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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 [email protected]@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
22
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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