Senior Design Project Report

Design of Microstrip Antennas for GSM, Wi-FI, and GPS

Integration

by

Eduardo Vargas

In partial fulfillment of the requirements for the degree

of

Bachelor of Science in Engineering Physics-Electrical

May, 2011

Department of Engineering

The University of Texas at Brownsville

Brownsville, TX-78520

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CERTIFICATE

It is certified that the work presented in this report by Mr. Eduardo Vargas

(0264529) was done under my supervision.

Senior Design Advisor Date

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Acknowledgments

I would like to express my sincere appreciation and admiration to Dr. Fabio Urbani who having

been my professor and tutor for several times, became my advisor not only for the project

described on this report but also for my academic and professional goals.

I wish to thank also Dr. Sanjay Kumar for his concerned support during both of the semesters

that constituted the development of my senior design project.

I thank as well the immeasurable support of my mother, and the core values she taught me, that

were of key importance throughout this project.

Thanks to Tomas Villarreal, an exceptional research partner and a friend of mine for several

years.

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Abstract

The “Age of Communications” requires technology to be developed up to the extent where

mobile communication offers more than one service or capability, such as Internet access and

Global Positioning System (GPS) all integrated in a single lightweight portable device.

To comply with the requirements of portable multiple communication protocol enabled hand-

held devices, a multiband antenna solution is highly desired.

Multiband Antennas allow manufacturers to shrink the size, reduce weight, and lower cost of

wireless devices as well as offering higher flexibility toward more aesthetic designs. But most

important, they represent one step more in the miniaturization race, which benefits the

manufacturer, services provider and finally, the most important the consumer.

The focus of the proposed project is to investigate the possibility to integrate three different

wireless applications such as GSM, Wi-Fi, and GPS in portable devices. The aim of the project is

to design a microstrip antenna (MSA) that will allow for multi-band operations. MSAs that

operate in multiple frequency bands are required in many wireless communication devices. In

this project, we will explore the possibility of employing different geometry solutions to achieve

multiband MSAs. Several numerical simulations using HFSS™ will be performed; if funds will

be available, the ones that are more amenable to fabrication will be manufactured and tested in

the Applied Microwave & Electromagnetic Laboratory.

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Contents

ACKNOWLEDGMENTS ...................................................................................................................................... - 3 -

ABSTRACT ............................................................................................................................................................ - 4 -

CHAPTER 1 INTRODUCTION ........................................................................................................................... - 6 -

SECTION 1.1 BASIC THEORY OF ANTENNAS ..................................................................................................................... - 6 - SECTION 1.2 TWO MAIN QUALITIES OF ANTENNAS .......................................................................................................... - 7 - SECTION 1.3 RESONANT AND RADIATION CAPABILITY OF MSA’S ........................................................................................ - 7 - SECTION 1.4 REMARKABLE ADVANTAGES AND DISADVANTAGES OF MICROSTRIP ANTENNA TECHNOLOGY ................................ - 10 -

CHAPTER 2 DESIGN OBJECTIVES AND MULTIPLE DESIGN CONSTRAINTS .................................. - 11 -

CHAPTER 3 SOFTWARE AIDED DESIGN .................................................................................................... - 12 -

SECTION 3.1 FIRST MSA SIMULATIONS ....................................................................................................................... - 12 - SECTION 3.2 DESIGN OF “RECTANGULAR PATCH” GEOMETRY .......................................................................................... - 13 - SECTION 3.3 “RECTANGULAR PATCH” GEOMETRY SIMULATIONS ...................................................................................... - 15 -

CHAPTER 4 MANUFACTURING PROCESS ................................................................................................. - 18 -

CHAPTER 5 RESULTS ...................................................................................................................................... - 20 -

CHAPTER 6 CONCLUSIONS ........................................................................................................................... - 23 -

BIBLIOGRAPHY ................................................................................................................................................. - 24 -

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Chapter 1 Introduction

Section 1.1 Basic Theory of Antennas

An antenna is an electrical transducer which transforms electric currents in to electromagnetic

waves. In general, any flow of electric charges produces disturbances in the electromagnetic field

which propagate as a wave, with certain speed depending on the medium. The reverse process is

possible and accounts for the capability of antennas to receive electromagnetic radiation. In free

space, for instance, the speed is that of the light. This result is a consequence of Maxwell

Equations which govern all electromagnetic phenomena, and similarly, makes up the

fundamentals of antenna theory.

Over the years antennas evolved from simple metal sticks, properly called dipoles, to quite

complex structures, ranging from several meters to a few millimeters in dimension. This

diversity in antenna dimensions, extending as well over the design technology, has been

motivated mainly by the need of telecommunications to send and receive higher amounts of

information, which implies increasing the frequency of the carrier channel. The way the

frequency relates to the size of the antennas is in fact one of the consequences of Maxwell

Equations, which apply a constraint on the antenna structures for the optimal emission and

reception of electromagnetic waves. Thus, recalling the discussion from the previous paragraph,

the wave length (λ) of an electromagnetic wave in a medium is [4]:

In the equation, εr is the relative permittivity of the medium, ƒ is the frequency of the wave, and

c is the speed of light in free space. This equation states that as the frequency increases the wave

length decreases proportionally and so do the dimensions of the antenna emitting or receiving

such a wave. For comparison, figure 1.1 [1] shows multiple Microwave antennas and figure 1.2

[2] shows an AM radio broadcast antenna which deals with smaller frequencies, meaning larger

wave lengths.

Figure 1.1 Several microwave antennas

Figure 1.2 Radio broadcast antenna

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Section 1.2 Two Main Qualities of Antennas

In general there are two main measures that define and qualify an antenna, the Resonant

Capability and the Radiation Capability. The Resonant Capability is the most important

characteristic of any antenna. It determines the specific frequency of electromagnetic waves (EM

Wave) at which the antenna structure favors the establishment of the varying electric field lines

which induce a potential difference between isolated parts of the antenna structure. This

phenomenon occurs when the dimensions of the antenna structure allow half, full waves, or

several periods of them. Then, the varying potential difference that those EM waves originate is

emphasized from among the infinite number of frequencies traveling in the space.

The Radiation Capability relates, in a single subject, two other important characteristics of an

antenna. Readily, these two qualities are Antenna Efficiency (Antenna Gain) and Radiation

Pattern. Together, Antenna Efficiency and Radiation Pattern talk about the way antennas use the

energy supplied to radiate it all, in any or a particular direction, ideally, or most of it in the best

real life scenarios. Radiation Capability, despite the short description given in this paragraph,

involves a whole lot of branches of the Engineering, Physics, and Chemistry fields, for its

analysis and development of improving technologies. As for instance, important research and

experimentation is being performed in private and academic Microwave and Electromagnetic

laboratories about the implementation of Double Negative Materials (DNG Materials),

Metamaterials indeed, for the gain improvement and size reduction of Microwave Antennas.

Keeping the focus to this project, it is important to mention that the technology employed in the

design of the multiband antenna, MSA, has lately become much popular with in this group of

Microwave Antennas.

Section 1.3 Resonant and Radiation Capability of MSA’s

In a MSA, the resonant capability is very well explained through the cavity model. Where, in this

sense, the MSA is considered a rectangular cavity due to its fundamental structure. MSA’s

basically consist of a pair of metallic sheets separated by a dielectric material with a particular

relative permittivity εr.

The derivation of the resonance formulas is born from Maxwell Equations, which, combined in

the explanation of the electromagnetic waves, become a second order differential equation

evaluated all around the space. Then, for such equations, the assumption of the cavity model,

simplifies the solution by giving discrete boundary conditions. In a simplified way, the pair of

metallizations (Patch and ground plane) act like perfect electric conductors, regions where the

electric field equals E=0. The four walls delimiting the sides of the dielectric material act like

perfect magnetic conductors, regions where the magnetic field equals H=0. These conditions,

evaluated together with a steady state wave solution, yield a formula for the resonance frequency

per each dimension of interest. Figure 1.3 shows the field lines of different waves extending

inside the cavity.

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Figure 1.3

The formula for the resonance frequency along the width of the cavity is [4]:

In this theoretical description, the patch geometry is rectangular. And even though the same

physical principles are valid for any patch geometry, the formulas derived for the resonance

frequencies apply only for rectangular geometries.

The Radiation Capability of MSA’s is the result of two physical mechanisms. One of them

demonstrated in a quantitative fashion and the other through a well known electric phenomenon

which makes sense through a short description. The first mechanism may be suited accounting

for the different types of losses in the Microstrip. This quantitative method develops in the model

of a bounded lossy system where the principle of energy conservation restricts the net power

consumption. The mechanism derives an equation which defines the “Effective Loss Tangent”

δeff. Such equation consists of similar loss coefficients that account for the dielectric damping,

conductor loss, and the loss due to radiation. Theoretically and realistically, a loss mechanism is

necessary for the antenna to radiate, otherwise all the energy supplied would be reflected back

and none of it would be radiated. The equation is as follows [4].

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Where tanδ is the dielectric loss tangent, Δ is the skin depth of the conductor, ωr =2πƒr with ƒr

being the resonant frequency, and WT is the total energy stored in the patch at resonance. It is

important to note that for a known effective loss tangent δeff, a small dielectric thickness h would

yield a similarly small amount of power radiated Pr. This result reveals the intrinsic low gain

characteristic of most MSA’s.

The second radiation mechanism occurs due to the fringing electric field lines between the edges

of the patch and the ground plane. Just like the electric field lines vary between the patch and the

ground plane, the fringing electric fields will do so. Since these later field lines have smaller

components normal to the patch, the net propagation direction is upward. Thus, if it is desired to

increase the radiation intensity directed above the patch, the dimensions of the ground plane

should be enlarged. Figure 1.4 exemplifies this mechanism.

Figure 1.4 Radiation of a MSA due to fringing electric fields

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Section 1.4 Remarkable Advantages and Disadvantages of Microstrip

Antenna Technology

MSA’s, as discussed earlier, are intrinsically light and small sized elements that can be fabricated

with quite inexpensive materials, even in the retail market. Moreover, they are easily

implemented with Microwave Integrated Circuits, as they are popularly built on Printed Circuit

Boards (PCB’s). And, in favor of the project, they are capable of dual and triple frequency

operation, as explained on the previous paragraphs.

However, compared with most popular antenna structures for wireless applications, MSA’s

suffer from two parameters which make them not the first option if high enough gain and wide

band response is a requirement.

Nevertheless, these disadvantages vary from geometry to geometry, which opens the possibility

to employ computer aided simulations. Then, through a series of parametric analyzes, we may

find the most convenient shape and dimensions, depending on the set goals and expectations for

the project. This is in fact the approach followed in the design of the Multiband MSA.

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Chapter 2 Design Objectives and Multiple Design Constraints

The design of the multiband MSA, in this particular project, is aimed to develop a triple

frequency operation suited for the bands of Global Positioning Systems (GPS), Global System

for Mobile Communication (GSM), and Wireless Internet Systems certified by Wi-Fi AllianceTM

(Wi-Fi).

More specific technical objectives are listed as follows:

The MSA must work at 1.575, 1.9, 2.4 GHz

Return Loss has to be smaller or equal to -15 dB

The total gain must be greater or equal to 3 dB(±10% )

The Bandwidth must be below 100 MHz ±10%

To such well defined objectives arises one of the critical constraints which, is introduced as one

more of the objectives:

The design should yield a reasonable prototype to be manufactured after a period of four

months.

This objective introduced strict periods of time to be invested in understanding and mastering the

simulation software, as well as the technology employed in the design.

In regard to environmental limitations, the weather conditions that affect the response of any

outdoor wireless application are in general:

Temperature

Humidity

These, in a simple scope, may vary the relative permittivity of the substrate, directly changing

the speed of propagation of any wave in the antenna, as well as its radiation and gain capabilities.

During the fabrication stage two other constraints are taken in to account:

Budget constraint-expenses cannot surpass 300 dll. Environmental constraint-appropriate procedures must be followed to dispose of chemical

substances used in the Photolithography for the Printed Circuit Board (PCB) etching.

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Chapter 3 Software Aided Design

Three different patch geometries were explored through simulations in search for the desired

multiband behavior. The software employed for the simulations is HFSS. However, to avoid

divergence out of the main subject, results discussion is omitted in this paper, for those

geometries that did not achieved the desired multiband behavior.

Section 3.1 First MSA Simulations

The first geometry simulated was “Coupled C-Shaped Patch”. An illustration of this patch shape

and a similar type is shown on figure 3.1. This geometry, drawn from the research paper

“Analysis of High Gain Multiband C-Shaped and Coupled C-Shaped Microstrip Patch

Antennas” [6], presumed of having a multiband operation, but no such behavior was ever seen

from the simulations around the frequencies of interest. Further careful reading of the text

evidenced the fact that the multiband behavior happened at higher frequencies than those

targeted, so the geometry was abandoned.

Figure 3.1 “C-Shaped and Coupled C-Shaped” patch geometry

Followed search of Multiband MSA geometries turned out the research paper titled “Multi-Band

Equilateral Triangular Microstrip Antennas” [7], which stated having multiband response around

frequencies closer to those targeted. The patch form chosen for simulations is shown in

figure3.2. This paper gave the user the freedom to find the appropriate antenna feeding position

of convenience. However, after several simulations, no significant control of the resonances was

obtained. Thus, the geometry was discarded.

Figure 3.2 “Equilateral Triangular” patch geometry

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Section 3.2 Design of “Rectangular Patch” Geometry

The third prototype employed a more popular geometry, “Rectangular Patch”, which also

provided the analysis with simplicity. Empirical formulas for its design are commonly found in a

variety of academic resources such as text books and papers on Microwave and Electromagnetic.

The major approach followed on this design was creating a simple rectangular patch antenna to

provide the main resonant mode at the first frequency band, one whose overall radiation is not

critically affected by the addition of any slot inside the rectangular patch. A microstrip antenna

calculator was employed to calculate the patch dimensions [3]. Then, to confirm the results the

following formulas may be used [4]:

Width of the patch:

Length of the patch:

The previous results however, would serve only as an initial size because the behavior would

change as the inner geometry of the patch was edited in pursuit of the two remaining resonances.

Feeding method chosen for this prototype was a microstrip transmission line network. Such

network consisted of a set of strip lines, one for the port and the other for a quarter wave

transformer. This last one, being of great importance, is implemented in order to match the patch

impedance with the characteristic impedance of the port, chosen to be Z0=50 Ω as it is a

microwave engineering standard. Computations of the port and quarter wave transformer

dimensions may be done numerically with the formulas shown below [5]:

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However, for simplicity and speedy of results, it was relied on a web microstrip line calculator to

obtain the port and quarter wave transformer dimensions. The dimensions computed with such

calculator should be consistent with those achieved with the previous formulas since the web

application is loaded with the same equations.

For any microwave transmission line, the characteristic impedance Z0 is independent of the

length of the line as long as it does not measures quarter of a wave length or multiples of it;

rather it just plays the role of a microwave phase shifter. As this parameter is irrelevant in the

design of the port, a random value may be chosen for it. In this project the value given by the

calculator was lp= 25.3mm.

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Section 3.3 “Rectangular Patch” Geometry Simulations

After solidifying the main resonant mode of the antenna, it would be a matter of trial and error

finding the next two resonant modes by the introduction of slots. Nevertheless, slots are not

simply opened on the patch with random sizes. The length is rather chosen sufficiently small as

we expect to excite the structure at higher order modes, belonging to smaller wavelengths than

the main mode. That is why the patch itself, which provides with the main mode, is designed

with the smallest frequency of the three bands.

The first slots configuration simulated was “Pair of parallel vertical slots”. The opening of the

parallel vertical slots, as shown in figure 3.3, influenced the appearance of a second resonance,

just around the second band of interest, 1.9 GHz. A plot of the negative return loss (RL) is

shown in figure 3.4, exemplifying the type of resonance response obtained with this

configuration.

Figure 3.3 Pair of parallel slots configuration

Figure 3.4 Best negative RL with “Pair of parallel slots”

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In this configuration, any variation of the length of the slots resulted in a shift of the second

resonance solely. And it was observed though further simulations, that the addition of smaller

slots, beside set of vertical slots, did not affect the second resonance, significantly. Thus it was

concluded that the creation of some other slots, beside the set already there, should provide with

control over a third resonance. This fact leaded to the next configurations.

The second slots configuration, “H-Slot Patch”, rendered considerable control over the higher

order modes of the patch, allocated at frequencies around multiples of the main mode frequency,

1.575 GHz. A picture of the “H-Slot Patch” is illustrated in figure 3.5, and the best resonance

behavior of this configuration is demonstrated in figure 3.6 with a plot of the negative RL vs.

frequency.

Figure 3.5 “H-Slot Patch” configuration

Figure 3.6 Best negative RL achieved with “H-Slot Patch” configuration

However, variation of the length of the additional apertures, between the vertical slots, had a

limit. Further extension turned in to suppression of the third resonance, which was never

achieved at the third band of interest, 2.4 GHz. Sometimes, even an undesirable shift in the

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apparently solid second resonance was observed. Therefore, this particular configuration of the

additional slots was discarded.

Several other slots configurations were analyzed. Short time after, the right multiband behavior

was found with this geometry in the following slots configuration, “Pair of parallel vertical- pair

of patch top slots”, depicted in figure 3.7 with the actual dimensions indicated.

Figure 3.7 Rectangular patch “Pair of parallel vertical- pair of patch top slots” configuration

The negative RL achieved with this prototype is plotted in figure 3.8.

Figure 3.8 Best negative RL obtained with rectangular patch geometry

This multiband behavior reached a satisfactory precision toward the marked goals. Therefore, in

abiding the schedule constraint, the design stage was concluded with this result

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Chapter 4 Manufacturing Process To obtain realistic results out of the best geometry a prototype was manufactured. The process is

described as follows:

1. A drawing of the patch and the feeding network simulated in HFSS was done with the

actual dimensions. AutoCAD was employed for this purpose and the drawing is shown

on figure 4.1.

2. The drawing was later printed on photographic paper and the ink of the print was

stamped on the surface of a double-sided PCB through ironing; the ironing lasted ten to

fifteen minutes and its result is displayed in figure 4.2. The PCB should be cropped to the

size of the ground plane, preferably before ironing. And after removing the photo paper, a

permanent marker may be used to cover and line up the imperfections.

Figure 4.1 AutoCAD drawing

Figure 4.2 Antenna design stamped on PCB after

ironing.

The stamped PCB should be subjected to a Ferric Chloride (copper etching solution) bath. This

will dilute the unprotected copper layer leaving the stamped area intact. For this antenna the

process required a preliminary bath to remove must of the copper around the patch and feeding

network. The preliminary and definitive etching results are depicted on figures 4.3 and 4.4.

Precautions were taken throughout this process since the handling of Ferric Chloride solution, a

corrosive substance, may cause irritation if. Also, due to its main function, metal corrosion, a

neutralization process must be followed prior to disposal as it can deteriorate the drainage

plumbing.

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Figure 4.3 Preliminary etching result

Figure 4.4 Definitive etching result

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Chapter 5 Results

As stated in Chapter 1 the two main measures of interest of an antenna are the resonant and

radiation capability. Therefore, the results discussion begins with a comparison between the

simulated and experimental resonant capabilities. In figures 5.1 and 5.2 this characteristic is

exposed through plots of negative RL versus frequency, just like they were employed in Chapter

3.

Figure 5.1 Negative RL of simulations

Figure 5.2 Manufactured antenna measured negative RL

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The Return Loss is the dB scale representation of the reflection coefficient. Such quantity is

defined as “the amplitude of the reflected voltage wave normalized to the amplitude of the

incident voltage wave” [5]. Thus, when at a particular frequency the intensity of the reflected

voltage wave is reduced by a factor of |Γ|=√10 (less than a third of the incident intensity) or

less, the antenna is assumed to be resonant and in radiation, as a significant amount of the energy

driven in is lost in radiation and not reflected back to the port.

The manufactured antenna demonstrated having a resonant behavior very similar to the

simulated prototype, differing in frequency by at most less than a hundred mega Hertz in the

worst case. For magnitude, however, fidelity was not a constraint, thus with all the three main

negative RL dips falling below –RL= -10, the goal was achieved.

In regard of the radiation capability, figures 5.3-5.5 show computer simulation results with 3D

Polar Plots of the Gain in dB at the three actual resonant frequencies of the simulated antenna:

(The directions where higher gain is found are the best for either transmission or reception)

Figure 5.3 Radiation pattern at 1.538 GHz

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Figure 5.4 Radiation pattern at 1.904 GHz

Figure 5.5 Radiation pattern at 2.447 GHz

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Chapter 6 Conclusions

The design of a Multiband Antenna suited for operation at the frequency bands of the three

different systems to integrate, is possible employing MSA technology, despite the narrow band

and low gain disadvantages it suffers from. Computer simulations proved to be vital in finding

and handling higher order resonances. And despite the very narrow band feeding method

employed, the manufactured Multiband MSA demonstrated a strong resonant response at the

frequency bands of focus, which again evidenced high reliability of simulation results.

The Geometry employed for the Multiband MSA (Rectangular Patch), may be just one of the

various different possible. And, as several outcomes from the literature research showed, there

are other patch shapes which are reported to have good enough gain and multiband operation,

with even smaller patch sizes than the one exposed on this report.

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Bibliography

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[2] http://www.flickr.com/photos/tossmeanote/5405258759/

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Engineering”,http://www.emtalk.com/mpacalc.php?er=4.22&h=1.52&h_

units_list=hmm&fr=1.575&Operation=Synthesize&La=&L_units_list=Lmm&Wa=&W_

units_list=Wmm&Rin=

[4] C. A.Balanis, “Antenna Theory: Analysis and Design”, Third Edition, Wiley, April 2005.

[5] D.M. Pozar, “Microwave Engineering”, Third Edition, Wiley.

[6] Madany, Y.M.; Elkamchouchi, H.; , "Analysis of high gain multiband C-shaped and

coupled C-shaped microstrip patch antennas," Antennas and Propagation Society

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&arnumber=1551311&isnumber=33054

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Antennas", Recent Advances in Microwave Theory and Applications, 2008.

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