraction eects


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FIGURE 12-7 Radio horion for direct space waves0






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Reected$art of the signal from the transmitter is bounced othe ground and reected back to the receivingantenna

Can cause problems if the phase between the direct

wave and the reected wave are not in phaseDetuning the antenna so that the reected wave is

too weak to receive


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FIGURE 12-9 %k!5wave propagation0






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Sky

WavesRadio waves radiated from thetransmitting antenna in adirection toward theionosphere

&ong distance transmissions

%k! wave strike theionosphere2 is refracted backto ground2 strike the ground2reected back toward theionosphere2 etc until it reachesthe receiving antenna

%kipping is he refraction andreection of sk! waves


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A!"s#$eri%&$e'"!e'"'

(hree la!ers>B0? ', and )*B*0J> ',


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8D';8re6uenc! division multiplex accessEarl! .$% s!stems

%everal channels

Earth station sends a signal re6uestingpermission to transmit2 a control signalresponds with the available fre6uenc! to

transmit on0


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(D';(ime division multiplex access%ingle satellite to service multiple earth

stations simultaneousl!

;ll stations use the same carrier but transmitone or more trac bursts in nonoverlappingtime frames


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(D'; ;dvantages)0 %ingle carrier for the transponder tooperate on

)0 &ess subject to intermodulation problems

>0 Can operate at a higher power output withsmaller range of fre6uencies

>0 ;chieve selectivit!

)0 %impler>0 &ess expensive

-0 %uited to digital communications


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:%;(:er! small aperture terminal "xed satellitecommunication s!stems

;llow multiple inexpensive stations to belinked to a large central installationSmart has :%;(s at over >+++ stores

linked to a mainframe computer in 'i0

;llows them to:erif! checks and credit cardsConve! data such as inventor!


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Dish is t!picall! ) m in diameter

$ower is just > to - watts

#mmune to optical "ber for another >+ !earsor until "ber replaces copper


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Chr!sler?+++ dealerships;ssists in

Repair

%ales

Deliver! dates

#nventor!

%pare parts autos


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Gary M. Miller, Jefrey S.Gary M. Miller, Jefrey S.

BeasleyBeasleyModern Electronic Communication,Modern Electronic Communication,

Cop!right H>++> b! $earson Education2Cop!right H>++> b! $earson Education2

#nc0#nc0Gpper %addle River2 Few Ierse! +BJ*AGpper %addle River2 Few Ierse! +BJ*A


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'%;(Gltrasmall aperture terminal mobile satellites!stems

:%;(1s on /heels

&arge national trucking "rms

;llows dispatching center to maintaincontinuous communication with each of its

trucks


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Description of ionospheric laers and theireects on sk! wave propagation

De"nitions

%kipping

Critical fre6uenc!

Critical angle

'G8


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Description and use of

:%;(

'%;(

%tud! guide

On5line Pui


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microwave and antenna theory

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