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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, [email protected] or 02-9668198, 089-6663156.