practical antennas material

8/13/2019 Practical Antennas material

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PRACTICAL ANTENNAS

FOLDED DIPOLE ANTENNA (FD)

The folded dipole antenna consists of two half wave dipoles one is continuousand the other split at the center. The split dipole is fed at the center by abalanced transmission line.

The two dipoles have the same voltages at the ends. The two dipoles are inparallel as far as the radiations field is concerned.

The radiation pattern of FD and a conventional dipole is same but the inputimpedance of FD is higher. The FD has directivity and high beam width.

If the radii of the two conductors are equal then the current flow in both theconductors are in the same direction. The currents are equal in magnitude andphase in two dipoles.

The total power developed by the FD is equal to the conventional dipole, theinput or terminal impedance of FD is greater than that of dipole.

It was found that input impedance at the terminals of FD is equal to the squareof the number of conductors comprising the antenna times the impedance atthe terminals of a conventional dipole.

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IMPEDENCE EQUATION

The equivalent circuit is show in fig

Let V be the emf applied at the antenna terminals !This is being equally divided in each dipole. "ence voltage in each dipole isV#$ as shown by the nodal analysis.

2121112

IZIZV +=

21, II are currents in dipoles % and $.

11Z is the self impedance of dipole %

12Z is the mutual impedance between dipole % and dipole $.

&ut 21 II = )(2

12111 ZZIV

+=

If two dipoles are close to each others, the spacing 'a! is of the order of

#%((, then 1211 ZZ =

1112

2

ZIV=

Input impedance 111

4ZI

VZ ==

For a half wave dipole 11Z ) *+

For FD input impedance - ) / *+ ) $0$

1imilarly for folded dipoles of + wires

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)3(3

111 ZIV =

11

1

9ZI

VZ ==

- ) 0 / *+ ) 23*

4enerali5e for n folded dipole

11

2

1

ZnI

VZ ==

THE V ANTENNA

The V6antenna is an e7tension of long wire antenna. Two wires are arranged inthe form of hori5ontal V fed at ape7.

If the angle between two sides of the V is equal to twice the angle that thecone and the a7is there produce a ma7imum radiation. 8hen the two wiresare fed with %9(: out of phase each other provides the gain and directivity."igher the length of the legs greater is the directivity and gain.

Traveling wave antennas are those antennas in which there is no standingwaves and waves travel in only one direction. Traveling wave antennas areobtained by terminating one end of the antennas by non inductive resistanceof the value equal to characteristic impedance. 1uch antennas are called anaperiodic or non resonant antennas. &y doing so, the bandwidth is increased.Inverted V antennas is used in high frequency band, is one of the traveling

wave antenna.

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dvantages;a< =rovides multiband operation.b< >an be fed by tuned feeders.c< ?atched by 4/ matching section or stub matchingd< >an be used for receiving and transmitting antennas.e< =roduces low angle beam, fi7ed frequency operation at "F band.

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RHOMBIC ANTENNA

@hombic antenna is a very efficient antenna of broad frequency capabilities. It

consists of four straight wires arranged in the form of rhombus or diamondsuspended hori5ontally above the surface of the earth.

It can be regarded as two inverted V antennas connected in series. If the twowires are pulled apart such that lobes &>A4 are combined together, thenadditional gain is achieved.

The tilt B is equal to 0(: minus angle of maCor lobe. The change of length from

to 9and change of angle of angle of the maCor lobe from %*: to $: onlyindicates the antenna operates over a wide band of frequency.

The fig gives the rhombic antenna hori5ontally installed over the ground at aheight h. when it is used for transmission the input is fed through a balancedline and terminating an inductive resister is adCusted so that traveling wavesare set up in the four legs of the rhombic.

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( )

{ }

sincos1

sincos1sinsin2

sincos2

2

=

Lh

E

66%haracteristics

%. If + element array is used it is called beam antenna$. It has unidirectional beam of moderate directivity with light weight, lowcost and simple fed system design.

3. 8ith spacing of (.%to (.%3, frequency bandwidth of $H is obtained.. It provides the gain of the order of 9 d&.3. It is also called as super directive or super gain antenna.2. If greater directivity is required, further elements may be used.

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*. It is a fi7ed frequency device ie frequency sensitive and a band widthof +H is achievable which is adequate for TV reception, its mostcommon use.

LOOP ANTENNA

lop antenna is a single turn coil carrying @F current. It has dimensions less

than a 8L < and hence current through it assumed to be in phase. The loop

is surrounded by a magnetic field everywhere perpendicular to the loop. Thedirectional pattern is independent of the e7act shape of the loop and isidentical to that of an elementary doublet. In loop antenna no radiation isreceived in the plane normal to the loop. This intern maes the loop antennasuitable for direction finding DF.The direction finding properties of the loop are Cust as good at ?F as those of

microwave antennas.

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If there are N turns of the oo!, the induced "ota#e $i %e N&fod

coscos

2b

ANEE o

N ==

'here

ANEb o

2=

h6L ) a ) area of the loop

8hen ) (, cos)%

) no of turn.

An ) If there are n turns of the loop, the induced voltage will be fold.

The instantaneous loop voltage will be ma7imum when the field is passingthrough 5ero.

LO' PERIODIC ANTENNA(FREQUENCY INDEPENDENT ANTENNA)

frequency independent antenna may be defined as the antenna for whichthe impedance and pattern remain constant as a function of frequency.

Therefore in order that the antenna to be frequency independent, the antennashould e7pand or contrast in proportion to the wavelength.

The geometry of log periodic antenna is so chosen that the electricalproperties must repeat periodically with logarithm of frequency independentcan be obtained when the variation of the properties over one period, andhence all the periods is small. "ear the ratio of length and spacing of adCacentelements should be constant each other. The length L and spacing @ or 1,

their ratio

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=2

1

R

R=

3

2

R

R=

4

3

R

R666666) ==

+

1n

n

R

R=

2

1

L

L=

3

2

L

L666 =

+1n

n

L

L

is called design ratio, or scalar ratio or periodic factor.

Kr

111 === ++ KL

L

S

S

n

n

n

nMN%, @)1

Typical values of )0(:, ) (.*.

If a graph is plotted between input impedance -in or 18@< and frequency arepetitive variation will be obtained. If this is against the logarithm offrequency, than this variation will be periodic, called log periodic which implythat the impedance is a logarithamatically periodic function of frequency. otonly this all the electrical properties under go similar periodic variation

particularly radiation pattern gain, side lobe level, beam width and beamdirection.

From equation < if the structure is e7tended from the verte7 of the angel at one end to infinite at the other end, than this periodicity would e7tend from

frequencies ( to &ut the transmission line for feeding has to be very smallconductor si5e and spacing. ll these conditions lead to impossibility. "encelog periodic structure must be terminated in either direction at some points.

These terminated ends in either direction determines high and low cutofffrequencies. &eyond these cutoff frequency the log properties ceases.

The two consecutive ma7ima of impedance variation occur at frequency F%and F$ where

21

1

2

1

2

1

1

FF

F

F

LogF

FLog

=

=

=

8hen the log periodic antenna is operated at a given frequencyit was observed that all the structure does not radiate but a certain portion

radiate nown as active region .

==LI>TIK1;

%. These lie in the frequency of "F common where multiband steerable andfi7ed antennas are generally used.

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$. sed for TV reception where only one log periodic design is sufficient forall the channels even up to "F band.

+. it is best used for all round monitoring in which a single LK8 =K8A@antenna cover all higher frequency bands if the cost in the installation isno problem .

HELICAL ANTENNA

eica antenna is a %road %and * and +* antenna $hich is used $hen it is desire to !ro"ide

circuar !oariation characteristics. +sed for e-traterrestria coons.

he heica antenna consists of oose $ound hei- %aced u! % a #round !ane. here are t$o

odes of radiation Nora and a-ia.

In the nora ode the radiation is in direction at an#es to the a-is of the hei-. In a-ia ode

is ore interestin# %ecause it !roduces a %road %and. If the hei- circuference a!!ro-iates a

' ), it a %e sho$n that a tra"ein# $a"e tra"es around turns of the hei-, and the radianto%e in this end&fire action is circuar !oaried.

eica antenna has !ro!erties, directi"it is 25. ea $idth is 90: %et$een nus and thefreuenc ran#e of a%out 20 on either side of center freuenc. he ener# in the circuar

!oaried $a"e is di"ided eua %et$een horionta and "ertica co!onents the are 90: outof phase.

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1) spacing between heli7D )diameter of the loop>)circumference of heli7) O DD)diameter of helical conductor )a7ial length )1L) length of one turnJ L)spacing of heli7 from ground) pitch angle

ormal mode of operation@adiation is ma7imum in broad way side normal to the heli7 a7isThis mode is obtained if the dimensions of the heli7 is small compared to the8L ie L ()&andwidth efficiency is lowThe efficiency is increased by increasing the dimensions of the heli7

For small loops

A) %$( O P I:sin B

;

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For short dipole 1)L)LA4T" KF DI=KLA

A) Q 2( I< 1I 1

7ial ratior ) A#A) 1#$ ) $1#;#A:) %, $1) ;


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The antennas consists of rectangular metal patch on a dielectric R coatedground plane .the dielectric substrate having r ) $ and thicness t /100.

o$ !rofie antennas are needed in s!acecraft or aircraft, $here sie, cost, $ei#ht, !erforances

and aerodnaic !rofie are constraints. Dicrostri! antennas eet these s!ecifications

Eractica a!!ications at the freuenc a%o"e 100 D. ( 3 c). Ft!ica !atch antenna $ith

diensionsen#th@ G $idth @'G thicness@t

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he horionta co!onents of the eectrica fieds at the eft and ri#ht ed#es are in the sae

direction #i"in# in !hase inear !oaried radiation $ith a a-iu %road side to the !atch.

he !atch acts as a resonant /2 !arae !ate icrostri! transission ine $ith characteristics

i!edance eua to the reci!roca of the nu%er n of !arae& ce transission ines.

Dicro stri! characteristic i!edance

r

r

c Z

nZ

0

1==

If 2,10,1 === rr n

= 6.26c

Z

In #enerat

wn= , the ore #enera reation is

r

o

cw

tZZ

=

MICROWAVE ANTENNAS

Fn antenna is an >D sste ca!a%e of radiatin# or recei"in# >D $a"es. heir function is to

cou!e the transitter or recei"er to s!ace. In case of icro$a"es, the transittin# and recei"in#antenna shoud %e hi#h directi"e.

Dicro$a"e recei"ers are nosier than the o$ freuenc recei"ers. herefore the si#na at the in!utof the recei"er shoud %e as ar#e as !ossi%e. Hni directiona antennas are not reuired, %ecause

%road castin# are not done at these freuencies. Dicro$a"e antennas are hi#h directiona %ecause

the are reuired in direction findin# eui!ent such as FD ener# ie an antenna in to s!ace, if it is suita%ee-cited at one end and o!ened at the other. he disad"anta#e of this o!en ended $/# is that on

sa !ortion of the for$ard ener# is radiated and uch of it is refected %ac % the o!en circuit,

%ecause o!en end is a discontinuit and atchin# is not !ro!er, and diffraction around the ed#es

#i"es a !oor, non&directiona radiation !attern. o o"ercoe these !ro%es, the outh of the o!en

end is fared so that it taes the sha!e of a horn it is caed horn antenna. It increases the directi"itand reduces the diffraction.

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?e"era horn confi#urations can %e used such as *i# 11. ?ectora horn 2. Eraida horn 3. Bonica horn.

he sectora horn fares out on one direction. he !raida horn $hich fares in sectora %oth >and !an direction and has a sha!e of truncated !raid it i!ro"es directi"it, %ecause it fares

in t$o directions. he conica horn $hich is used as a terination for circuar $/#. if the fare

an#e is "er sa then it resuts a shao$ horn. han the $a"efront ea"in# this horn $i %e

s!herica and not !an. he radiated %ea $i not %e directi"e. he fare an#e shoud %eo!tiu $hich is cose reated $ith en#th of the horn easured in ' (). Eractica

"aries fro 40: when L#) 2 to %(: when L# ) 3(.

=yramid and conical horns gives pencil beams that has pronounced directivity

in both vertical and hori5onatal planes. Fan shaped beams results when onedimension of the horn mouth is much smaller than the other lie sectoral horn.

In addition to these, there are some specially designed antennas which arebetween horn antenna and parabolic reflections called cross horn and hoghorn antennas. Fig$. It is a combination of a parabolic and horn. It is a low

noise microwave ant2enna. It consists of a parabolic cylinder Coined topyramidal horn, with the rays eminating from or being received at the ape7 ofthe horn.

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The aperature of the parabola is the ratio of the focal length to the mouthdiameter.If a source of radiation is placed at the focus, all the waves coming from it andreflected by the parabola will have traveled the same distance by the time

they reach the directory. They will thus be in phase. Thus the radiation is verystrong and concentrated along the a7is. The cancellation taes place in anyother direction because of path length differences. This leads to concentratedbeam of radiation.

paraboloid reflector antenna has nearly same characteristics both fortransmission and receiption. It is directional because only rays coming from Adirection i.e normal to the directri7 are brought together at the focus. 8hereas the rays from any other direction are cancelled at that point, because ofpath length differences. The antenna has a high gain lie the mirror ofreflecting telescope, because it collects radiation from a large area and

concentrates it all at the focal point.

The capture area is related to the mouth area of the parabolic reflectoro ) M

8here o is the capture area is the mouth areaM is the constant ) (.23 for parabolap the power gain with respect to a resonant half wave dipole.

=ower gain02

4 AAP

=

KAAP 2

4

=

22

2 5.6

4

4

==

DK

DA

P

The radiation pattern of an antenna using g parabolic reflector has a verysharp main lobe and surrounding the main lobes are minor lobes. The minorboles are very small in si5e compared to the main lobe. The primary or feed

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antenna is non6 directional then the apraboioid produces a beam of radiationwhose width is given by

)*( /< U@2

'here < @outh diaeter of refector@ %ea $idth %et$een haf !o$er !oints in de#rees o @ %ea $idth %et$een nus, in de#rees.

FEED SYSTEMS

In #enera the !riar antenna is !aced at the focus of the !ara%oic for transission andrecei!tion. If the refectors not used, the di9rect radiation fro the feed s!reads out in a directions

$hich s!ois the directi"it .an ethods can %e used to !rotect the directi"it. Hne of the

%ein# a sa s!herica refector to redirect a such radiations %ac to the !ara%oic.

Hther ethods used to refect are, a sa di!oe arra at the focus, such as o#i or an end firearra !ointin# at the !ara%oic refector. F ost coon used !riar feed is $ith the he! of ahorn antenna !ointin# at the ain refector.

*i# 1

Fnother ethod is casse#rain feed. F second h!er%ooid refector is used in this case one of

$hose foci coincides $ith the !riar !ara%oic refector. he ras fro the feed horn are

refected fro the !ara%oic irror. his t!e of feed is used $hen it is reuired to !ace the

!riar antenna in a con"enient !osition and to shorten the en#th of transission ine or $/#connectin# recei"er or transitter to the !riar feed.

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Fnother $a of feed sste is to !ace the acti"e !art the acti"e !art of the transitter or recei"erat the focus. 'ith transitters, this is not !ossi%e at a, %ecause of the ar#e sie, and it is asodifficut to !ace the * a!ifier of the recei"er .ho$e"er this t!e of feed causes other

difficuties such as that of ser"icin# and re!aceent. ?o the casse#rain feed is the %est feasi%e

soution.

he fu !ara%oic is not used in !ractice. ?e"era t!es of !ure%oods are

*i# 3.(a) truncated or cut !ara%oic

(%) cindrica !ara%oic

(c) !i %o- section

In the !i %o- refector, the %ea is "er narro$ horionta, %ut not so in "ertica direction.It is a dra$%ac. ut it is used in a nu%er of a!!ications $here the a%o"e dra$%ac is not

so i!ortant. It is used in shi! to shi!. adar $here aiuth and directi"it ust %e e-ceent

%ut ee"ation seecti"it is iateria.

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SLOT ANTENNA

F sot antenna is nothin# %ut a sot of en#th /2 cut in a eta sheet. F sot antenna a %eener#ied $ith a coa-ia transission ine. he sot antenna $ors on the !rinci!e that ener# is

radiated $hen a * fied e-it across a narro$ sot in a conductin# !ane. Ft hi#her freuencies ieicro$a"e re#ion, sot antennas are found to %e "er con"enient radiators. he >D fied

distri%ution in the sot can %e o%tained % usin# asinetJs !rinci!e.

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Basinets principle.

asinetJs !rinci!e is %asica a!!ied in o!tics. It states that the su of the fieds %eond an t$o

co!ientar a%sor%in# screens ad to !roduce then fied that $oud e-ist there $ithout anscreen.

his !rinci!e is e-tended to the conductin# screens and !oaried fieds.

et a source s1 to the eft of an infinite screen ?1 !roduce a fied on its ri#ht. et +1 %e the ratio of

this fied to that $oud e-ist there in the a%sence of ?1. ?iiar, et +2 %e the ratio of the fied to

the ri#ht of the co!eentar screen ?2 !roduced % a source s2 $hich is conCu#ate to s1 to the

eft of ?2, to the fied that $oud e-ist there I the a%sence of ?2.hen, +1 K +2 @ 1 . ?2 conCu#ate to s1, is defined as that for $hich the distri%ution of eectric

and a#netic source currents and char#es i.e. interchan#in# of > and for the incident fieds.

If +1 @ >1/>i then u2 @ 2 / i

he %asis of %asinetJs !rinci!e is the duait !ro!ert of the >D fied. he Da-$eJs euations in

a source free re#ion

L > @ &C$

L @ M'>

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If a fied (>1, 1) satisfies these euations, then a second fied (>2, 2) o%tained fro it % the

transforations

>2 @ K(#


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In general an array of slots may be cut in the wave guide so as to producedirectional ration pattern.

pplications1lot arrays provide a desirable form of "F antenna for high speed aircraft. &y

closing the aperture with an insulating material, such an antenna does notaffect the streamlining of the plane.The disadvantage of a slot radiator is its limited use at relatively "F, becauseto radiate effectively, the slot length must be of the order of half wave lengthwhich a reasonable dimension only in V"F.

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practical antennas material

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