A WIDEBAND SLOT ANTENNA ARRAY WITH CPW-FED

INDUCTIVELY COUPLED STRUCTURE

Mr. JEERASAK CHUANGCHAI

A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF MASTER OF SCIENCE

IN COMMUNICATION ENGINEERING

SIRINDHORN INTERNATIONAL THAI-GERMAN GRADUATE SCHOOL OF ENGINEERING

(TGGS)

GRADUATE COLLEGE

KING MONGKUT'S INSTITUTE OF TECHNOLOGY NORTH BANGKOK

ACADEMIC YEAR 2007

COPYRIGHT OF KING MONGKUT'S INSTITUTE OF TECHNOLOGY NORTH BANGKOK

ii

Name : Mr. Jeerasak Chuangchai

Thesis Title : A Wideband Slot Antenna Array with CPW-Fed Inductively

Coupled Structure

Major Field : Communication Engineering

King Mongkut's Institute of Technology North Bangkok

Thesis Advisor : Associate Professor Dr. Prayoot Akkaraekthalin

Academic Year : 2007

Abstract

A wideband slot antenna array with coplanar waveguide (CPW)-fed inductively

coupled structure is designed and compared to the CPW-fed inductively coupled slot

antennas using uniform impedance resonator (UIR). The slot antenna array with

CPW-fed inductively coupled structure can increase the impedance bandwidths

(VSWR < 2) from 4% to 38% with respect to traditional CPW-fed inductively

coupled slot antennas using UIR operating at the same frequency. The measurement

that shows the bandwidth of the prototype antenna is higher than 38% (|S11|≤ 10 dB).

The characteristics of the prototype antenna have been calculated using simulation

software IE3D. Simulated results are verified with measurements.

(Total 52 pages)

Keywords : slot antenna array, CPW-fed, inductively coupled structure, IE3D

______________________________________________________________Advisor

iii

ชอ : นายจระศกด ชวงชย ชอวทยานพนธ : สายอากาศแบบรองแถวล าดบทปอนดวยสายน าสญญาณระนาบรวมแบบ

ตวเหนยวน าส าหรบชวงความถกวาง สาขาวชา : วศวกรรมโทรคมนาคม สถาบนเทคโนโลยพระจอมเกลาพระนครเหนอ ทปรกษาวทยานพนธ : รองศาสตราจารย ดร.ประยทธ อครเอกฒาลน ปการศกษา : 2550

บทคดยอ

วทยานพนธฉบบน ไดน าเสนอการออกแบบและสรางสายอากาศแบบรองแถวล าดบทปอนดวยสายน าสญญาณระนาบรวมแบบตวเหนยวน าส าหรบชวงความถกวาง ซงไดจ าลองการท างานดวยโปรแกรมออกแบบสายอากาศยานความถไมโครเวฟ (IE3D) โดยออกแบบใหสายอากาศเปนแบบแถวล าดบเพอใหสามารถใชงานในยานความถสงขนได สายอากาศทสรางขนไดเปรยบเทยบคณสมบตกบสายอากาศแบบไดโพลทปอนดวยสายน าสญญาณระนาบรวม ซงสายอากาศตนแบบทสรางขนสามารถใชงานในยานความถเพมขนจาก 4% เปน 38% เมอเปรยบเทยบกบกบสายอากาศแบบไดโพลทปอนดวยสายน าสญญาณระนาบรวม ตนแบบสายอากาศทไดจากการวจยนจะมยานความถมากกวา 38 % (S11≤-10 dB) หรอเรยกวา สายอากาศแบบยานความถกวางยงยวด และมอตราขยายอยท 2-8 dBi โดยผลงานวจยทไดจะถกยนยนทงจากโปรแกรมจ าลองการท างาน (IE3D) และผลจากการทดสอบสายอากาศตนแบบ

(วทยานพนธมจ านวนทงสน 52 หนา)

ค าส าคญ : slot antenna array, CPW-fed, inductively coupled structure, IE3D

_____________________________________________อาจารยทปรกษาวทยานพนธหลก

iv

ACKNOWLEDGMENTS

This thesis would not be completed without the supports from my advisors,

professor, friends and my family. Thanks to those who devoted me their time and

information. Everyone was very helpful and enthusiastic for my thesis success.

I am greatly indebted to my advisor, Associate Professor Dr. Prayoot

Akkaraekthalin, for their helpful guidance, suggestion and encouragement throughout

this study. Moreover this thesis would have not been finished without the endless

support and tolerance of my friends at KMITNB Wireless Communications Research

Group.

Finally, I would like to express my thank to my commander and colleagues of

electronics engineering technology depart, college of industrial technology, KMITNB,

respectively Assistant Professor Preecha Ongaree, who had allowed me to join in this

college. Last but not the least, it would be impossible for me to work on this thesis

completely without the encouragement from my family. Therefore, I am greatly

appreciated for both physically and mentally supports they gave me.

Jeerasak Chuangchai

v

TABLE OF CONTENTS

Page

Abstract (in English) ii

Abstract (in Thai) iii

Acknowledgements iv

List of Tables vi

List of Figures vii

List of Abbreviations and Symbols ix

Chapter 1 Introduction 1

1.1 Propose of the study 1

1.2 Scope of the study 1

1.3 Method 2

1.4 Tools 2

1.5 Utilization of the study 2

Chapter 2 Background and theory 3

2.1 Microstrip patch antenna with coplanar waveguide (CPW) feed line 3

2.2 The various shapes of coupling slot in CPW-fed microstrip antennas 4

2.3 Miniaturized CPW-fed slot antenna 7

2.4 CPW inductively coupled slot antenna 10

2.5 Wide-band slot antennas with CPW feed lines 15

Chapter 3 Design of a wideband slot antenna array with CPW-fed inductively

coupled structure 19

3.1 Methodology 19

3.2 Design of the CPW feedline 19

3.3 Design of the SIR slot antenna CPW-fed with inductively

coupled structure 20

3.4 Design of the UIR slot antenna CPW-fed with inductively

coupled structure 22

3.5 Design of a wideband slot antenna array with CPW-fed

inductively coupled structure 23

Chapter 4 Experimental results 31

Chapter 5 Conclusion and future prospects 40

5.1 Conclusions 40

5.2 Problem and suggestion for future work 40

References 41

Appendix A Spectrum utilization 3-7 GHz 42

Appendix B Simulation and designing program 44

Appendix C EECON 30 47

Biography 52

vi

LIST OF TABLES

Table Page

2-1 Dimensions of antennas 10

2-2 Dimensions of the wideband antenna on r = 4.3 and h = 1.58 mm 16

2-3 Dimensions of the wideband antenna on r = 12.5 and h = 1.27 mm 16

3-1 Dimensions of prototype antenna 26

vii

LIST OF FIGURES

Figure Page

2-1 Configuration of the coplanar fed microstrip patch antenna 3

2-2 CPW-fed aperture-coupled microstrip antennas 4

2-3 Return loss and F/B versus frequency for different slot lengths 5

2-4 The geometry of square slot-loops exciting aperture for a CPWFA 6

2-5 Return loss and F/B versus frequency for different sizes of square slot

loops excitation 6

2-6 Half-wavelength resonators in slot line configuration 7

2-7 CPW-fed SIR antenna 8

2-8 The return loss of CPW-fed SIR antenna 8

2-9 Geometry of CPW-fed inductively coupled slot antenna using stepped

impedance resonator with open stub 9

2-10 Simulated and measured return loss of the antennas 10

2-11 Geometry of the inductively coupled slot antenna 11

2-12 Layout of single-slot element for 5GHz operation 12

2-13 Return loss of the single-slot element for 5 GHz operation 12

2-14 Layout of the three-element uniform excited linear array 13

2-15 Return loss of the three-element uniform excited linear array 13

2-16 Layout of the three-element log periodic slot array 14

2-17 Return loss of the three-element log periodic slot array 14

2-18 Layout of the CPW-fed HSA 15

2-19 Theoretical and measured return loss 16

2-20 Theoretical and measured return loss of CPW-fed HAS 17

2-21 Layout of the CPW-fed LPSA 17

2-22 Theoretical and measured return loss of CPW-fed 5 elements LPSA 18

2-23 Theoretical and measured return loss of CPW-fed 7,9 and 11 elements

LPSA 18

3-1 Physical parameters of the CPW feedline 20

3-2 Layout of CPW-fed SIR slot antenna 21

3-3 Return loss of CPW-fed SIR slot antenna 21

3-4 Layout of CPW-fed UIR slot antenna 22

3-5 Return loss of CPW-fed UIR slot antenna 22

3-6 Layout of wideband slot antenna array with CPW-fed inductively

coupled structure 23

3-7 The simulated return losses for various width of the step impedance

resonator 24

3-8 The simulated return losses for various length of the step impedance

resonator 25

3-9 The simulated return losses for various gaps between the conventional

antenna and step impedance resonator 25

3-10 Simulated return loss of the prototype antenna 26

3-11 Normalized input impedance 27

3-12 Simulated VSWR of prototype antenna 27

3-13 Current distribution of the antenna at 2.8 GHz 28

3-14 Current distribution of the antenna at 3.4 GHz 28

viii

LIST OF FIGURES (CONTINUED)

Figure Page

3-15 Current distribution of the antenna at 4.0 GHz 28

3-16 Radiation pattern of the antenna at 2.80 GHz 29

3-17 Radiation pattern of the antenna at 3.40 GHz 29

3-18 Radiation pattern of the antenna at 4.00 GHz 29

3-19 Antenna efficiency and radiation efficiency of the antenna 30

3-20 Gain of the antenna 30

4-1 The prototype of a wideband slot antenna array with CPW-fed

inductively coupled structure 31

4-2 Measurement setup for return loss 32

4-3 Measured return loss 32

4-4 Simulated and measured return loss 33

4-5 Measurement setups for radiation patterns 33

4-6 Measurement setups for co-polarization in X-Z plane 34

4-7 Measurement setups for cross-polarization in X-Z plane 34

4-8 Radiation patterns in X-Z plane at 2.8 GHz 35

4-9 Radiation patterns in X-Z plane at 3.4 GHz 35

4-10 Radiation patterns in X-Z plane at 4.0 GHz 36

4-11 Measurement setups for co-polarization in Y-Z plane 36

4-12 Measurement setups for cross-polarization in Y-Z plane 37

4-13 Radiation patterns in Y-Z plane at 2.8 GHz 37

4-14 Radiation patterns in Y-Z plane at 3.2 GHz 38

4-15 Radiation patterns in Y-Z plane at 4.0 GHz 38

A-1 Spectrum utilization 3-7 GHz 43

C-1 Simulation program for microwave frequency devices (IE3D Zeland) 45

C-2 Simulation of a wideband slot antenna array with CPW-fed inductively

coupled structure 45

C-3 Basic parameters assignment 46

ix

LIST OF ABBREVIATIONS AND SYMBOLS

g Guide wavelength

eff Effective relative dielectric constant

r Relative dielectric constant

Relative frequency span

f0 Fundamental frequency

ADS Advances design system

BJT Bipolar junction transistor

BPF Bandpass filter

CBCPW Conductor-backed coplanar waveguide

CPS Coplanar striplines

CPW Coplanar waveguide

HSA Hybrid structure arrays

LO Local oscillator

LPF Lowpass filter

LPDA Log-periodic dipole arrays

LPSA Log-periodic structure arrays

MESFET Metal-semiconductor field effect transistor

PCB Printed circuit board

SIR Stepped impedance resonator

SMA Subminiature version A

Sij S-parameter characterization between port i and j of a network

TE Transverse electric

TM Transverse magnetic

TSA Tapered slot antenna

VSWR Voltage standing wave ratio

UIR Uniform impedance resonator

CHAPTER 1

INTRODUCTION

In microwave applications, slot antennas fed by coplanar waveguide (CPW)

lines are receiving increasing attention. Numerous advantages have been obtained by

feeding a radiating element with coplanar waveguide feeds; such as lower radiation

leakage and less dispersion than microstrip lines. CPW feed lines also facilitate

parallel as well as series connection of both active and passive components on one

side of the planar substrate thereby eliminating via hole connections. Many slot

antenna elements suitable for a CPW-fed configuration have been reported in

literature. Open-end CPW-fed microstrip antennas have been studied experimentally

[1]. Similar geometries of microstrip antennas inductively and capacitively coupled to

CPW have also been investigated [2].

The conventional CPW-fed slot antenna is a one wavelength center fed slot

antenna. Antennas of this type have been reported in literature for various

applications and have impedance bandwidth between 15 to 20 % [3]. An alternative

to this design is an open-end CPW structure which can be modified to a so-called half-

wave capacitively coupled slot antenna giving an impedance bandwidth of 10% to

15%. This type of structure has been modeled as a radiating element and is referred to

as a CPW-fed slotline dipole antenna. Stepped impedance resonator technique is used

to reduce antenna size and giving an impedance bandwidth of 4 % [4, 5]. A wide

bandwidth can be obtained by arraying different narrow bandwidth resonators, each

having its own frequency of operation. Various configurations of low profile,

conformal antennas have been developed [6].

This master thesis proposes a new antenna using a combination of a step

impedance resonator (SIR) and uniform impedance resonator (UIR) array antenna in

order to increase bandwidth. The most interesting approach for this is a modified

antenna using many proposed techniques to obtain advantages of each technique. The

proposed antenna is fabricated on FR-4 substrate and is demonstrated to achieve a

wideband bandwidth (more than 38% for 10 dB return-loss). Both of IE3D simulation

and measurement will confirm the results.

1.1 Propose of the study

1.1.1 Investigate the CPW-fed antennas technology,

1.1.2 Study and design a wideband slot antenna array with CPW-fed inductively

coupled structure.

1.2 Scope of the study

1.2.1 Investigate the CPW-fed antennas technology,

1.2.2 Design of a CPW-fed wideband inductively-coupled antennas using IE3D,

1.2.3 Study and design of a CPW-fed wideband inductively-coupled antennas

using array slot antennas,

1.2.4 Perform simulation, measurement, validation and conclusion.

2

1.3 Method

1.3.1 Literature survey of the design theory for CPW-fed wideband antennas,

1.3.2 Design CPW-fed wideband inductively-coupled antennas,

1.3.3 Study and design of a CPW-fed wideband inductively-coupled antennas

using array slot antennas,

1.3.4 Perform circuit simulation, fabricate circuit measurements, validate the

results, and conclusions.

1.4 Tools

1.4.1 Personal computer

1.4.2 Zeland software

1.4.3 FR4 substrate

1.4.4 SMA connector

1.4.5 Coaxial cable (RG-142)

1.4.6 PCB engraving machine

1.4.7 Horn antenna

1.4.8 Network analyzer

1.4.9 RF sweep generator

1.4.10 Spectrum analyzer

1.5 Utilization of the study

1.5.1 A design technique for wideband slot array antenna with CPW-fed

inductively-coupled structure,

1.5.2 A knowledge base for deployment of wideband slot array antenna with

CPW-fed inductively coupled structure.

CHAPTER 2

BACKGROUND AND THEORY

In this chapter, basic concept and theory for coplanar waveguide (CPW)-fed slot

antenna are described. In addition, the important structures such as uniform

impedance resonator (UIR) and step impedance resonator (SIR) are also discussed.

Finally, important literatures are reviewed and design strategies are concluded.

2.1 Microstrip patch antenna with coplanar waveguide (CPW) feed line Microstrip antennas have found widespread applications for microwave as well

as millimetre wave systems. On the other hand, for components including active

devices, especially Monolithic Microwave Integrated Circuits (MMICs), coplanar line

is gaining an increasing interest. Coplanar line allows the realization of series as well

as shunt connections on one side of the planar substrate avoiding via hole

connections. Furthermore, the substrate can be relatively thick. This fact, on the other

hand, matches well with good efficiency and improved bandwidth of microstrip

antennas integrated on the same substrate. To combine the advantages of coplanar line

and microstrip patch antennas, two antenna configurations are shown in Figure 2-1. A

patch resonator is placed on one side of the substrate, while a slot is arranged opposite

to the patch in the ground plane. A coplanar line then feeds the slot. The inner

conductor of the coplanar line may either be connected directly across the slot

forming an inductive type of feeding as shown in Figure 2-1(a), or it may be coupled

to the slot in a capacitive way as shown in Figure 2-1(b).

(a) (b)

FIGURE 2-1 Configuration of the coplanar fed microstrip patch antenna [1]

Patch on

backside

r = 2.22 22

ls

3.12

3

18.3

1.58

4

In the case of the inductive coupling, the return loss depends strongly on the slot

width, while in the capacitive coupled arrangement; the return loss is low over a wide

range of slot widths. The antenna bandwidth (10 dB return loss) was around 3.5% for

the inductive coupling, but only 2.8% for the capacitive coupling. The return loss for

both structures can be achieved at the same slot length but at different frequencies.

2.2 The various shapes of coupling slot in CPW-fed microstrip antennas A numerical model of CPW-fed aperture-coupled microstrip antennas

(CPWFA) is using the integral equation technique to realized and validated. This

model is based on two coupled integral equations solved by the method of moments

(MoM). The first integral equation is derived from the magnetic field continuity in the

aperture at the ground-plane level. In the first equation, the unknown is the magnetic

current distribution in the apertures. The second integral is based on the tangential

continuity of the electric field at the patch level. The unknown of the second equation

is the electric current distribution on the patch. The field is expressed in terms of

vector and scalar potentials of electric and magnetic types. The potentials are

expressed as Sommerfeld integrals using the appropriate Green’s functions. The

surface currents are expanded into rooftop basis functions. The weight of every basis

function is obtained by applying the Galerkin method and solving the resulting MoM

matrix equation. The numerical excitation of the odd mode in the CPW line is

provided by two magnetic charges located in both slot-lines. The reflection coefficient

is calculated from the study of the standing wave in the line. The radiation patterns are

determined from the knowledge of the magnetic and electric current distributions.

Figure 2-2 shows the geometry of the CPW-fed aperture-coupled microstrip

antennas. The antennas are excited by the same size of square patch. The line

dimensions are calculated to obtain 50-Ω characteristic impedance. The reference

plane is taken at the slot level. As these antennas are all symmetrical with respect to

the feeding CPW line.

FIGURE 2-2 CPW-fed aperture-coupled microstrip antennas [2]

ls

10 mm

Ws

ls

Ws

20 mm 20 mm

G G

W

WC WC

W

r = 2.2

tg = 0.001

2.3 mm

(a) (b)

5

The front-to-back (F/B) radiated power ratio is defined as the ration between the

maximum co-pol power radiated in the main direction to the maximum co-pol power

radiated in the back direction. The F/B is used to describe the quality of the coupling

between the excitation slot and the patch. A low F/B means that a significant part of

the power provided by the slot is radiated directly to the backside of the antenna

instead of exciting the patch antenna.

FIGURE 2-3 Return loss and F/B versus frequency for different slot lengths

In Figure 2-3, the return loss S11 is represented in the neighbourhood of the

resonant frequency. In the same figure, The F/B is represented versus the frequency. It

can be observed that the F/B variation does not depend on the slot length. Figure 2-3

shows that an increase of the slot length shifts the loaded resonant frequency toward

the lowest frequencies where the F/B is smaller. The antenna bandwidth (10 dB return

loss) was around 4.0-4.5%.

Figure 2-4 shows the square slot-loop excitation aperture for a CPWFA. The

simplification is brought by the slot loop to the biasing circuitry of active antennas.

As the bias network must provide the energy to the active device without disturbing

the RF circuit, its realization can be quite cumbersome and delicate. For instance, in

oscillating slot antennas, a metal-insulator-metal capacitor must be fabricated for

isolating bias voltages. On the other hand, a slot loop can be used as both the

antenna’s excitation and the isolation for dc bias. The gap around the loop has not

been left constant on the top and button edges (0.2 mm and 1 mm) because of the

constraints on the cells size imposed by the line width (0.2 mm).

The effect of the size of the square loop on the return loss and the front-to-back

radiated power ratio of a CPWFA are shown in Figure 2-5 for several loop

dimensions. It may be observed that both return loss and F/B depend on the slot

dimensions. The back radiation increases strongly with the loop size, which means

that the antenna is poorly excited for large dimensions of the loop. For example, an

increase of the length of the loop edges from 10-18 mm diminishes the loaded

resonant frequency from 4.40-4.20 GHz only, while the F/B drops from 16 to 5 dB.

The antenna bandwidth (10 dB return loss) was around 4.5 %. A comparison between

Figure 2-3 and Figure 2-5 also indicates that the loop coupled patch has a larger

bandwidth that the patch excited by a capacitively coupled slot.

6

FIGURE 2-4 The geometry of square slot-loops exciting aperture for a CPWFA

Figure 2-5 Return loss and F/B versus frequency for different sizes of square slot

loops excitation

20 mm

ls

20 mm ls

1 mm

4.4

4.8

7

2.3 Miniaturized CPW-fed slot antenna

A conventional CPW on a substrate consists of a center strip conductor and two

semi-infinite ground planes on either side. CPW has several advantages over the

conventional microstrip line. Compared with the microstrip uniform impedance

resonator (UIR), the microstrip stepped impedance resonator (SIR) has several

advantages including compact size, harmonic suppression, and low insertion loss.

SIRs have been widely used in realization of filters. Sharing the same property, the

slot line SIR as shown in Figure 2-6(b) also has a compact size over the slot line UIR

as shown in Figure 2-6(a) [4].

Half-wavelength capacitive CPW-fed UIR slot dipole is useful due to its

coplanar characteristics and compact size when compared with one-wavelength

inductive CPW-fed UIR slot dipole.

FIGURE 2-6 Have-wavelength resonators in slot line configuration

The input impedance Zi of the SIR as shown in Figure 2-6(b) can be derived by

using transmission line equations. The input impedance is given by

𝑍𝑖 = 𝑍2

𝑍′𝑖 + 𝑗𝑍2𝑡𝑎𝑛𝜃2

𝑍2 + 𝑗𝑍′𝑖𝑡𝑎𝑛𝜃2

Where

𝑍′𝑖 = 𝑍1

𝑍′′𝑖 + 𝑗𝑍1𝑡𝑎𝑛(2𝜃1)

𝑍1 + 𝑗𝑍′′ 𝑖𝑡𝑎𝑛(2𝜃1) ,and 𝑍′′

𝑖= 𝑗𝑍2𝑡𝑎𝑛(𝜃2)

Where Z1 and Z2 are the characteristic impedances of the inner and outer slot

lines, and 1 and 2 are the electrical lengths of the inner and outer slot lines,

respectively. The resonant frequency can be found when Zi=0, which implies that the

numerator of (2-1) should be equal to zero, which reduces to

𝑅𝑧 =𝑍1

𝑍2= 𝑡𝑎𝑛𝜃1𝑡𝑎𝑛𝜃2

Equation (2-2) implies that in addition to the length of dipole, the impedance

ratio Rz of two slot lines is also related to the resonant frequency. The relation ship

between the electrical length 1 and the normalized resonator electical length

Eq.2-1

Eq.2-2

L L2(2) 2L1(21)

L2(2)

W2 W1 Z1

Z2 Z2

Zi Zi’

Zi’’

(b) UIR (a) SIR

8

LN=T/π, which is the normalized electrical length of SIR with respect to that of UIR

(in this case, the length is π )

𝜃𝑇 = 2 𝜃1 + 𝜃2 = 2 𝜃1 + 𝑡𝑎𝑛−1 𝑅𝑧

𝑡𝑎𝑛𝜃1

It is clear that Rz=1 is for the case of UIR, and the length of UIR and SIR are the

same. When Rz<1, there is a minimum, and when Rz>1, there is a maximum. The

optimal 0 can be derived by differentiating (2-3) and set it equal to zero.

𝜃1 = 𝜃2 = 𝜃0 = 𝑡𝑎𝑛−1 𝑅𝑧

Therefore, to make a compact SIR, the impedance ratio Rz should be smaller

than one. The electrical length 1 and 2 can be calculated by (2-4) and then the

coressponding slot lines are realized by using the transmission line calculation tool.

FIGURE 2-7 CPW-fed SIR antenna

FIGURE 2-8 The return loss of CPW-fed SIR antenna

Eq.2-3

Eq.2-4

L2(2) L2(2) 2 L1(21)

LF

W2 W1

Z

Y

X

9

Figure 2-7 shows the configuation of the CPW-fed antenna desiged at the center

frequency of 4.1 GHz. The dimensions of the antenna are L1= 4.35 mm, L2 = 3.6 mm,

W1=1 mm, W2 =5 mm, and LF =40 mm. The center conductor width and gap of the

CPW are 2.3 mm and 0.3 mm for a 50 Ω line. In addition, it should be mentioned that

the center conductor of CPW extrudes into the SIR by 0.5 mm for impedance

matching. The substrate is 25-milRT/Duroid 6006 with the dielectric constant of 6.15.

The total length of SIR is 2(L1+L2) = 15.9 mm. For comparison, a CPW-fed UIR is

also designed at the same center frequency with its total length of 22.9 mm.

Compared with SIR, about 31% length reduction is acheive.

Figure 2-8 shows the simulated and measured return losses. The center

frequency shifts from 4.1 to 3.6 GHz, and the -10 dB fractional bandwidth is 3.6 %.

The geometry of a miniature CPW-fed inductively coupled slot antenna

(CICSA) using stepped impedance resonator with tuning slot stub loading is shown in

Figure 2-9 [5]. The antenna is center-fed inductively coupled slot where the slot has a

length (L-Wf) and width S. The slot length (L) determines the resonant frequency,

while the slot has a width (S) which may be adjusted to achieve a wider bandwidth.

The length L is approximately one-guide wavelength (g)at the slot line resonance.

The wavelength, g, in the slot is determined to be about 0.78 1 + 𝜀𝑟 /2𝜀𝑟 free

space wavelength. The dimensions of antenna are chosen to be L = 50 mm, T = 10

mm, S = 10 mm, P = 40 mm, and the ground size 60 mm x70 mm. The antenna is

designed on a single-layer PCB substrate with dielectric constant(r) is 4.4, loss

tangent (tan ) is 0.002 and the thickness of substrate (h) is 1.6 mm. A CPW-fed,

which consists of a signal strip width of 3.0 mm and a gap(g) of 0.3 mm for

approximate 50Ω characteristic impedance between the signal strip and the coplanar

ground plane, is used for feeding the antenna.

FIGURE 2-9 Geometry of CPW-fed inductively coupled slot antenna

using stepped impedance resonator with open stub

70 mm

60 m

m

L=50 mm T

S

P

W2

Ls Gs

Wst

Lst

h r Wf

y

x z g

10

Three different CICSAs-SIR with tuning slot stub loading with difference

parameters are designed, fabricated, and measured. The geometrical parameters of the

antennas are given in Table 2-1.

TABLE 2-1 Dimensions of antennas

Antenna L(mm) S(mm) Gs(mm) W2(mm) Ls(mm) Lst(mm) Wst(mm)

Ref 50 10 - - - - -

Ant1 50 10 4 4 15 - -

Ant2 50 10 6 4 15 - -

Ant3 50 10 4 4.7 15 5 2

FIGURE 2-10 Simulated and measured return loss of the antennas

Figure 2-10 shows a comparison between the simulated and measured return

losses of the reference antenna and the prototype antenna. The simulated and

measured data of the prototype and conventional antennas are approximately the

same. For comparison, a convenional CPW-fed inductively coupled slot antenna

(UIR) with SIR, about 46% length reduction is acheive. The antenna bandwidth was

around 4.0%.

2.4 CPW inductively coupled slot antenna

The antenna is formed by etching two half-wave slots located symmetrically on

both sides of the CPW line as shown in Figure 2-11, at a distance G from the outer

CPW edges. The slots have a bent section of length l in order to increase the coupling

to the CPW line. The magnetic field flux of the CPW’s propagated wave through the

slots excites them by inducing an electric field. Due to the opposite direction of the

magnetic field on both sides of the CPW line, the electric fields induced in both slot

have the same direction in the vertical segments of length L-l and the radiation pattern

is broadside. In the horizontal segments of length l, the electric fields in the two slots

have opposite directions and the radiation from the bent sections are cancelled in the

broadside direction. Actually their radiation only contributes to the cross polarization.

The coupling region identified in Figure 2-11 can be seen as a section of two

coupled transmission lines, one being the feeding CPW line and the other one being

formed by the two outer slots of width W. The line formed by the outer slots is

11

terminated at one end by a short circuit and at the other end by the vertical radiating

slots forming an antenna with radiation resistance Rs. Each of these vertical sections is

approximately half-wavelength long and it can be modelled as lossy resonant circuit.

The presence of a short circuit at the end of each outer horizontal slot favours a strong

current around the slots and, thus, inductive coupling between these slots and the

feeding CPW line. The radiation of the antenna will, therefore, be mostly dependent

on the feeding CPW line’s current rather than on its voltage. Consequently, the

antenna loading can be modelled as equivalent impedance in series along the feeding

line.

FIGURE 2-11 Geometry of the inductively coupled slot antenna [6]

The parameters of geometry which sensitive to input are:

1. G , the width of the gap between the slot and the CPW line

2. l/L, the ratio between the CPW-coupled section and the overall length of

the slots

3. W, the slot width

The propagated wave in the feeding CPW line has its fields well confined

around the line with a rapid decay in the plane transverse to the direction of

propagation. Therefore, the coupling region is limited to a small area around the line.

The coupling to the outer horizontal slots is proportional to the flux of the magnetic

field generated by the incident CPW wave through the aperture of the outer slots in

the coupling region. Thus, the metallic strip of width G. Increasing G decreases the

coupling, which decreases the radiation resistance Rant directly affects it.

The coupling section of length l does not contribute significantly to the

radiation. Therefore, for a given slot length L, increasing l decreases the length of the

vertical radiating sections, which leads to an increase of the intrinsic radiation

resistance Rs of the vertical slot and, therefore, an increase of the equivalent series

resistance Rant on the CPW line. The Q factor of the antenna at resonance will also

increase. It should be recalled that l must be kept to a minimum value in order to

minimize the level of cross polarization.

L

W

G D

l

Coupling

region

Substrate: r’,h

Feeding

CPW line

Coupled

outer line

Radiating slot

12

Increasing the slot width W leads to two opposite effects on Rant. First, it favors

a better coupling between the CPW line and the antenna, which should lead to a larger

Rant. On the other hand, a wider slot has a smaller radiation resistance (Rs), which also

means a smaller Rant. In most of the cases studied, the second effect is predominant

and the radiation resistance and the Q factor of the antenna decrease.

Three antennas have been designed using this new topology and built on 50-mil

substrate (r=10.2, h =1.25mm). The characteristic impedance of the feeding CPW

transmission lines is 50 Ω.

The antenna in Figure 2-12 resonates at 5GHz with an equivalent resistance

match to the CPW line (Rant = 50Ω). The transmission line is terminated by a short

circuit located at a half-guided wavelength from the coupling region in order to

present a short circuit in series with Rant in this region. The return loss is presented in

Figure 2-13. The measured resonance frequency is slightly shifted compared to the

prediction. The bandwidth for a -10 dB return is approximately 4%.

FIGURE 2-12 Layout of single-slot element for 5GHz operation

FIGURE 2-13 Return loss of the single-slot element for 5 GHz operation

1

1.15

12.42

3.35

0.2

13

Figure 2-14 shows a linear three elements array slot antenna with series-feed.

The slots are exited in phase since the electrical length in the CPW line between the

slots is about 0.42free-space at 5 GHz. All lines have characteristic impedance of 50 Ω.

At the resonance frequency, the radiation resistance of each simple antenna presents a

series load on the transmission line. In order to match the antenna to 50 Ω generator

and a uniform array excitation, the value of each radiation resistance must be Rant =

50/3 Ω. The chosen dimensions for the elements are L =12.7 mm, l/L = 0.24, G = 0.4

mm, W = 1 mm, and D = 0.81 mm.

The measured and simulated return losses are shown in Figure 2-15. The

measured resonance frequency is slightly shifted compare to the simulation. The

bandwidth for a -10 dB return loss is nearly 6%.

FIGURE 2-14 Layout of the three-element uniform exited linear array

FIGURE 2-15 Return loss of the three-element uniform exited linear array

1

0.81

0.4

12.7

3

12.7

14

A well-known approach to increase the bandwidth of an antenna is to realize a

log-periodic (LP) structure as shown in FIGURE 2-16. By applying, a scaling factor

to the first element dimensions for design the other elements. The scaling procedure

must be applied to the slot and the CPW transmission line in order to maintain the

log-periodic frequency characteristics of the CPW-to-slot coupling region. The

scaling factor used in the design was 1.1. At designed frequency, the adjacent active

element should be exited as mush in phase as possible to maintain a broadside

radiation. The best configuration is one in which antenna element n is located at dn

(dn = (2n-1) n-cpw/2) from the CPW short circuit at the end of the feed line. The

wavelength in the CPW line at the resonance frequency of element n and n = 1

corresponds to the low frequency element. The measured and simulated return losses

are shown in Figure 2-17. The bandwidth for a -10 dB return loss is about 20%.

FIGURE 2-16 Layout of the three-element log periodic slot array

FIGURE 2-17 Return loss of the three-element log periodic slot array

1.25

0.22

14.3

3.86

dn

1.25

15

2.5 Wide-band slot antennas with CPW feed lines

The impedance bandwidth of the generalized CPW open-end slot antenna can

be increased by combining it with the standard CPW slot antenna as shown in Figure

2-18. The structure is referred to as the Hybrid Structure Antenna (HSA) [7]. The

center frequencies of two structures were kept slightly apart to increase the bandwidth

of the overall structure. The following procedure is used to design such a hybrid CPW

fed slot antenna.

2.5.1 Design a standard CPW antenna at a frequency slightly below the center

frequency of the desired band, roughly between 8% to 10% below the center

frequency for higher dielectric constants (r > 6) and between 11% to 15% for lower

dielectric constants (2 < r < 6).

2.5.2 An output port from the standard CPW antenna is used to feed the

generalized CPW open-end antenna with equal excitation amplitude and phase at the

center frequency with the standard CPW antenna. The separation distance between the

two antennas is given by /2 at the center frequency.

2.5.3 Design a generalized CPW open-end antenna with a CPW feedline with

dimensions the same as those of the output CPW line of the standard CPW antenna.

The frequency of this antenna should correspond to the center frequency of the

desired band.

2.5.4 Optimize the widths of both structures so that an optimum match is

obtained over the entire bandwidth.

FIGURE 2-18 Layout of the CPW-fed HSA

Ss

Gs Gs

Ws

Ls G G

S D=0/2

Metal

L

W

16

The antenna in Figure 2-18 was designed and built on 1.58 mm thick substrate

of dielectric constant 4.3. The dimensions of the structure are shown in Table 2-2 and

the theoretical and measured return loss is shown in Figure 2-19. The standard CPW

antenna was designed to resonate at 4.4 GHz, and the generalized CPW open-end

antenna at 4.8 GHz. The two antennas were then combined to be fed in phase at

frequency of 4.8 GHz. The overall impedance bandwidth of the structure obtained

from both simulation and measurement is 49%.

TABLE 2-2 Dimensions of the wideband antenna on r = 4.3 and h = 1.58 mm

r = 4.3 h = 1.58 mm

S = 11 mm SS = 1.26 mm

G = 0.5 mm GS = 0.25 mm

L = 21.8 mm LS = 40.3 mm

W = 6.7 mm WS = 4.3 mm

FIGURE 2-19 Theoretical and measured return loss of CPW-fed HSA for r = 4.3,

h = 1.58 mm and f = 4.8 GHz

Using the same procedure, another wide-band hybrid structure was designed on

a substrate with dielectric constant of 12.5. The dimensions are shown in Table 2-3.

The center frequency of the structure is 4.7 GHz. Figure 2-20 shows measured and

computed VSWRs between 3 and 6 GHz. The bandwidth for a VSWR < 2 is 1.4 GHz,

yielding an impedance bandwidth of 33 %.

TABLE 2-3 Dimensions of the wideband antenna on r = 12.5 and h = 1.27 mm

r = 12.5 h = 1.27 mm

S = 11 mm SS = 0.88 mm

G = 0.5 mm GS = 0.5 mm

L = 17.5 mm LS = 32.26 mm

W = 6.5 mm WS = 4.5 mm

17

FIGURE 2-20 Theoretical and measured return loss of CPW-fed HSA for r = 12.5,

h = 1.27 mm and f = 4.7 GHz

A 5-element prototype array, shown in Figure 2-21, was designed on a substrate

of dielectric constant 12.5 and thickness 1.27 mm. First, the element corresponding to

the highest frequency of the desired bandwidth was designed. Different scaling factors

from 0.75 to 0.95 were chosen to design the remaining elements. The wideband nature

of such a structure is achieved using a scaling factor of 0.95; for the design is 5o.

FIGURE 2-21 Layout of the CPW-fed LPSA

Wn

Rn Rn+1

Ln

Ln+1

Wn+1

18

FIGURE 2-22 Theoretical and measured return loss of CPW-fed 5 elements LPSA

for r = 10.2, h = 1.27 mm and f = 4.8 GHz

FIGURE 2-23 Theoretical and measured return loss of CPW-fed 7,9 and 11 elements

LPSA for r = 2.2, h = 1.6 mm

Figure 2-22 illustrates the theoretical impedance bandwidth for VSWR < 2 of

33% and the measured value is 38%. LPSA with 7, 9, and 11 elements were designed

on a substrate of dielectric constant of 2.2 and thickness of 1.57 mm. The theoretical

return losses for these antennas are shown in Figure 2-23. An impedance bandwidth

of 32% is obtained for the 7-element design. As the number of elements is increased

to 9, the bandwidth increased to 41%. The bandwidth for the 11-element log-periodic

structure is 48%.

CHAPTER 3

DESIGN OF A WIDEBAND SLOT ANTENNA ARRAY WITH

CPW-FED INDUCTIVELY COUPLED STRUCTURE

In this thesis, design of a wideband slot antenna array with CPW-fed inductively

coupled structure. The prototype antenna is designed and compared with the CPW-fed

inductively coupled slot antennas using uniform impedance resonator (UIR).

3.1 Methodology

The following procedure is used to design the CPW fed slot antenna.

3.1.1 Use the LineGuage program to design a CPW feedline at resonance

frequency. The characteristic impedance of the feedline is 50 Ω. 3.1.2 Design a CPW-fed SIR slot antenna at a frequency slightly below the

center frequency of the desired band. An output port from the CPW-fed SIR slot

antenna is used to feed the CPW-fed UIR slot antenna with equal excitation amplitude

and phase at the center frequency with the standard CPW antenna. The separation

distance between the two antennas is given by g/2 at the center frequency.

3.1.3 Design a CPW-fed UIR slot antenna with a CPW feedline with

dimensions the same as those of the output CPW line of the CPW-fed SIR slot

antenna. The frequency of this antenna should correspond to the center frequency of

the desired band.

3.1.4 Optimize the widths of both structures so that an optimum match is

obtained over the entire bandwidth.

The commercial software IE3D based on method of moments (MOM) is used to

simulate the characteristic of prototype antenna. The prototype antenna is designed

and built on FR4 substrate with dielectric constant (r) of 4.4, The loss tangent (tan )

of 0.02, the conductor thickness of 0.002 mm and the substrate thickness (h) of 1.6

mm.

3.2 Design of the CPW feedline

The CPW feedline is designed to resonant frequency at 3.6 GHz. The

parameters for calculation are

Frequency = 3.6 GHz

Relative Permittivity = 4.4

Substrate Height h = 1.6 mm

Metal Thickness t = 0.002 mm

Spacing S = 0.3 mm

Zc(Ohm) = 50

Electrical Length (Degree) = 90

The calculation results of the LineGuage is shown in Figure 3-1. The physical

parameters from LineGuage program are

Strip Width w = 2.96151 mm

Length = 13.3087 mm

Guide Wavelength = 53.1993 mm

20

For easier fabricated, the strip width is chosen to 3 mm. Then, recalculate the

electrical parameters from the chosen strip width. The electrical parameters from

LineGuage program are

Zc(Ohm) = 49.8773

Electrical Length (Degree) = 89.9425

Effective Permittivity = 2.44732

Guide Wavelength = 53.2333 mm

Figure 3-1 Physical parameters of the CPW feedline

3.3 Design of the SIR slot antenna with CPW-fed inductively coupled structure

Figure 3-2 shows the layout of CPW-fed SIR slot antenna. The antenna was

designed to resonate at 2 GHz. The dimensions of antenna are chosen to be

LS = 63 mm, Lf = 45 mm, W1=2 mm, G = 2 mm, S = 10 mm, and the ground size

65 mm x 70 mm. The antenna is designed on a single-layer PCB substrate with

dielectric constant(r) of 4.4, loss tangent (tan ) of 0.002 and the thickness of

substrate (h) of 1.6 mm. A CPW-fed, which consists of a signal strip width (Wf) of

3.0 mm and a gap (g) of 0.3 mm for approximate 50 Ω characteristic impedance

between the signal strip and the coplanar ground plane, is used for feeding the

antenna.

Figure 3-3 illustrates the simulated return loss of the antenna. The impedance

bandwidth for VSWR < 2 is about 30 %. The minimum return loss is -20.77 dB at the

frequency 1.9 GHz.

21

Figure 3-2 Layout of CPW-fed SIR slot antenna

Figure 3-3 Return loss of CPW-fed SIR slot antenna

LS = 63 mm

W1 = 4 mm

G = 2 mm

h Wf

g

y

x z

Lf = 45 mm

70 mm

65 m

m

S =

10

22

3.4 Design of the UIR slot antenna with CPW-fed inductively coupled structure

Figure 3-4 shows the layout of CPW-fed UIR slot antenna. The antenna was

designed to resonate at 3.6 GHz. The dimensions of antenna are chosen to be

L = 50 mm, P = 45 mm, S = 10 mm,T = 15 mm, and the ground size 65 mm x 70 mm.

The antenna is designed on a single-layer PCB substrate with dielectric constant(r) of

4.4, loss tangent (tan ) of 0.002 and the thickness of substrate (h) of 1.6 mm. A

CPW-fed, which consists of a signal strip width (Wf) of 3.0 mm and a gap (g) of 0.3

mm for approximate 50 Ω characteristic impedance between the signal strip and the

coplanar ground plane, is used for feeding the antenna. Figure 3-5 illustrates the

simulated return loss of the antenna. The impedance bandwidth for VSWR < 2 is

about 3 %. The minimum return loss is -11 dB at the frequency 3.4 GHz.

Figure 3-4 Layout of CPW-fed UIR slot antenna

Figure 3-5 Return loss of CPW-fed UIR slot antenna

23

3.5 Design of a wideband slot antenna array with CPW-fed inductively coupled

structure

A wideband slot antenna array with CPW-fed inductively coupled structure is

designed. The designed slot antenna geometry is shown in Figure 3-6. In additional,

the SIR, which resonance at lower frequency, is arrayed with the conventional slot

antenna. The slots extrude by W2 and Ls to create the slot SIR and the width W1 is the

distance between the conventional slot antenna and the slot SIR. The distance W1, W2,

and Ls are three key parameters for obtaining the resonant frequency and input

impedance matching. The antenna was designed to resonate at 3.6 GHz. The

dimensions of antenna are chosen to be L1 = 50 mm, L2 = 63 mm, Lf = 30 mm, T = 15

mm, GS = 2 mm, S = 10 mm, and the ground size 65 mm x 70 mm. The antenna is

designed on a single-layer PCB substrate with dielectric constant(r) of 4.4, loss

tangent (tan ) of 0.002 and the thickness of substrate (h) of 1.6 mm. A CPW-fed,

which consists of a signal strip width (Wf) of 3.0 mm and a gap (g) of 0.3 mm for

approximate 50 Ω characteristic impedance between the signal strip and the coplanar

ground plane, is used for feeding the antenna. The return loss for L = 50 mm, S = 10

mm, and T = 15 mm, denoted as reference antenna, and the calculated return loss has

also been demonstrated for comparison.

Figure 3-6 Layout of wideband slot antenna array with CPW-fed inductively coupled

structure

Lf = 30 mm

24

Figure 3-7 shows the simulated effects on the antenna’s frequency response by

changing the width W2. The length of the SIR slot (Ls) and the gaps between UIR slot

and SIR slot (W1) is constant. The length Ls is set to 20 mm and the gap W1 is set to

2.5 mm. The value of W2 is varied from 2.0 to 4.0 mm, the fundamental frequency is

shifted to lower frequency when increase W2. However, when W2 is increased more

than 4.0 mm, there was decreased the impedance bandwidth of the antenna. A large

frequency shift occurred in the fundamental resonance when changing the parameter

W2. The minimum return loss is -27.5 dB at 2.9 GHz and the impedance bandwidth

about 16% when W2 is varied to 4 mm.

Figure 3-7 The simulated return losses for various width of the step impedance

resonator (W2)

Figure 3-8 shows the simulated return loss versus frequency for different length

of patches Ls. The value of Ls is varied from 10.0 to 26.0 mm, the fundamental

frequency is shifted to higher frequency and the bandwidth is increased when increase

the length Ls. However, the length Ls is limited by the antenna size. When increased

the length Ls more than 26.0 mm, the return loss of the antenna is wider but the return

loss is decreased.

Figure 3-9 shows the effect of varying the gap between conventional slot and

SIR (W1). The gap is varied from 0.5 to 3 mm, the bandwidth and the return loss are

depended on the gap. When decreased the gap, the bandwidth is increased but the

return loss is decreased, and the resonant frequency is shifted to lower frequency.

When the gap is less than 0.5 mm, the antenna is achieved to two resonant

frequencies.

W1 = 2.5 mm

Ls = 20.0 mm

25

Figure 3-8 The simulated return losses for various length of the step impedance

resonator (Ls)

Figure 3-9 The simulated return losses for various gaps between the conventional

antenna and step impedance resonator (W1)

W1 = 2.5 mm

W2 = 4.0 mm

W2 = 4.0 mm

LS = 26.0 mm

26

After many optimization processes using commercial software IE3D, the

structural parameters of the antenna show in Table 3-1. The slot length, L1, is defined

by one-guide wavelength of the center frequency. The width of feedline is calculated

from LineGuage program. The values of length, T, S and gap (g) is chosen from the

physical of material. The value of length, L2, Lf, Lf, W1 and W2 are optimization value

from program IE3D. The simulated return loss is shown in Figure 3-10. The

bandwidth for a VSWR < 2 is 1.29 GHz, yielding an impedance bandwidth of 38 %.

The minimum return loss is -18 dB at frequency 3.1 GHz. The center frequency of

antenna is 3.4 GHz.

The slot antenna array with CPW-fed inductively coupled structure can increase

the impedance bandwidths (VSWR < 2) from 3% to 38% with respect to traditional

CPW-fed inductively coupled slot antennas using UIR operating at the same

frequency and same size of ground plane. The minimum return loss is decreased from

-11 dB to -18 dB. That means the prototype antenna has performed matching better

than reference antenna.

Table 3-1 Dimensions of the prototype antenna

Element Unit (mm)

L1 50.0

L2 63.0

Lf 30.0

LS 26.0

W1 2.0

W2 4.0

Wf 3.0

T 15.0

S 10.0

g 0.3

Gs 2.0

Figure 3-10 Simulated return loss of the prototype antenna

1.29 GHz

27

Figure 3-11 shows the simulated normalized input impedance of the antenna.

The normalized input impedance is directly related to return loss and VSWR. The

simulated VSWR of the antenna is shown in Figure 3-12.

Figure 3-11 Normalized input impedance

Figure 3-12 Simulated VSWR of prototype antenna

Figure 3-13 shows the distribution current on the antenna at the lowest resonant

frequency of 2.8 GHz. The maximum current is 15.206 A/m but the current is not

smooth distributed in structure, it is densely in the feedline. The current distribution

on the antenna at the center frequency is shown in Figure 3-14. The maximum current

is 14.705 A/m. The current is smoothly distributed on SIR and UIR slot more than the

lowest and highest resonant frequencies. Figure 3-15 shows the current distribution of

28

the highest resonant frequency of 4.0 GHz. The maximum current is 16.257 A/m. The

current is densely on the UIR slot, caused to the normalized cross-polarization level of

the highest frequency is higher than lower frequency.

Figure 3-13 Current distribution of the antenna at 2.8 GHz

Figure 3-14 Current distribution of the antenna at 3.4 GHz

Figure 3-15 Current distribution of the antenna at 4.0 GHz

29

The radiation patterns of the antenna are shown in Figure 3-16 – Figure 3-18.

The radiation patterns of all frequencies are bi-directional in x-y plane.

Figure 3-16 Radiation pattern of the antenna at 2.80 GHz

Figure 3-17 Radiation pattern of the antenna at 3.40 GHz

Figure 3-18 Radiation pattern of the antenna at 4.00 GHz

30

Figure 3-19 Antenna efficiency and radiation efficiency of the antenna

Figure 3-20 Gain of the antenna

Figure 3-19 shows antenna and radiation efficiency. The antenna efficiency in

the frequencies band is ≥ 80 %. The radiation efficiency in the frequencies band is

about 82-92 %. Antenna gain is shown in Figure 3-20. Gain of antenna is the

minimum at the lowest resonant frequency of 4.5 dBi and the maximum at the highest

resonant frequency band of 5.9 dBi.

31

CHAPTER 4

EXPERIMENTAL RESULTS

A wideband slot antenna array with CPW-fed inductively coupled structure was

designed and implemented. The antenna is connected with a network analyzer to

measure return loss and input impedance. The measurement results were compared

with the simulation results. The discussions are given with the results.

Figure 4-1 shows the prototype of a wideband slot antenna array with CPW-fed

inductively coupled structure. The dimension of the antenna prototype closes to the

dimension from designing.

FIGURE 4-1 The prototype of a wideband slot antenna array with CPW-fed

inductively coupled structure

A network analyzer, Agilent 8719ES, is connected with the antenna prototype to

measure the return loss by using coaxial cable 50 Ω type RG-142 connecting together.

The experimental setup is shown in Figure 4-2. The measurement has been performed

at frequency range of 1-5 GHz.

32

FIGURE 4-2 Measurement setup for the return loss

Figure 4-3 shows the operation bandwidth starting from 2.80 GHz to 4.10 GHz.

The center frequency is 3.4 GHz. The impedance bandwidth at -10 db return loss is

1.3 GHz (38.235%). Therefore, this figure confirms that the prototype antenna has a

wideband operation.

FIGURE 4-3 Measured return loss

Figure 4-4 shows comparison between simulated and measured return loss

results, which a good agreement is obtained. However, measured result is shifted from

33

simulated result to higher frequency a little bit, because simulation program had been

set 5 cells/wavelength, which faster than 20 cells/wavelength (default value) but the

result was a decreased accurate result. In addition, dimensions of the prototype

antenna have some errors in designing dimensions and error in fabricated the antenna.

FIGURE 4-4 Simulated and measured return loss

FIGURE 4-5 Measurement setups for radiation patterns

34

For radiation patterns measurement, system setup is shown in Figure 4-5. The

RG generator was used to generate signal at frequency 2.8 GHz, 3.4 GHz, and 4.0

GHz. Horn antenna was connected to RF generator by coaxial cable 50 Ω type

RG-142 for transmit the signal. The prototype antenna was connected to the spectrum

analyzer by coaxial cable 50 Ω type RG-142 for receive the signal from horn antenna.

The distance between transmitted and received antennas is 120 cm and 100 cm height.

Then the received antenna is turned around from 0 degree to 360 degrees and then

value of received power was recorded every 5 degree. This thesis shows measurement

in X-Y plane (H plane) and X-Z plane (E plane). Each plane was measured for both of

co-polarization and cross-polarization.

The measurement setup for measured co-polarization in X-Z plane is shown in

Figure 4-6. Both transmit and receive antennas are arranged in same direction. Figure

4-7 shows the measurement setup for cross-polarization in X-Z plane. The received

antenna is arranged orthogonally to the transmitted antenna.

FIGURE 4-6 Measurement setups for co-polarization in X-Z plane

FIGURE 4-7 Measurement setups for cross-polarization in X-Z plane

35

Figure 4-8, 4-9 and 4-10 show the measured radiation patterns in X-Z plane at

frequency 2.8, 3.4 and 4.0 GHz, approximately corresponding to the lower end, center

and upper end frequencies of the prototype antenna, respectively. A well-defined

bidirectional pattern is observed. The F/B is better than 10 dB for all frequencies.

FIGURE 4-8 Radiation patterns in X-Z plane at 2.8 GHz

FIGURE 4-9 Radiation patterns in X-Z plane at 3.4 GHz

36

FIGURE 4-10 Radiation patterns in X-Z plane at 4.0 GHz

The radiation pattern at 4.0 GHz shows the cross-polarization in some direction

is higher than co-polarization and the pattern will be inclined. That means, the

antenna patterns are poor radiation pattern when operate in high frequency.

The measurement setup for measured co-polarization in Y-Z plane is shown in

Figure 4-11. Both transmit and receive antennas are arranged in same direction.

Figure 4-12 shows the measurement setup for cross-polarization in Y-Z plane. The

received antenna is arranged orthogonally to the transmitted antenna.

FIGURE 4-11 Measurement setups for co-polarization in Y-Z plane

37

FIGURE 4-12 Measurement setups for cross-polarization in Y-Z plane

Figure 4-13, 4-14 and 4-15 show the measured radiation patterns in Y-Z plane

at frequency 2.8, 3.4 and 4.0 GHz, approximately corresponding to the lower end,

center and upper end frequencies of the prototype antenna, respectively. The front to

back ratio of the antenna is better than 20 dB for all frequencies.

FIGURE 4-13 Radiation patterns in Y-Z plane at 2.8 GHz

38

FIGURE 4-14 Radiation patterns in Y-Z plane at 3.4 GHz

FIGURE 4-15 Radiation patterns in Y-Z plane at 4.0 GHz

39

The radiation patterns in Y-Z plane (H-Plane) at 4.0 GHz are in good

agreement with simulation results. For all patterns, the co-polarization is higher than

the cross-polarization for 20 dB. That means the front to back ratio in H-plane is more

than 20 dB for all frequencies.

This chapter shows measurement results of the wideband slot antenna array with

CPW-fed inductively coupled, which is based on the proposed techniques. The

prototype antenna has linear polarization in Z-direction. The prototype antenna

achieves a wideband, there were about 38% measured (BW=1.3 GHz) with starting

frequency as 2.8 GHz to end frequency as 4.1 GHz, which is agree well with

simulation results. Additionally the proposed antenna is also providing bidirectional

pattern radiation. The antenna is compact and easy to fabricate, and achieves

extremely wide bandwidth and bidirectional radiation characteristics.

CHAPTER 5

CONCLUSION AND FUTURE PROSPECTS

In this thesis, a wideband slot antenna array with CPW-fed inductively coupled

structure is proposed. The idea is to add a dipole step impedance resonator arrays on a

dipole uniform impedance resonator. To validate this design technique, the proposed

antenna is demonstrated. The prototype antenna has been designed on a single layer

FR4 substrate with dielectric of 4.4, loss tangent of 0.002 and dielectric thickness of

1.6 mm and using IE3D to simulate the antenna characteristics. Measurements have

been performed by using a network analyzer and spectrum analyzer.

5.1 Conclusions

The purpose of this thesis is successfully completed as studied and designed the

antenna using a step impedance CPW-fed inductively coupled dipole arrays with

uniform impedance CPW-fed inductively coupled dipole structure to increase an

operation bandwidth. Methods of research were designed to create a uniform CPW-

fed dipole first. The simulated result of the return loss of the uniform CPW-fed dipole

yields 3% bandwidth. Second step, combination of array SIR has been studied.

Adding of array SIR was made to obtain a wider bandwidth. After a lot of

experimental optimization, the measured operation bandwidth of 38% is obtained,

which agrees well with the simulated results (36%).

The radiation pattern is bidirectional pattern for all of operation bandwidth,

although the main lobe at the end of operation frequency shows a little bit lifting.

Additionally, the prototype antenna shows high gain. It can be observed that the peak

gain can be higher than 5.9 dBi at 4 GHz. Also at other frequencies, more than 4 dBi

gain is obtained.

An addition of SIR arrays using a connection of coplanar waveguide can

increase operation bandwidth of classical UIR antenna and it still keeps the pattern of

bidirectional radiation.

5.2 Problem and Suggestion for future work

Deviation from measurement compared with simulation results may be caused

by a few problems. The first problem is a differentiation of return loss at the center

frequency of operation bandwidth because there are dimension’s errors between the

design and the prototype. Secondly, measurement tools may cause the problem, since

the low loss coaxial cable and connection tools were used. Finally, the last problem

may be caused by human errors on fabrication and using measurement tools.

This structure can be designed to suitable for mobile phone and Wireless LAN.

The number of operation frequency can increase by adding the slot to this structure.

But, it may caused to larger ground plane size.

REFERENCES

1. Menzel W. and Grabherr W. “A microstrip patch antenna with coplanar feed

line.” IEEE Microwave Guided Wave Letters. November 1992: (340–342).

2. Giauffret L., Laheurte J.-M., and Papiernik A. “Study of various shapes of the

coupling slot in CPW-fed microstrip antennas.” IEEE Trans. Antennas Propagat. April 1997: (542–547).

3. Liu H.-C., Horng T.-S., and Alexopoulos N. G. “Radiation from aperture antennas

with coplanar waveguide feed.” Proc. IEEE AP-S Symp. Dig., 1992: (1820–

1823).

4. Wen-Hua Tu and Kai Chang, “Miniaturized CPW-fed slot antenna using stepped

impedance resonator.” IEEE Trans. Antennas Propagat., July. 2005: (351–

354).

5. Chaimool S. and Akkaraekthalin P. “Miniaturized CPW-fed inductively coupled

slot antennas using stepped impedance resonator with tuning slot stub

loading.” International Symposium on Antennas and Propagation., 2006.

6. Sierra-Garcia S. and Laurin J.-J. “Study of a CPW inductively coupled slot

antenna.” IEEE Trans. Antennas Propagat., January 1999: (58–64).

7. Bhobe A.-U., and Holloway C.-L. “Wide-band slot antennas with CPW feed lines:

Hybrid and log-periodic designs.” IEEE Trans. Antennas Propagat., October

2004: (2545-2554).

APPENDIX A

SPECTRUM UTILIZATION 3-7 GHz

43

FIG

UR

E A

-1 S

pec

trum

uti

liza

tion 3

-7 G

Hz

APPENDIX B

SIMULATION AND DESIGNING PROGRAM

45

Antenna Designing Programs

FIGURE B-1 Simulation program for microwave frequency devices (IE3D Zeland)

FIGURE B-2 Simulation of a wideband slot antenna array with CPW-fed inductively

coupled structure

46

Basic Parameters Assignment of IE3D Program

FIGURE B-3 Basic parameters assignment

APPENDIX C

EECON 30

48

49

50

51

52

BIOGRAPHY

Name : Mr. Jeerasak Chuangchai

Thesis Title : A Wideband Slot Antenna Array with CPW-fed inductively coupled

Structure

Major Field : Communication Engineering

Biography

I graduated with Bachelor of Industrial Technology, Industrial Electrical

Technology, Faculty of Engineering, King Mongkut’s Institute of Technology North

Bangkok, Bangkok in 1997. I have studied in Master of Science in Communication

Engineering at King Mongkut’s Institute of Technology North Bangkok in the

Department of Communication Engineering, Sirindhorn International Thai-German

Graduate School of Engineering (TGGS) since 2004. My Current position is lecturer

in electronics engineering technology department, college of Industrial Technology,

King Mongkut’s Institute of Technology North Bangkok. My interested topics are

microwave circuit design and antenna technology.

My work experiences:

1997-2007: Electronics engineering technology department, college of

Industrial Technology, King Mongkut’s Institute of Technology North Bangkok

My Contact is 16/27, Watkampeang Soi. 6, Phibulsongkram road., Muang,

Nonthaburi, jcc_liverbird@hotmail.com or 02-9668198, 089-6663156.