Low-Proﬁle Antennas and Personal Communication Antennas_new

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Antennas Lecture Notes, prepared by Y. A.S. Dama, An-Najah National UniversityAntennas Lecture Notes, prepared by Y. A.S. Dama, An-Najah National University

Low-Profile Antennas andPersonal Communication

AntennasDr Yousef Dama

An-Najah National University

Antennas Lecture Notes, prepared by Y. A.S. Dama, An-Najah National University

Wireless Personal Communications (WPC)

Which employ a communication device that a person can carry oreasily move from place to place.

These antennas should be low-profilethat is, of smallphysical thickness.

Applications are cellular telephone communications; Wi-Fi,Bluetooth, and UWB communications; RFID (radio frequencyidentification); position location (such as GPS) and asset tracking;and body area networks (BAN).

A few of the applications permit the use oftunable antennasthatmove a narrow frequency band over a large frequency range;examples are TV and radio reception in which only one channel isreceived at a time

The essence of the handset antenna challenge is to accommodatean ever increasing number of wireless services and frequencybands within a shrinking allocated volume.


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In a handset there are certain regions of the internal volume thatare designated as keep-outzones, where an antenna cannot belocated, thus restricting the possible antenna volume.

A typical volume maximum is about 5 with dimensions of

3 5 3 5 4

The printed circuit board size for a bar shape phone is 100 40 but smart phones often are larger.

The antenna is expected to support several cellular telephonebands as well as Wi-Fi, TV and FM radio reception, MIMO modes,and so on, in this small volume.

One technique used to relax the size constraint is to make use ofother handset parts for the antenna ground plane, such as aprinted circuit board.

However, handset antenna performance requirements are lessdemanding than in many other areas of antenna design.


Typical performance metrics for handset antennas are,

The low gain specification is due to the need for a very lowdirectivity pattern to minimize signal variations as handset

position is varied.

The handsets themselves are generally one of two shapes, clamshell (that fold in the middle) or bar (single block). The antennadesign differs for these two form factors.

Impedance match: < 3, > 6

Antenna gain: 0 dBi

Radiation efficiency: 50%

Bandwidth: 8 to 12% (depending on the band)


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The perfect handset antenna, albeit unobtainable, would be onethat can be made as small as desired to fit into any allocatedvolume; presents no biohazard; and experiences no frequency

shifting, pattern distortion, gain loss, or impedance mismatch dueto the presence of the human operator.


MICROSTRIP ANTENNA ELEMENTS

The Microstrip antenna is a special type of printed antenna.

The Microstrip antenna (MSA) consists of a metallic patch printed on

top of a thin substrate with a ground plane on the bottom of the

substrate


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The MSA is usually less than 0.05 thick, where is the freespace wavelength.

The basic feeds for the patch are a probe feed using a coaxialtransmission line below the ground plane or an edge feed using a

coplanar microstrip transmission line connected to an edge of thepatch.

The edge-fed patch is a very low profile antenna that also caninclude other components using microwave integrated circuittechniques and the feed network when arrayed.

This offers the advantage of low-cost, controlled-dimensionconstruction.

The radiation pattern is a single, broad unidirectional beambroadside to the patch due to the ground plane greatly reducingthe back radiation (zero for an infinitelylarge ground plane).

The MSA it is narrowband, leading to its biggest designchallengeachieving adequate bandwidth.


Other disadvantages include false radiation from the feed, poorcrosspolarization purity, limited power handling, and adjustmentdifficultyafter fabrication.

The radiation pattern of the basic MSA varies slowly withfrequency, the impedance does not. Thus, impedance bandwidthis the limiting factor of the basic MSA, because it is only a fewpercent.


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Microstrip Patch Antennas

Microstrip patch antenna consists of a radiating patch on one side of a

dielectric substrate which has a ground plane on the other side.

The patch is generally made of conducting material such as copper or goldand can take any possible shape

In order to simplify analysis and performance prediction, the patch is

generally square, rectangular, circular, triangular, elliptical or some other

common shape


Rectangular Microstrip Patch Antennas

The simplest MSA is a rectangular patch on top of a substratematerial with relative dielectric substrate of thickness and backed by a large ground plane

The edge feed is a Microstrip transmission

line on the left side of the patch.

The patch is long (in the plane of the

feed line) and wide


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For rectangular patch the length, L, of the patch is usually 0.3333 < < 0.5, where is the free space wavelength

The patch is selected to be very thin


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Most of the electric field lines reside in the substrate and parts of somelines in air.

As a result, this transmission line cannot support pure transverseelectric-magnetic (TEM) mode of transmission, since the phasevelocities would be different in the air and the substrate.

Instead, the dominant mode of propagation would be the quasi-TEMmode.

An effective dielectric constant must be obtained to account firthe fringing and the wave propagation in the line.

The value of is slightly less than , because the the fringing fieldsaround the edge of the patch are not confined in the dielectric substratebut also spread in the air.


The effective dielectric constantgiven by,

Where

: is the width of the Patch

: is the height of the dielectric

: is the dielectric constant of substrate

=+ 1

2 +

1

2 1 +

10

.

. ( 1)


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Now lets consider a rectangular microstrip patch antenna as shownbelow,

Fringing makes the patch effectively longer than its physical length, sothe resonant length is less than a half-wavelength

In order to operate in the fundamental TM10 the length of the patchmust be slightly less than where is the wavelength in the

dielectric

0.49 = 0.49

(half -wave patch).(2)


The microstrip patch antenna is represented by two slots, separated bya transmission line of length L and open circuited at both the ends.

Along the width of the patch, the voltage is maximum and current

is minimum due to the open ends.

The fields at the edges can be resolved into normal and tangential

components with respect to the ground plane.


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Thenormal components of the electric field at the two edgesalong the width are in opposite directionsand thus out of phase

since the patch is2/long and hence they cancel each other inthe broadside direction.

Thetangential components, which are in phase, means that theresulting fields combine to give maximum radiated field normal tothe surface of the structure.

Hence the edges along the width can be represented as two

radiating slots, which are 2/apart and excited in phase andradiating in the half space above the ground plane.


In practice, this formula is used as a starting point for anexperimental hardware model or for simulation.

Next, the input impedance is measured or calculated as afunction of frequency spanning the design frequency, and theresonant frequency is noted.

Then, the model is scaled in size to shift the resonance to thedesired frequency, followed by a second measurement orsimulation for verification.

The fringing fields along the width can be modeled as radiatingslots and electrically the patch of the microstrip antenna looksgreater than its physical dimensions.

The dimensions of the patch along its length have now beenextended on each end by a distance , which is given empiricallyby Hammerstad as,


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= 0.412+ 0.3

+ 0.264

0.258

+ 0.8

(3)

Where is fringing length

The design formulas begin with a more accurate equation for theresonant patch length:

= 0.5

2 ..(4)

The effective length of the patch L now becomes

= + 2 ..(5)


The resonant frequency of narrow band patches is sensitive tovariations of material thickness and dielectric constant.

Thus, it is important to have a material that is manufactured withproper quality control over material uniformity (especially ),haslow loss, and has a selection of thicknesses and sheet sizes.

As frequency increases, these are even more importantconsiderations.

The total fringing length, 2, due to the fringing on both edgesis the amount the patch length should be reduced below a halfwavelength to achieve resonance.

Viewed another way,is the amount to be added to each edgeto form an effective patch length.

increases with substrate thickness approximately linearly.

This is because as the thickness increases, the electric fieldsextend farther and fringemore.


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As an example, at 10 GHz ( = 3 ) for a 0.01thick substratewith = 2.2 the preceding three formulas yield = 0.98 ,which is in good agreement with the approximate formula (2)that gives 0.99 cm.

Note that the Microstrip feed line will not radiate because thewidth of the upper conductor of the Microstrip transmission lineis small compared to a wavelength, whereas the patch is largerelative to a wavelength.

Thus, the electric field lines on the two sides of the upperconductor of the Microstrip transmission line are oppositelydirected


The radiated electric field is linearly polarized in the planecontaining the feed.

The Microstrip antenna pattern follow the form as,

= cos

, = 0..(6)

= cos

, = 90(7)

This simple H-plane pattern expression is based on a symmetric

standing wave with uniform fieldsalong the radiating edges. As a result there are no cross-polarized fields,but in reality cross-

polarization can be significantin MSAs


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The input impedance at the edge of the resonant rectangularpatchranges from 100 to 400

An approximate expression for the input resistance (reactance iszero at resonance) of a resonant edge-fed patch is,

= 90

half -wave rectangular patch antenna .(8)

This formula reveals that input resistance decreases as the patchis widened.

For example, for a dielectric with = 2.2, a patch width tolength ratio of = 1 gives an input resistance of 363 .

A very wide patch with = 2.7is required to obtain an inputimpedance of 50 .


A narrow band antenna such as a Microstrip antenna is limited byimpedance match to the connecting transmission line.

As operating frequency moves off resonance, the patch inputimpedance increases rapidly, causing large mismatch.

The efficiencyof the MSA improves as the impedance mismatchis reduced, which is accomplished by widening the bandwidth.

An empirical formula for the bandwidth is,

= 3.77 1

where B is the fractional bandwidth relative to the resonantfrequency


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The final important parameter is radiation efficiency .

Radiation efficiency is as high as 95% for thin, low dielectricconstant substrates and decreases with increasing substratethickness.

The design process for a MSA focuses on optimizing performancevalues for the application at hand such as having an inputimpedance of a certain value (often 50 O), achieving a specifiedbandwidth, or having high efficiency.

The firststep is to findthe patch length L for resonance at thecenter frequency of the desired operating band using,

0.49 = 0.49

Next, the patch width is found by back solving,

= 90


for achieving a desired bandwidth,

If high efficiency is the goal, the following width is selected atresonance,

=

2

+ 1

2

The length is then recalculated using the more accurate formulasin (1), (3) and (4)

= 3.77 1

If the substarit is assumed to be PTEF (Teflon), = 2.1),then to obtain a 50 input impedance we need

= 0.3723,

since 0.49, we have = 1.316


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Techniques for feeding Microstrip patchantennas

Microstrip Line Feed

In this type of feed technique, a conducting strip is connected directly to

the edge of the microstrip patch


Matching patch antenna to 50 coaxial cable


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If is the Microstrip patch, is the coaxial cable or Microstripline and the impedance of the short quarter-wave lineconnecting the microstrip line and the patch should be,

= This special case is called the quarter-wavelength transform since

the load impedance istransformed (after a quarter wavelength)to the input impedance given by this simple equation.

Hence, the length of the transition should be

4 =

4

It provides ease of fabrication and simplicity in modeling as wellas impedance matching

as the thickness of the dielectric substrate increases the

bandwidth of the antenna hampers


Coaxial Feed OR Probe Feed

The Coaxial feed or probe feed is a very common technique used for

feeding Microstrip patch antennas.

The inner conductor of the coaxial connector extends through the dielectric

and is soldered to the radiating patch, while the outer conductor is

connected to the ground plane


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The main advantage of this type of feeding scheme is that the feed

can be placed at any desired location inside the patch in order to

match with its input impedance.

This feed method is easy to fabricate and has low spurious radiation Its major disadvantage is that it provides narrow bandwidth and is

difficult to model

Also, for thicker substrates, the increased probe length makes the

input impedance more inductive, leading to matching problems

As the probe from the patch edge, ,

is increased the input resistance decreases

as follows,

= 90

1


Example: Design a rectangular Microstrip antenna using a substrate(RT/duroid 5880) with dielectric constant of 2.2, t = 0.1588 cm so as toresonate at 2 GHz, taking into account the fringing, also calculate theachievable bandwidth and the input impedance.

=

=

3 1 0

2 1 0= 15

0.5 = 0.5

= 0.5

15

2.2= 5.06

For high efficiency the following width is selected at resonance,

=

2

+ 1

2

=15

2

2 . 2 + 1

2

= 5.929

=+ 1

2 +

1

2 1 +

10

.

=2 . 2 + 1

2 +

2 . 2 1

2 1 +

10 0.1588

5.929

.

= 2.133


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= 0.412+ 0.3

+ 0.264

0.258

+ 0.8

= 0.412 2.133 + 0.3

5.9290.1588

+ 0.264

2.133 0.258 5.929

0.1588+ 0.8

0.1588 = 0.084

= 5.06 2 0.084 = 4.892 (patch length include fringing)

= 3.77

1

= 3.77 2 . 2 1

2.2

5.929

4.892

0.1588

15 = 0.012


Note that 0.012 is the fractional Bandwidth

= 90

= 90 .

.

.

.

= 247.124

IfthePatchistobeconnectedtoa50ohmMicrostriplineusingthesamePCBboard,designthefeedtothisantenna.

The characteristic impedance of the transition sectionshould be

= = 50 247.124 = 111.16


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Then we use the following equation to determine the transition linewidth,

= 60ln 8

+

4

=60

2.2ln

8 0.1588

+

4 0.1588 = 111.16

By solving the above equation we get,

= 9.833 0.082

We take the shortest wdith


To obtain its length, we need to calculate the relative effectivepermittivity for the line as follows,

=+ 1

2 +

1

2 1 +

10

.

=2 . 2 + 1

2 +

2 . 2 1

2 1 +

10 0.1588

0.082

.

= 1.73

Hence, the length of the transition should be

4

= 4

= 154 1.73

= 2.85


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The width of the 50 Microstrip feed line can be found by,

=120

+ 1.393 + 0.667 ln

+ 1.44

L=4.892 cm

W=5.929 cm2.85 cm

WT=0.082

cmWm

Generally speaking, a /4 extension from the edge of the patch is

required for the ground plane.


Since the width of the quarter-wave transformer might be toothin to make properly in practice, an alternative design is toemploy an probe feed


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Example:A probe-fed rectangular patch antenna of 0.3-cm-thick dielectricof 4.53 dielectric constant is 1.74 cm long and 2.31cm wide. The resonantfrequency is 3.72 GHz.

(a) Calculate input impedance in Ohms for an edge feed.

(b) Calculate input impedance for a probe feed 0.55 cm in from the edge.

=

=

3 1 0

3.72 10= 8.065

a)

= 90

= 90 .

.

.

.

= 296.85

b) = 0.55 cm

= 90

1

= 296.85 0.55

1.74= 88.68

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