final kamran thesis

77
NATIONAL UNIVERSITY OF COMPUTER AND EMERGING SCIENCES Thesis By Kamran Zahid MS(10I-1123) A THESIS SUBMITTED TO THE FACULTY OF ELECTRICAL ENGINEERING IN FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER IN ELECTRICAL ENGINEERING ELECTRICAL ENGINEERING DEPARTMENT NATIONAL UNIVERSITY OF COMPUTER AND EMERGING SCIENCES ISLAMABAD October 2012

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Coverage Area Enhancement ofRFID System Using Patch Antenna Array

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Page 1: Final Kamran Thesis

NATIONAL UNIVERSITY OF COMPUTER AND EMERGING SCIENCES

Thesis

By

Kamran Zahid MS(10I-1123)

A THESIS

SUBMITTED TO THE FACULTY OF ELECTRICAL ENGINEERING

IN FULFILMENT OF THE REQUIREMENTS FOR THE

DEGREE OF MASTER IN ELECTRICAL ENGINEERING

ELECTRICAL ENGINEERING DEPARTMENT

NATIONAL UNIVERSITY OF COMPUTER AND EMERGING SCIENCES

ISLAMABAD

October 2012

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NATIONAL UNIVERSITY OF COMPUTER AND EMERGING SCIENCES

The undersigned certify the acceptance, a thesis entitled “Coverage Area Enhancement of

RFID System Using Patch Antenna Array " submitted by Kamran Zahid (EE -10I-1123)

in fulfilment of the requirements of the degree of Masters.

Supervisor, Respected Mr.KASHIF SIDDIQUE Assistant Professor,

Department of Electrical Engineering, FAST-National University, Islamabad.

Date

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ABSTRACT

The thesis is aims to design an improved RF front end for RFID reader systems to

enhance its coverage area over the conventional broad-beam, single-antenna readers. The

system is designed to operate at 2.4 GHz frequency (American standard for RFID). The

RF front end consists of a 1×4 circularly polarized micro-strip patch antenna array, three

Wilkinson power dividers and switched line phase shifters. The switching has been

accomplished using PIN diodes. The structure is developed on very low-cost FR4

dielectric substrate. The patch antenna array is a phased array optimized to achieve

maximum possible gain near 10 dB. The direction of the main beam of the phased array

is steerable in the range of ±28o. The design is simulated and optimized in antenna design

software HFSS 11. Practical measurements have been performed to characterize the

fabricated circuit. Results show an improved performance in terms of angular coverage

and range, over the conventional RFID systems. The design will be able to use in

commercial RFID reader systems.

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ACKNOWLEDGEMENTS

First of all I would like to thank ALLAH almighty for helping me and providing me

courage to complete this work. I offer my sincere gratitude to my supervisor, Mr Kashif

siddique, who has guided me throughout my thesis work with his valuable supervision. I

appreciate his knowledge, advice and skill in this regard. Without his persistent kindness

and efforts this work would not have been completed. One simply could not wish for a

better supervisor than him. I am also very thankful to all my teachers in the National

University of Computer & Emerging Sciences (NUCES FAST).

I would like to thank my elder brother Zeeshan Zahid for his valuable assistance,

encouragement and guidance throughout my studies. He never hesitated to provide

relentless support and motivation all the time. It was though his companionship that I

completed my Masters degree. No doubt, I would have been lost without him.

I specially acknowlegde my wife for her patience, understanding and time to complete

my thesis. She always held my hand in the time of worry and sorrow. No doubt she is the

best support for me.

Finally, I would like to thank my parents and grandparents. They have been a constant

source of support in each and every way of my life. Their love and prayers are value-able

assets for me. No doubt this thesis would not have existed without their prayers. I

dedicate this thesis to my parents.

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Dedicated to

My Parents

&

My Grand Father

~ Amanat Ali (Late)~

Who taught me simplicity and patience

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TABLE OF CONTENTS

Approval Page .................................................................................................................... .ii Abstract .............................................................................................................................. iii Acknowledgement ............................................................................................................. iv Table of Contents ............................................................................................................... vi List of tables ...................................................................................................................... vii List of Figures and illustration ......................................................................................... viii List of Symbols, Abbreviations and Nomenclature. .......................................................... ix

CHAPTER 1:INTRODUCTION ......................................................................................1

RFID Technology ................................................................................................................1

RFID Tags ............................................................................................................................2

Design of RFID ....................................................................................................................3 CHAPTER 2:ANTENNA DESIGN .................................................................................6

Single Patch Dimension .......................................................................................................6

Circular Polarization ............................................................................................................9

Patch Antenna And Circular Polarization ..........................................................................10 Circula Patch Antenna Design ...........................................................................................12 CHAPTER 3:POWER DIVIDER NETWORK ...........................................................18

Power Divider Network .....................................................................................................18

Wilkinson Power Divider Basics .......................................................................................18

Wilkinson Power Divider Advantages ...............................................................................23 CHAPTER 4:PATCH ANTENNA ARRAY .................................................................25

Phased Array RFID ............................................................................................................25

Phase Shifting ....................................................................................................................26

PIN Diode Switch ..............................................................................................................28

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PIN Diode SMP1345.........................................................................................................32

C Series Capacitors (Type: C1005,C1608)........................................................................33

Design Sequence ................................................................................................................34

Efficiency of proposed antenna .........................................................................................41

CHAPTER 5:CONCLUSIONS AND RECOMENDATIONS ....................................45

Future Work .......................................................................................................................46

APPENDIX-I:Matlab Codes ...........................................................................................47

Code for Array Factor ........................................................................................................48

Code for patch ...................................................................................................................49

Code for the transmission line ..........................................................................................50

Code for theoretical array pattern......................................................................................50

APPENDIX-II: Data Sheets ............................................................................................53

Pin Diode data sheet...........................................................................................................54

Capacitor data sheet ...........................................................................................................59

REFERENCES ..................................................................................................................64

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List of Tables

Table 2.1: Calculated dimensions.......................................................................................7

Table 2.2: Single truncated Patch calculated dimensions..................... ...........................13

Table 3.1: Comparison of different passive power divider...............................................24

Table 4.2: Efficiency of Proposed Design........................................................................41

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List of Figures and Illustrations

Figure 1.1 : RFID tag with antenna and IC chip ................................................................2

Figure 1.2 : Internal sections of RFID tag’s IC .................................................................3

Figure 2.1 : RFID antenna with Phase delay input using divider network ........................6

Figure 2.2 : Single patch Antenna......................................................................................8

Figure 2.3 : Return Loss plot of single patch Antenna ......................................................8

Figure 2.4 : 3D plot of Gain of single patch Antenna ........................................................9

Figure 2.5 : Electric Field vectors of circularly polarized................................. ..............10

Figure 2.6 : Dual-Feed circularly polarized Patch antenna ..............................................11

Figure 2.7 : Single Feed circularly polarized Patch antenna ...........................................11

Figure 2.8 : Single truncated patch Antenna ....................................................................12

Figure 2.9 : Return Loss plot of single patch Antenna ....................................................13

Figure 2.10 : Manufactured single patch Antenna .............................................................14

Figure 2.11 : VNA Return Loss plot of single patch Antenna...........................................14

Figure 2.12 : Circularly polarized wave and targeted tag ..................................................15

Figure 2.13 : Port Impedance of Circularly polarized Patch Antenna ...............................16

Figure 2.14 : Axial Ratio of Truncated Patch ....................................................................17

Figure 2.15 : 3D Gain plot of single truncated patch Antenna .........................................17

Figure 3.1 : Wilkinson power divide ...............................................................................18

Figure 3.2 : A two way Wilkinson power divider ...........................................................19

Figure 3.3 : Wilkinson power divider in HFSS ...............................................................20

Figure 3.4 : Manufactured Wilkinson power divider .......................................................20

Figure 3.5 : (a) Wilkinson Power divider testing with RF signal generator and

spectrum analyzer (b) Magnified view of the circuit .........................................................21

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Figure 3.6 : Wilkinson Power divider results (a) S11 Plot of 1×4 divider (b) S12 Plot of

1×4 divider (c) S13 Plot of 1×4 divider (d) S14 Plot of 1×4 divider (e) S15 Plot of 1×4

divider................................................................................................................................22

Figure 4.1 : Feeding techniques of Patch array (a) Series feed (b) Corporate Feed ........25

Figure 4.2 : Relationship between radians (2π), degrees (°), wavelength (λ) and

phase shift (β).. ...................................................................................................................27

Figure 4.3 : Switched line Phase Shifters .......................................................................28 Figure 4.4 : PIN diode switch in series configuration ....................................................29 Figure 4.5 : PIN diode switch series configuration in ADS (Forward Biased) ..............29 Figure 4.6 : ADS results of PIN diode switch (Forward Biased) ...................................30 Figure 4.7 : Manufactured PIN diode switch in series configuration .............................31

Figure 4.8 : Switching circuit testing and results (a) Testing with RF signal generator

and Spectrum analyzer (b) Magnified view of switch circuit(c) VNA result S21

forward biased (d) VNA result S12 reverse biased ...........................................................31

Figure 4.9 : SC-79 PIN diode .........................................................................................32 Figure 4.10 : SC-79 PIN diode typical performance ........................................................33 Figure 4.11 : Equivalent model of Pin Diode ...................................................................33

Figure 4.12 : C Series Capacitors (Type: C1005, C1608) ................................................34

Figure 4.13 : Complete Patch phased array with Wilkinson divider ...............................35 Figure 4.14 : Steering of pattern ......................................................................................36 Figure 4.15 : Two different states of array design (a) State 1 (b) State 2 ........................36 Figure 4.16 : Current distribution on the patch at 2.4 GHz .............................................37

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Figure 4.17 : Theoretical pattern of array .........................................................................37

Figure 4.18 : Return Loss plot of Antenna array with Wilkinson Divider (a) state1 (b)

state 2..... ............................................................................................................................38

Figure 4.19 : 2D-Gain plots with beam steering(a) state1 (b) state 2 ...............................39

Figure 4.20 : Axial ratio of complete array .......................................................................39

Figure 4.21 : 3D-Gain plots with beam steering(a) state1 (b) state 2 ...............................39

Figure 4.22 : Manufactured patch Antenna array with Wilkinson Divider ......................40

Figure 4.23 : VNA testing of Manufactured patch Antenna .............................................40

Figure 4.24 : VNA Return Loss plot of patch Antenna array (a) state1 (b) state 2 ..........42

Figure 4.25 : Anechoic radiation pattern of patch Antenna array in state 1 (a) 3D (b)

2D........... ............................................................................................................................43

Figure 4.26 : Anechoic radiation pattern of patch Antenna array in state 2 (a) 3D (b)

2D........... ............................................................................................................................44

Figure 4.27 : Final design with complete dimensions .......................................................44

Figure 5.1 : Array Factor plots with beam steering .........................................................49

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List of Symbols, Abbreviations and Nomenclature

RFID Radio Frequency Identification

HPBW Half Power Beam Width

HFSS High Frequency Structural Simulator

RF Radio Frequency

IC Integrated Circuit

AC Alternating Current

DC Direct current

CP Circular polarization

LCP Left-handed circular polarization

RCP Right-handed circular polarization

FET Field Effect Transistors

MEMS Microelectromechanical Systems

LNB Low-noise block

WLAN Wireless Local Area Network

VNA Vector Network Analyzer

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CHAPTER 1: INTRODUCTION

Radio frequency identification (RFID) technology was first developed in 1920’s. In

World War II, similar technology was used by British to identify jet planes as “friend or

foe”. In 1948 Harry Stockman first explored the vast potential for RFID technology.

Mario Cardullo presented a patent in 1973, who was the first ancestor of latest and

modern RFID. The first patent to be associated with the abbreviation RFID was granted

to Charles Walton in 1983.

RFID is the technology that uses electromagnetic coupling in the radio frequency to

identify different objects. RFID is a part of automatic identification systems. These

systems are more popular in business and commercial areas. It is a recognition method

relying on receiving and storing the data remotely.

RFID TECHNOLOGY

RFID technology is based on modulation that is used by manipulating the sequence at

which the reflection occurs. The design of RFID tags are made in such a way that it

reflects the RF signals that are interpreted in the form of digital data, called information.

RFID systems work at a specified frequency bands. A frequency band is a range of

frequencies that are much close to center frequency. This can be explained as the 915

MHz frequency band has the set of frequencies having range from 902 MHz to 928 MHz,

which have fifty (50) channels that are used in communication. The frequency bands that

are widely used in these days are 2.4 GHz, 915 MHz, 868 MHz, 13.56 MHz and 125

KHz. This Thesis focuses on American standard of RFID at 2.4 GHz but the principles

will be the same for other RFID bands [2].

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In RFID technology, the RFID system communicates over the channel frequency (e.g. 2.4

GHz) which is known as carrier wave, because it is used to carry data. RFID tag operates

in specified range of frequencies (carrier frequencies). So the RFID tag is able to absorb

and reflect back the energy to the source at the operating range of frequencies [2].

RFID TAGS

RFID tag has an IC Chip that is connected to very small antenna. There are two major

types of RFID tags, active and passive. Passive tags are used very commonly, because

there is no need of internal power (battery). These tags take the power from the carrier

wave that is transmitted by interrogator. RFID tag is shown in the Figure 1.1.

An antenna senses the modulated wave which is radiated by the interrogator. The carrier

wave generates the small current in the antenna. In the IC Chip, regulator and power

rectifier converts the alternating current (AC) to direct current (DC) and uses it to drive

the IC chip [2].

Figure 1.1: RFID tag with antenna and IC chip

In the IC chip there are different sections e.g. clock extractor, logic, memory and

modulator etc. From the carrier wave, clock extractor takes the clock pulses and uses

these pulses to synchronize the sections like modulator, memory and logic. The main

function of logic section is to separate the 0’s and 1’s from the carrier wave. It also

compares the data with its internal program to get the desired response. If logic section

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approves that the data is valid then it gets the access to memory. Clock extractor pulses

are used by the logic section to encode the data. After that, the data stream is fed into the

modulator. The modulator mixes the carrier wave with the data stream [2]. Internal

sections of RFID tag is shown in the figure 1.2.

Figure 1.2: Internal sections of RFID tag’s IC

DESIGN OF RFID

RFID systems are being used in large number of areas. Due to the small size of tags and

antennas in RFID systems, wide coverage area and long range operation are troublesome.

In this scenario to enhance the coverage area of an RFID system, one approach is to

implement multiple antennas to cover the necessary area which is a complex and costly

approach. The other technique is to use the phased array system. Higher gain can be

achieved by using array of antennas for single RFID reader, which also covers the larger

area [2]. This work proposes phased array patch antenna system for enhancing the

coverage range of RFID system.

Conventional RFID readers have fixed antenna patterns and they have very small

coverage area. In this work, the main approach is to make an efficient RFID reader that

will steer its beam in two different directions. To cover larger area than a conventional

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systems 1x4 patch antenna array is used along with Wilkinson power divider and delay

line phase shifters [3].

The main focus will be on the design, tests, and fabrications of the RFID system. The

feeding network of Patch antenna array is quarter wavelength transmission line-fed type,

utilizing Wilkinson power divider. Wilkinson power dividers are very important for

antenna array systems and act as power splitting networks. The Wilkinson power divider

is a device that divides power among n output ports with equal distribution, keeping

equal path lengths from input to output ports. Two ways power dividers allow the use of

multiple stepped sections to meet the design requirements [6].

If antenna impedance and gain requirements are not fulfilling the coverage performance

then it suffers. Similarly, if the size, ease of assembly and material cost of the RF front

end are not reliable, then the device customer will never use the said system in their

product. That is why to make an efficient RFID antenna engineer should have to be more

careful in designing such wireless products. Therefore in this design, the parameters are

selected to fulfil all the criteria to make a useful product.

Moreover, the results of the proposed design will not only be shown in simulation but

measured results as well. Finally, there is a final hardware design product for commercial

purpose. For designing and simulation HFSS 11, Matlab and ADS 2008 software are

used.

ADVANTAGES

The proposed design of RFID front antenna has many advantages. It has good RF signal

transfer and has the ability to charge tag coil efficiently. This is cost effective and robust

design. The system can be used in tracking of goods, persons and airport baggage, access

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management, contactless payment toll collection and authentication. Moreover, it can

also be used in the massively distributed sensor networks, WLANs, personal

communication (Bluetooth, Zig-Bee etc.) and machine readable travel documents.

TDMA technique can also be used to avoid data collisions among these communicating

devices that are working at the same frequencies. It is widely used in military

communication system.

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CHAPTER 2: ANTENNA DESIGN

SINGLE PATCH DIMENSIONS

The patch antenna is one of the types of antenna, having low profile that can be placed on

a flat surface. It consists of a rectangular metallic sheet of length L and width W and h is

the height of the substrate with the microstrip transmission line placed over a larger sheet,

which is called a substrate as shown in the Figure 2.1.

Figure 2.1: Single patch Antenna

The length of the sheet is approximately one half wavelengths of the radio waves. Before

designing the patch, the dimension is calculated by using its mathematical equations. Due

to the radiations at edges, the patch antenna acts electrically larger than its physical

dimensions. So length ∆L is included in its dimensions. Patch antennas are relatively

inexpensive as compare to other antennas [8]. Single patch antenna has the ability to

provide a maximum gain of around about 6 - 8 dB. It is very easy to make an array using

Patch antennas that are low profile, compact in size that can be easily integrated in RF

devices. They are easy to modify, simple to fabricate and customize [17]. The calculated

dimensions of patch are given in the table 2.1.

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The general design equations are as follows;

22

901

rr

r

LRW

εε⎛ ⎞⎛ ⎞= ⎜ ⎟⎜ ⎟− ⎝ ⎠⎝ ⎠

290( 1)

r

r r

W LR

εε

=−

, o

r

0.49L λε

=

Table 2.1: Calculated dimensions

Parameters Dimensions

Width (W) 3.88cm

Length (L) 2.88 cm

Height (h ) 1.6mm

Permittivity (εr) 4.4

Inset Feed Point Distance (Yo) 0.625 cm

Effective dielectric constant (εreff) 3.59

Extended Incremental length (∆L) 0.238 cm

Width of feed line (Wo) 0.26 cm

Ground Plane Size 13.5 x 13.5 cm2

Loss Tangent (FR4) 0.02

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Figure 2.2: Single patch Antenna

The radiations from discontinuities arise at edges of the micro strip patch. It is

constructed on a dielectric substrate, so for this design FR4 epoxy substrate is used. The

operating frequency of the patch is 2.4 GHz. The design is simulated in the HFSS and the

results are given in the Figure 2.3 and Figure 2.4.

Figure 2.3: Return Loss plot of single patch Antenna

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Figure 2.4: 3D plot of Gain of single patch Antenna

CIRCULAR POLARIZATION

Basically, there are three types of polarization that are horizontal polarization, circular

polarization and elliptical polarization. In RF communication, the polarization is very

important in antenna designing. For example horizontally polarized antenna does not

communicate with the antenna that is vertically polarized. Because, vertically polarized

antenna radiates and receives in vertically polarized fields. Similarly horizontally

polarized antenna always communicates with a horizontally polarized antenna. Antenna

radiates and receives in exactly the same manner due to the reciprocity theorem.

So it is more feasible for the antenna to communicate with other antennas or radiating

material if the reader is circularly polarized. There will be very less chance to miss the

target.

Figure 2.5: Electric Field vectors of circularly polarized wave

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Circular polarization is a polarization in which the electric field of the passing wave does

not change strength but only changes direction in rotation as shown in Figure 2.5. The

direction of an electric field is defined by an electric field vector. In circularly polarized

wave the tip of electric field vector describes a circle as time proceeds, at a given point in

space. If wave is frozen the electric field vector describes circle or helix along the

direction of propagation.

Circular polarization can be left-handed circular polarization (LCP) or right-handed

circular polarization (RCP). These types of polarization can be obtained by using proper

shape of the antenna and feeding method.

PATCH ANTENNA AND CIRCULAR POLARIZATION

It is also possible to fabricate patch antennas that radiate circularly polarized waves.

There are different approaches to make the patch antennas circularly polarized.

First approach is used to excite square patch using two input feeds. In the dual feed

method 90° delay is given in one feed with respect to the other as shown in Figure 2.6. In

doing this the horizontal current becomes zero and vertical current will be maximum. So

the electric field radiated will be vertical, similarly after one quarter cycle the situation

will reverse and field will be horizontal. At the end, the radiated field will rotate in time

that will produce circularly polarized wave.

Second approach is single feed method that introduces different types of asymmetric slots

or other features on the patch that causes current distribution in the patch. A patch which

has perturbed slightly to produce a rectangular microstrip patch antenna can be driven

along a diagonal and produce polarization as shown in Figure 2.7 [7]. At driving point of

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patch one mode is +45ºand the other mode -45º to produce 90º phase shift for circular

polarization.

Instead of rectangular patch, circular patch antennas can also be used for such techniques.

Circular patch antenna does not radiate circular polarized waves. A single feed circular

patch creates linear polarized radiation [11]. Moreover the circular patch can be perturbed

into an ellipse and fed properly it can be used as circularly polarized antennas as shown

in Figure 2.6 [7].

Figure 2.6: Dual-Feed circularly polarized Patch antenna

Figure 2.7: Single Feed circularly polarized Patch antenna

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Circular polarization can be achieved by feeding patch with two orthogonal input signals

that have different directions and using the patch antenna right in between the two input

signals [6]. It is very important that the two input modes are energized equally, with the

phase difference of 90°. Such a scheme is slightly complex to make the input signal that

are different to each other. So in this research work the design that is preferred is “corners

truncated patch”, which is shown in Figure 2.8. There is no need of dual input or

asymmetric perturbed parts of the metallic patch sheet. In such technique, by cutting two

corners off the patch element the circular polarization can easily be achieved.

CIRCULAR PATCH ANTENNA DESIGN

Circular polarization has the large number of advantages that is why in this thesis the

corner truncated design technique is used. There is no complexity in this design as other

techniques have. It is very easy to implement [11].

Figure 2.8: Single truncated patch Antenna

The calculated dimensions of patch are given in the table 2.2.

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Table 2.2: Single Truncated Patch Calculated dimensions

Parameters Dimensions

Width (W) 2.6 cm

Length (L) 2.6 cm

Height (h ) 1.6mm

Permittivity (εr) 4.4

Mitered edge 0.48cm

Effective dielectric constant (εreff) 3.59

Extended Incremental length (∆L) 0.238 cm

Width of feed line (Wo) 0.135/0.26 cm

Ground Plane Size 13.5 x 13.5 cm2

Loss Tangent (FR4) 0.02

The S11 curve taken by testing the patch in HFSS 11 is shown in Figure 2.9.

Figure 2.9: Return Loss plot of single patch Antenna

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The truncated patch antenna is also manufactured and tested using Agilent 8362B VNA.

Agilent 8362B VNA has the ability to test the devices or RF modules in the frequency

range up to 26GHz. The manufactured truncated patch antenna is shown in figure 2.10

and VNA result is shown in figure 2.11.

Figure 2.10: Manufactured single patch Antenna

Figure 2.11: VNA Return Loss plot of single patch Antenna

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Circular polarization plays a very vital role in RFID communication. Tag antenna can be

intended to take complete benefit of the circularly polarized reader antenna. The target

tag will be accessible whether it is placed in any direction. It will be extremely responsive

to all phases of circular polarization. This can be represented in the figure 2.12

Figure 2.12: Circularly polarized wave and targeted tag

The quality of circular polarization is quantified as the axial ratio (AR) which is

expressed in dB. It is the ratio of orthogonal components of an E-field. As we know that,

due to the two orthogonal E-field components of equal amplitude, the circularly polarized

field is made-up and these field components have equal magnitude. Therefore, the axial

ratio will be equal to 1 or 0 dB. Moreover the port impedance is also set to 50 ohms that

is shown in Figure 2.13.

Usually axial ratio of 3 dB is considered enough for most applications [14]. The axial

ratio has an optimum value of 0 dB in between resonance frequencies.

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Figure 2.13: Port Impedance of Circularly polarized Patch Antenna

In the direction of z-axis the axial ratio is optimal at broadside and towards lower

elevations it degrades. The antenna geometry plays a very important role so the degree of

degradation is much dependent on it [6]. Axial ratio measurements are very important

when dealing with circularly polarized antennas. For circularly polarized fields, the ideal

value of axial ratio is 0 dB. From the main beam of an antenna, axial ratio always tends to

degrade away.

Axial ratio can also be defined and given as; Axial ratio = major axis (Emax) = 20 log [Emax/Emin]

minor axis (Emin)

The axial ratio that is calculated in HFSS is shown in figure 2.14.

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Figure 2.14: Axial Ratio of Truncated Patch

The 3D-gain plot of patch is given in figure 2.15.

Figure 2.15: 3D Gain plot of single truncated patch Antenna

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Chapter 3: POWER DIVIDER NETWORK

POWER DIVIDER NETWORK

The Wilkinson power divider was invented by Ernest Wilkinson. It divides the input

signal to equal halves of the output signals and also have the ability to combines two

equal signals into one in the opposite direction. The Wilkinson power divider is shown in

the Figure 3.1. For matching the split ports to the common port this network has quarter

wave transformers [6].

Figure 3.1: Wilkinson power divider

An ideal Wilkinson power divider has 100% efficiency. Firstly, the 1×2 divider is

designed after checking its results the 1×4 divider is designed. Wilkinson power dividers

are widely used these days because of its efficiency and better isolation between the

ports. On the other hand, the conventional resistive dividers have no such type of

advantages.

WILKINSON POWER DIVIDER BASICS

Wilkinson power divider concept can also be used for an n way system. It is very easy to

check how it works as two way system. Quarter wave transformers is used in Wilkinson

power divider that split input signal to two output signals that are in phase with each

other [18] [20].

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Figure 3.2: A two way Wilkinson power divider

The purpose of the resistor is to provide the isolation between the output ports at the

operating frequency. It enables the two output ports to be matched that provides isolation

also. Ideally, there is no resistive loss in the power divider network or dissipate any

power, as a result the divider can be lossless. In practice there are some losses also

present, but these are very low [19].

The values of the components of the Wilkinson divider can be calculated as follows ;

R = 2 ×Zo

Zmatch = SQRT 2 ×Zo

= 1.414 ×Zo

Where,

R = Resistor connected between the ports

Zo = Characteristic impedance of system

Zmatc = Quarter wave transformers impedance

Now, we see how this divider works. Consider that, a signal entering the port 1 in the

previous diagram. The signal arrives the physical split and enters in port two and three of

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divider. The two arms of splitter are identical, so the signals entering in the ports 2 and 3

have the same phase. It means that ports 2 and 3 are at the same potential and there will

be no current flow in the resistor. The wilkinson power divider is shown in the figure

3.2, 3.3 and 3.4. It is very necessary to check that the impedances of the divider are well

balanced and matched. In achieving this, the two ports 2 and 3 must appear equal to the

impedance of (2 Zo). Now, impedance transformation can be achieved by setting the

transmission line of quarter wave between the input and the output. The impedance of

transmission line is 1.414 Zo. The Wilkinson divider is widely used in RF applications

and it is an ideal form of divider.

Figure 3.3: Wilkinson power divider in HFSS

Figure 3.4: Manufactured Wilkinson power divider

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(a) (b)

Figure 3.5: (a) Wilkinson power divider testing with RF signal generator and

Spectrum analyzer (b) Magnified view of the circuit

The design is simulated in the HFSS and the results are given in the Figure 3.5.

VNA results are shown in dotted line.

(a) (b)

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(c) (d)

(e)

Figure 3.5: Wilkinson Power divider results (a) S11 Plot of 1×4 divider (b)

S12 Plot of 1×4 divider (c) S13 Plot of 1×4 divider (d) S14 Plot of 1×4 divider

(e) S15 Plot of 1×4 divider

Ideally, each output port of the 1×4 Wilkinson divider should be at -6 dB. But the

measured result shows that these values vary from -7.7 to -7.5 dB. The S11 value is -19.8

dB which meets the required value.

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WILKINSON DIVIDER ADVANTAGES

Wilkinson Divider has the following advantages that is why it is widely used in RF

equipments.

Advantages:

• Simplicity: The Wilkinson divider can easily be realised on a printed circuit

board using printed components and it has very simple structure. Lumped

capacitor and inductor elements can also be used, but this complicates the design.

• Cost: When The Wilkinson divider is realised on a printed circuit board using

printed components, the cost will be very low. The only increase in cost is due to

the single resistor that also increases the board area. To overcome such problem

printed elements can be used. Moreover, to minimize losses, lossless substrate

may be used that also increases the cost.

• Loss: The Wilkinson divider does not introduce any additional loss if perfect set

of components are used

• Isolation: The Wilkinson divider provides high isolation among the output ports.

• Moreover advantages and disadvantages with other divider networks [6] can be

shown in the table 3.1.

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Table 3.1: Comparison of different passive power divider

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Chapter 4: PATCH ANTENNA ARRAY

PATCH PHASED ARRAY RFID

In conventional RFID system fixed antenna pattern is used that operates in confined area,

because of greater HPBW and lower gain [1]. Multiple antennas are required to cover

larger area. So in this purpose high input power is needed that increases the overall cost.

To overcome such draw backs the technique used in this thesis is Phased array. In

achieving the phased array we should have to be more careful in feeding the patch.

There are two main feeding techniques used in these days first one is series feed and

second is corporate feed, this can be shown in Figure 4.1. In this design, corporate feed

technique is used. As “Phased array is a set of radiating elements in which the phases of

the respective input signals are changed in such a way that the overall radiation pattern of

the array can be steered in a desired direction”.

So, phased array RFID has beam steer advantage by giving the phase delay in the

transmission line by using corporate feed technique [5] [12].

Figure 4.1: Feeding techniques of Patch array (a) Series feed (b) Corporate Feed

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PHASE SHIFTING

Phase Shifters are the devices that can change phase of an electromagnetic wave, when it

propagates through the transmission line. Phase Shifters are widely used in beam forming

networks, phase discriminators, power divider networks and phase array antennas. Before

going to the circuit demonstration first we have to understand the relationship between,

phase shift, propagation constant delay, and wavelength.

In the transmission line propagation constant is a complex number having two main

parts:

(1) The real portion α which is the attenuation constant (where α is neper / unit length).

(2) Similarly, the imaginary portion (βx) is the phase constant (where β is radians / unit

length)

The α (attenuation constant) determines the way a signal is reduced in amplitude as it

propagates down the line and the β (phase constant) describes the phase difference in the

voltages at the sending end and at distance x of the line. The β(x) (phase constant)

represents the phase shift of the current or voltage at a distance x with respect to sending

current or voltage along the transmission line. Phase shift of 2π radians (360°) is equal to

one wavelength as shown in Figure 4.2. From the figure it is clear that λ = 2π / β.

Moreover, by giving the proper length in delay line the phase shift can be adjusted

accordingly. The phase shift and the time delay can be relate as follows:

Time delay = Phase Shift (°) / 360 × frequency (Hz)

In the proposed design switched line approach is used. It is the most straight forward and

uncomplicated approach to provide phase shift between two paths. It uses the simple

difference of time delay. For switching in phase shifters PIN diodes, mechanical switches

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or relays, microelectromechanical systems (MEMS) and Field Effect Transistors (FET)

are commonly used.

Figure 4.2: Relationship between radians, degrees (°), wavelength (λ) and phase

shift (β)

The standard phase shifter is used by setting switched line segments, having different

path of different lengths. The phase shifter (or switched line phase shifters) is totally

dependent upon the lengths of the micro-strip line. Two transmission lines are used to

feed a single patch, one of the two transmission lines is known as a reference line and the

second one is a delay line as shown in Figure 4.3. In the figure the L1 is the reference line

while L2 is the delay line.PIN diodes can be placed at points P1 and P2. An advantage of

such a circuit is that phase shift will be linear.

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Figure 4.3: Switched line Phase Shifters

PIN DIODE SWITCH

In the phase shifters PIN diodes are commonly used because of their high switching time,

speed response, low loss and simple bias circuits. Classification of PIN diodes switched

line phase shifters based on the following points:

(1) Number of bits used in the circuit.

(2) Type of the transmission line (regular, irregular, and coupled)

(3) Structure ( reflecting or nonreflecting)

(4) No. of switched inputs and outputs (SPDT, SPST, SP3T, etc.)

(5) Connection of PIN diode with transmission line (shunt, series, series-shunt)

(6) Bandwidth

(7) Element configuration

A PIN diode can be used in shunt and series configuration for single RF switch. But in

this thesis series configuration has been used. As shown in the following Figure 4.4.

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Figure 4.4: PIN diode switch in series configuration

The Switch will be on when diode is forward biased. The input power will be reflected

back if the switch is off. Capacitors that are used as DC blocker should be very low

impedance at the operating frequency and RF choke inductors should have very high RF

impedance. To get better result the above mentioned design is simulated and tuned in

ADS as shown in Figure 4.5 and results are shown in Figure 4.6[10].

Figure 4.5: PIN diode series configuration in ADS (Forward Biased)

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Figure 4.6: ADS results of PIN diode switch (Forward Biased)

The switch should have zero insertion loss in forward biased and maximum attenuation in

the off state. In practice, the switching element results in some insertion loss in on state

and finite attenuation in off state [10]. The insersion loss of the circuit in series

configuration can be calculated as follows,

IL -20

Zd , is the diode impedence for reverse and forward bias and can be given as follows;

Insersion losses in both on and off states can be improved by adding the external

reactances. Inductors can also be replaced with the quarter wave transmission line when

operating at high frequencies. So transmission lines are used instead of inductors. PIN

diode driver circuit is manufactured as shown in Figure 4.7 and tested by using RF signal

generator, RF spectrum analyzer and Agilent VNA. Results are shown in Figure 4.8.

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Figure 4.7: Manufactured PIN diode switch in series configuration

(a) (b)

(c) (d)

Figure 4.8: Switching circuit testing and results (a) Testing with RF signal generator

and Spectrum analyzer (b) Magnified view of switch circuit(c) VNA result S21

forward biased (d) VNA result S12 reverse biased

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Ideally, the insertion loss should be at the 0 dB. By using RF signal generator, Spectrum

analyzer and VNA, the measured result shows -2.2dB, this is acceptable insertion loss. It

is due to the material and manufacturing error. In the reverse biased state the measured

value of insertion loss is -15.78 dB.

PIN Diode SMP1345

The SMP1345 surface mountable PIN diodes are used in applications like WLAN, LNB

and many other RF switching applications in frequency range from 10 MHz to 6 GHz.

The short carrier lifetime of the diode is 100 ns. It is because of thin I-region, having the

width of 10 µm. It helps in fast switching of PIN diode. The performance PIN diode is

guaranteed because of its very low resistance 1.5 Ω at 10 mA and low capacitance 0.15

pF. Some very important specifications and of PIN diode is shown in Figure 4.9, 4.10.

Figure 4.9: SC-79 PIN diode

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Figure 4.10: SC-79 PIN diode typical performance

According to the above specification we can define the values of components in the

equivalent model of diode in reverse as well as in forward bias [10]. The equivalent

model of diode is shown as follows;

Figure 4.11: Equivalent model of Pin Diode

The required values of Li=0.7nH, Rf = 1.5 Ω and C = 0.12 pF at 10 mA are taken from

the data sheet.

C SERIES CAPACITORS (TYPE: C1005, C1608)

These capacitors are widely used in mobile communications equipment, electronics

equipment, high frequency RF modules, Matching and coupling circuits, test or

measurement equipments and tuning circuits. Some important specification of this

product is given as follows;

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Different shapes of the capacitors are given in figure 4.12.

Figure 4.12: C Series Capacitors (Type: C1005, C1608)

DESIGN SEQUENCE

In the design sequence, firstly the single patch antenna is designed. After checking its

results and simulations the 1×4 patch array is designed. Now there is a need of proper

feeding network, for that purpose 1×2 Wilkinson power divider network is designed that

is extended to 1×4 Wilkinson power divider network. The power divider network is then

used as the feed network of the complete Array. The overall design is shown the figure

4.13.

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Figure 4.13: Complete phased array

For proper beam steering towards desired direction progressive phase shift technique is

used. The amount of phase shift required is obtained by writing the code of array factor in

MATLAB. The delay line phase shifts is switched by using the switching circuitry where

the proper length of delay line is employed. Capacitors are used in the transmission line

to block the DC components. All the design parameters are chosen according to their

mathematical model.

The scheme is used for concurrent monitoring of several adjacent areas. This technique

removes the need of multiple, conventional fixed reader systems. By using array greater

gain can also be achieved that will cover larger areas. As the main focus of the design is

to cover the larger area that is established using beam steering. So the required radiation

pattern on which the thesis is based is given as in the Figure 4.14.

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Area 1 Area2

Antenna

Figure 4.14: Steering of pattern

The above technique can be used by proper switching in delay lines to cover larger areas.

So the array is designed in such a way that it works in two states in which larger front

area is covered. Two states are shown in the figure 4.15.

(a) (b)

Figure 4.15: Two different states of array design (a) State1 (b) State2

The transmission line length is adjusted in such a way that it gives the delay 0, 450, 900,

1800 gradually. The current distribution on the patch at 2.4 GHz can be shown in the

Figure 4.16. Theoretical pattern of array is simulated using MATLAB, which is shown in

the Figure 4.17.

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Figure 4.16: Current distribution on the patch at 2.4 GHz

Moreover, the design is also simulated in the HFSS and the results are given in the Figure

4.18 and 4.19 and axial ratio is given in the Figure 4.20.

Figure 4.17: Theoretical pattern of (a) single Patch (b) Array

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(a)

(b)

Figure 4.18: Return loss of complete Antenna array in (a) State1 (b) State2

(a)

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(b)

Figure 4.19: 2D Gain plots with steered beam in (a) State 1 (b) State 2

Figure 4.20: Axial ratio of complete array

(a) (b)

Figure 4.21: 3D-Gain plots with steered beams (a) State 1 (b) State 2

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The value of axial ratio of complete array is 2.07 dB and gain is round about 10 dB. After

simulating the design in HFSS and ADS, it is manufactured and tested by using VNA.

Manufactured array design is shown in the Figure 4.21 and measured results are shown in

the Figures 4.22 and 4.23.

Figure 4.22: Manufactured patch antenna array

Figure 4.23: VNA Testing of Manufactured patch Antenna

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The results of VNA testing are given as in the Figure 4.23.

EFFICIENCY OF PROPOSED DESIGN

Efficiency of the patch antenna can be given in the table 2.2.

Table 2.2: Efficiency of Proposed Design

Patch Array Gain (G) = 10dB, λ = 12.5cm

Effective Area , Ae Gλ2/4π 124.33 cm2

Physical Area, AP Height × Width 155 cm2

Efficiency Ae/Ap 80 % Single Patch

Gain (G) = 8dB, λ =12.5cm Effective Area , Ae Gλ2/4π 78cm2

Physical Area, AP Height × Width 80 cm2

Efficiency Ae/Ap 97.5 %

(a)

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(b)

Figure 4.24: VNA Return Loss of patch Antenna array (a) State1 (b) State2

The design is also tested in the Anechoic chamber and the results meet its requirement,

which are shown in the Figure 4.24 and 4.25. The measured axial ratio of the array is 2.3

dB.

(a)

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(b)

Figure 4.25: Anechoic pattern of patch Antenna array in state 1 (a) 3D (b) 2D

(a)

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(b)

Figure 4.26: Anechoic pattern of patch Antenna array in state 2(a) 3D (b) 2D

As from the figures 4.24 and 4.25, it is clear that the practical results are much better. No

doubt manufacturing error and material loses are also present. Figure 4.26 shows the

overall design with complete dimensions.

Figure 4.27: Final design with complete dimensions

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CHAPTER 5: CONCLUSIONS AND RECOMENDATIONS

Patch antennas are particularly versatile antennas that can be used for large number of

applications due to its simple geometries and easy to fabricate. There are many different

feed systems, shapes, and array configurations. These design options make patch

antennas an attractive area of study. The distinctive property of the patch antenna is its 2-

D structure. As a flat antenna, arrays may have high gain, but volume and weight will be

very low [11]. Patch antennas are also being used in PCB technologies and in advanced

substrate to produce best communication networks in all over the world.

The 1x4 RFID patch antenna array that consists of power dividers, phase shifters, and

patch antennas working at frequency of 2.4GHz, is designed, simulated, implemented and

measured. The direction of the main beam of the phased array is steerable in the range of

±28o.Micro strip Patch antennas, Wilkinson power divider, Patch antennas and micro

strip transmission line of 50 Ω characteristic impedance, have been realized using HFSS

11software.

An extended version of the Wilkinson power divider has been demonstrated by

maintaining the ideal performance of being reciprocal, isolated, and matched between the

output ports. The design show outstanding performance at operating frequencies, even

when put into operation by using low-cost technology. The design is very robust with

simple hand calculations that provide reliable dimensions. The measured performance of

all these components was admirable as had been projected and an exceptional agreement

between the measurement and prediction.

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FUTURE WORK

There are many improvements and extensions that can be made in the proposed design.

Firstly, the beam switching has been executed by manual controlled switches where the

switching time may be the critical factor. This issue can be resolved by incorporating a

microcontroller to control the switches automatically with a predefined switching time.

The time may be decided by the application in which the RFID front end is to be

employed.

As the reader is the RF receiver and transmitter, that will be controlled by a

microcontroller or digital signal processor. By using an attached antenna, the reader will

capture the data from tags and it will pass the data to the computer for processing.

The tag corresponding to the design features can also be proposed or the existing tags can

be improved for error free results. Currently the angular area covered by the design is

±28o. The coverage area can be enhanced by increasing the number of beam switching

states. The switched beam technique can be employed by using Butler matrix technique.

It has been tried to make the hardware compact and portable. There is a room to compact

the physical dimensions even further.

The project can be extended to design a complete receiver/transmitter circuit using FPGA

technology.

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APENDIX-I

MATLAB CODES AND RESULT

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Code for the Array Factor

% Code for Array Factor

% by KAMRAN ZAHID

% Date: 09-10-2011

%===============================

clc

close all

clear all

lambda=0.125; %Lambda at 2.4 GHz

N=4;

d=lambda/2;

% Enter the value of phase from 0 to 2 for steering the beam to left side

% Enter the value of phase from 0 to -2 for steering the beam to right side

Phase=0;

k=2*pi/lambda;

theta=[0:.01:pi];

s=k*lambda/2*cos(theta)+Phase;

Af=abs(sin(N*s./2)./sin(s./2)); %Array Factor

subplot(2,1,1)

polar(theta,Af)

Title('Array Factor Plots')

xlabel('polar plot')

subplot(2,1,2)

plot(theta,Af),grid

xlabel('\theta (rad)')

ylabel('AF')

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Figure 5.1: Array Factor plots with beam steering

Code for the Patch design

% Patch antenna design (units in cm)

% By KAMRAN ZAHID

%% Date:09-10-2011

%======================================

clear all

close all

fr =2.4*10^9; % in GHz

eps=4.4;

h=0.16; % in cm

c=3*10^10;

lambda = (c/fr)

Third_of_lambda = (c/fr)/3;

W=c/(2*fr)*sqrt(2/(eps+1)); % Width of the Patch

Width=W

eps1 = (eps+1)/2+(eps-1)/2*(1+12*h/W)^(-0.5); %Effective eps

d1=(eps1+0.3)*(W/h+0.264);

d2=(eps1-0.258)*(W/h+0.8);

dL = h*0.412* ( d1/d2 );

Length=c/(2*fr*sqrt(eps1)) - 2*dL %Length of the patch

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Code for the Transmission Line Impedance

% Transmission line impedence

% By Kamran Zahid

%% Date:09-10-2011

% ======================================

t=1*10^(-6); % Tx line thickness

eps=4.4; % Permittivity of substrate

h=.16; % Height of substrate

wp=0.265; % Width of tx line

clc

W=4.94; % Patch width

eps1 = (eps+1)/2+(eps-1)/2*(1+12*h/W)^(-0.5);

ZO_Balanis = 120*pi / ( sqrt(eps1) * (wp/h + 1.393 + 0.667*log(wp/h+1.444)))

Code for theoretical array pattern

clc

close all

clear all

lambda=12.5; %Lambda at 2.4 GHz

N=4;

d=lambda/2;

W=2.6; L=2.6; h=0.16;

fi=[0:.01:pi];

k=2*pi/12.5;

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Phase = 0; r = 1;

% Enter the value of phase from 0 to 2 to steer left & and 0 to -2 to

% steer right

s=k*lambda/2*cos(fi)+Phase;

Af=abs(sin(N*s./2)./sin(s./2)); %Array Factor

E_fi=k*W/(pi*r)* ( sin(k*h*cos(fi)/2) / ( k*h*cos(fi)./2 ) ).* cos (k*L/2.* sin(fi));

pattern=Af.*E_fi;

subplot(2,2,1)

polar(fi,E_fi),grid

xlabel('\phi (rad)')

Title('Single Patch Rad Pattern (E)')

xlabel('\phi (rad)')

subplot(2,2,2)

polar(fi,pattern),grid

xlabel('\phi (rad)')

Title('Array Rad Pattern')

subplot(2,2,3)

polar(fi,-10*log10(E_fi)),grid

xlabel('\phi (rad)')

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Title('Single Patch Rad Pattern (dB)')

xlabel('\phi (rad)')

subplot(2,2,4)

polar(fi,1000*log10(10*pattern)),grid

xlabel('\phi (rad)')

Title('Array Rad Pattern (dB)')

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APPENDIX-II

DATA SHEETS

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CAPACITOR Data Sheet

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REFERENCES

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[2] Richard Moscatiello, “Basic Concepts in RFID Technology”,

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[3] Mehmet Abbak, “RFID Coverage Extension Using Microstrip Patch Antenna

Array” IEEE Antennas and Wave propagation, Vol 51, pp 185-199, Feb 2010.

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[5] Iulian Rosu, “Phase Shifters”, YO3DAC / VA3IUL, http://www.qsl.net/va3iul/

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[9] Gary Breed, “The Fundamentals of Patch Antenna Design and Performance”,

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[12] R. C. Hansen, “Phased Array Antennas” John Wiley & Sons, ISBN-13: 978-

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[17] R. Garg, “Microstrip Antenna Design Handbook”, 2nd ed, ISBN-10: 0890065136,

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[20] S. H. Ahn, J. W. Lee, C. S. Cho, T. K. Lee, “A Wilkinson Power Divider with

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