MICROSTRIP ANTENNAS MICROSTRIP ANTENNAS

By :

Dr ADr A DastranjDastranjDr. ADr. A. . DastranjDastranjSchool of Electrical & Computer School of Electrical & Computer EngineeringEngineering

Shiraz UniversityShiraz University

adastranj@shirazu ac iradastranj@shirazu ac iradastranj@shirazu.ac.iradastranj@shirazu.ac.ir

1May 28, May 28, 2011 2011

Chapter 5Chapter 5Chapter 5Chapter 5Microstrip antennas

Microstrip antennas Are considered theAre considered the

most practical antennasFor mobile communication !

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Mi iMi iMicrostripMicrostripAntennasAntennas

OutlineOutlineOutlineOutline

11-- Components Components 22-- Types of microstripTypes of microstrip22-- Types of microstrip Types of microstrip

AntennasAntennas33-- Feeding techniquesFeeding techniques

44-- Advantages Advantages 55-- Disadvantages Disadvantages

66-- Techniques to overcomeTechniques to overcomeDisadvantages Disadvantages

77-- Microstrip arraysMicrostrip arrays77 Microstrip arrays Microstrip arrays 88-- Feeding of arrays Feeding of arrays

99-- Microstrip vs. reflectors. Microstrip vs. reflectors.

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6 1 C t6 1 C t6.1. Components 6.1. Components

A microstrip antenna consists of :

Patch ( radiating Element ) FeedFeed

DielectricDielectricGround planeGround planeGround planeGround plane

coppercopper

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The patch ( radiating element ) may be circular , rectangular or any other shape .

C t D i tC t D i tComponents : Design parameters Components : Design parameters

Design parameters : ( W , L , f , ) , o = c / f , g = /

The microstrip antennas have a main radiating edge , the other edge is weaker .

W W

5L L

6 2 T f i t i t6 2 T f i t i t6.2 . Types of microstrip antennas6.2 . Types of microstrip antennas

Open circuit microstripOpen circuit microstrip Short circuit microstripShort circuit microstrip

-The patch is totally isolatedFrom the ground plane -The patch is connected toFrom the ground plane

-Higher efficiency than shortCircuit microstrip antennas .-Side length of the patch is

pThe ground

-Have only one radiating Edge .

Side length is g / 4

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g pg / 2. - Side length is g / 4 .

T f i t i tT f i t i tTypes of microstrip antennasTypes of microstrip antennas

As it is difficult to manufacture a short circuit microstrip antenna , we use shortingshortingPostsPosts instead .

Shorting posts have :-Inductance in each one

-Capacitance between them> As number of posts increaseResonant frequency increase ShortingShortingResonant frequency increase . ShortingShorting

PostsPosts

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6 36 3 F di t h iF di t h i6.3.6.3. Feeding techniques Feeding techniques

Feeding by couplingFeeding by coupling

Microstrip line Microstrip line FeedFeed

Feeding by coupling Feeding by coupling

Direct feeding by coaxialDirect feeding by coaxialFeed line ( probe ) Feed line ( probe )

Feed Feed

ApertureAperturecoupledcoupled

Proximity Proximity coupledcoupledpp

feedfeedpp

feedfeed

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Feeding techniques : Feeding techniques : Direct feed by Direct feed by g qg q yycoaxial fees linecoaxial fees line

The inner ( central ) of the coax is attached to the patch while The outer ground is welded to the ground of the microstrip

( like the monopole ) .

PatchPatch

CoaxialCoaxial

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Coaxial Coaxial

Equivalent circuit Equivalent circuit

F di t h iF di t h i Mi i f d liMi i f d liFeeding techniques : Feeding techniques : Microstrip feed lineMicrostrip feed line

It is a conducting strip of much smaller width compared to thePatch , it is easy to fabricate and simple to match ..y p

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F di t h iF di t h i f di b lif di b liFeeding techniques : Feeding techniques : feeding by couplingfeeding by coupling

ApertureAperturecoupledcoupled

feedfeed

Proximity Proximity coupledcoupled

feedfeed

The most difficult to fabricate And has a narrow band ,

Depends on two substrates andA d ith l t

Has a band width of 13% ,however it is difficult to fabricate.

A ground with a slot .

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6 4 Ad t6 4 Ad t6.4 . Advantages 6.4 . Advantages

1 1 –– High accuracy in manufacturing , the design is executed byPhoto etching g

2 2 –– Easy to integrate with other devices

3 3 –– An array of microstrip antennas can be used to form a Pattern that is difficult to synthesize using a single element.Pattern that is difficult to synthesize using a single element.

44 –– We can obtain high directivity using microstrip arrays

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4 4 –– We can obtain high directivity using microstrip arrays

Ad tAd tAdvantages Advantages

5 5 –– Have a main radiating edge , this makes it useful for mobilePhones to avoid radiation inside the devicePhones to avoid radiation inside the device .

66 S ll i d li bl f h dh ld t bl i ti6 6 –– Small sized applicable for handheld portable communication

7 7 –– Smart antennas when combined with phase shifters .

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6 5 Di d t6 5 Di d t6.5 . Disadvantages 6.5 . Disadvantages

1 1 –– Narrow band width ( 1% ) , while mobiles need ( 8% )

2 2 –– Low efficiency , especially for short circuited microstrip antenna

3 3 –– Some feeding techniques like aperture and proximity Coupling are difficult to fabricate

4 4 –– An array suffers presence of feed network decreasing Efficiency , also microstrip antennas are relatively expensive .

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6.6 . Techniques for overcoming6.6 . Techniques for overcomingq gq gdisadvantagesdisadvantages

Conventional techniquesConventional techniques Non conventional techniquesNon conventional techniques

1- Decreasing dielectric Constant

2- Increasing thickness 3- Increasing width

1- Aligned parasitic elements2- Using stacked parasitic

Elements.3 Increasing width .

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Techniques for overcomingTechniques for overcomingq gq gdisadvantages : disadvantages : Aligned parasitic elementsAligned parasitic elements

Feeding one patch by coax Patch #1 : Patch #1 : F d bF d bProbe and the other two

Patches are fed by coupling ,This makes the antenna has

Three resonating frequencies

Fed by coax Fed by coax Feed lineFeed line

Patch #2 , 3 : Patch #2 , 3 : Fed byFed byThree resonating frequencies

And the ultimate resonance Is of a wider band width.

Fed byFed byCoupling.Coupling.

Single element Single element Parasitic elements Parasitic elements

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Techniques for overcomingTechniques for overcomingq gq gdisadvantages : disadvantages : Stacked parasitic elementsStacked parasitic elements

Rather than aligning them , We can even combine the two

Methods and modulate the Patch’s shape to yield widest

Band width .

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6 7 Mi t i A6 7 Mi t i A6.7 . Microstrip Arrays 6.7 . Microstrip Arrays

2 ^ n2 ^ n

2 ^2 ^Feed

Network

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2 ^ n2 ^ n

Microstrip ArraysMicrostrip ArraysMicrostrip Arrays Microstrip Arrays

The optimum spacing is 0.8, length must be <= o to avoidMultiple grating lobes and also must be >= lambda / 2 .p g g

Advantages of microstrip arrays Advantages of microstrip arrays

1 1 –– Used to synthesize a required pattern difficult to achieve with A single element.

33 –– Increases directivity .

2 2 –– Used to scan the beam of an antenna system

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3 3 Increases directivity .

Microstrip ArraysMicrostrip ArraysMicrostrip Arrays Microstrip Arrays

Disadvantages of microstrip arrays Disadvantages of microstrip arrays

1 1 –– Narrow bandwidth ( 1 % ) .

2 2 –– Low efficiency y

3 3 –– If the separation is more than lambda , grating lobes appear

4 4 –– Feed network decreases efficiency .

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6 8 F di f6 8 F di f6.8 . Feeding of arrays 6.8 . Feeding of arrays

A microstrip antenna uses feed network which may be either :

1 1 –– Series feed Series feed

2 2 –– Corporate feed . Corporate feed .

Sometimes feed networks are synthesized with the antenna !

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F di f S i f dF di f S i f dFeeding of arrays : Series feed Feeding of arrays : Series feed

Series feed Series feed

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F di f C t f dF di f C t f dFeeding of arrays : Corporate feed Feeding of arrays : Corporate feed

Corporate feed Corporate feed

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Fl t l Mi t i A tFl t l Mi t i A tFlat plane Microstrip AntennaFlat plane Microstrip Antenna

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