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CHAPTER 1

INTRODUCTION

1.1 OVERVIEW

The recent rapid growth in technology and the successful commercial deployment of

wireless communications are significantly affecting our daily lives. The transition

from analog to digital cellular communications, the rise of third- and fourth-

generation radio systems, and the replacement of wired connections with Wi-Fi and

Bluetooth are enabling consumers to access a wide range of information from

anywhere and at any time. As the consumer demand for higher capacity, faster

service, and more secure wireless connections increases, new enhanced technologies

have to find their place in the overcrowded and scarce radio frequency (RF) spectrum.

This is because every radio technology allocates a specific part of the spectrum; for

example, the signals for TVs, radios, cell phones, and so on are sent on different

frequencies to avoid interference to each other. As a result, the constraints on the

availability of the RF spectrum become more and stricter with the introduction of new

radio services. Ultra-wideband (UWB) technology offers a promising solution to the

RF spectrum drought by allowing new services to coexist with current radio systems

with minimal or no interference. This coexistence brings the advantage of avoiding

the expensive spectrum licensing fees that providers of all other radio services must

pay.

In spite of all these advantages, one of the practical problems faced by UWB

systems is the interference due to the coexistence of UWB with several other

narrowband services. For example, IEEE 802.11a which works around 5 GHz band is

a main source of interference to indoor UWB systems. Another example is the effect

of worldwide interoperability for Microwave access (WiMAX) and up link frequency

band for Standard C-band (5.8-6.4 GHz), Extended C-band (6.4-6.7 GHz), and

INSAT/Super Extended C-band (6.7-7.0 GHz), which is used for satellite application,

creates interference to the UWB systems as its operating frequency overlaps with

UWB system band. So, antenna with notch characteristics is enviable when other

services coexist with the UWB.

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The main aim of this project work is to develop UWB slot antennas to meet all

of the above requirements. In order to achieve these, a UWB antennas and notch

antennas for the rejection of UP link of C-band satellite and WiMAX, bands between

3.1 GHz to 10.6 GHz are developed and presented in this report. Behaviour of UWB

antenna and band notch antenna characteristics such as return loss, radiation pattern

and gain are analysed. The dimensions of the proposed antennas are appropriately

chosen to improve the performance.

1.2 HISTORY AND BACKGROUND

Ultra-wideband communications is fundamentally different from all other

communication techniques because it employs extremely narrow RF pulses to

communicate between transmitters and receivers. Utilizing short-duration pulses as

the building blocks for communications directly generates a very wide bandwidth and

offers several advantages, such as large throughput, covertness, robustness to

jamming, and coexistence with current radio services.

Ultra-wideband communications is not a new technology; in fact, it was first

employed by Guglielmo Marconi in 1901 to transmit Morse code sequences across

the Atlantic Ocean using spark gap radio transmitters. However, the benefit of a large

bandwidth and the capability of implementing multiuser systems provided by

electromagnetic pulses were never considered at that time.

Approximately fifty years after Marconi, modern pulse-based transmission

gained momentum in military applications in the form of impulse radars. Some of the

pioneers of modern UWB communications in the United States from the late 1960s

are Henning Harmuth of Catholic University of America and Gerald Ross and K. W.

Robins of Sperry Rand Corporation [1]. From the 1960s to the 1990s, this technology

was restricted to military and Department of Defence (DOD) applications under

classified programs such as highly secure communications. However, the recent

advancement in microprocessing and fast switching in semiconductor technology has

made UWB ready for commercial applications. Therefore, it is more appropriate to

consider UWB as a new name for a long-existing technology.

As interest in the commercialization of UWB has increased over the past

several years, developers of UWB systems began pressuring the FCC to approve

UWB for commercial use. In February 2002, the FCC approved the First Report and

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Order (R&O) for commercial use of UWB technology under strict power emission

limits for various devices [2]. Figure 1.1 summarizes the development timeline of

UWB.

FCC approves the use Spark Gap Military Radars and of Unlicensed UWB for Transmission, Hertz covert communications commercial purposes and Marconi

1900 1960 1990 2002

standardization

efforts continues...

Figure 1.1: A brief history of UWB developments

1.3 FEEDING METHODS

There are many methods of feeding a Microstrip antenna. These methods can be

classified into two categories-

a) Contacting.

b) Non-contacting.

In the contacting method, the RF power is fed directly to the radiating patch using a

connecting element such as a microstrip line/Coaxial Cable.

In the non-contacting scheme, electromagnetic field coupling is done to

transfer power between the microstrip line and the radiating patch such as aperture

coupling and proximity coupling [3].

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1.3.1 Microstrip Line Feed.

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

the Microstrip patch. The conducting strip is made smaller in width as compared to

the patch. Advantage is that the feed can be etched on the same substrate to provide a

planar structure.

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Figure 1.2: Microstrip line feed

Actually this kind of feeding has very high resonant input impedance. The

Resonant input impedance can be decreased by using an inset feed recessed by some

distance from the input slot. Impedance matching leads to less reflection loss at the

feed-slot junction. However, the inset feed introduces a physical notch and this notch

introduces junction capacitance and this junction can affect the resonant frequency.

1.3.2 Coaxial Feed (Coplanar Feed)

The Coaxial feed or Probe feed is also used for feeding Micro-strip antennas. The

inner conductor of the coaxial connector extends through the dielectric and is attached

to the radiating patch, while the outer conductor is connected to the ground plane.

The main advantage is that the feed can be placed at any desired location

inside the patch in order to match with its input impedance. This is easy to fabricate

and has low spurious radiation.

However, the main disadvantage is that it provides narrow bandwidth and it is

difficult to model since a hole has to be made in the substrate and the connector

bulges outside the ground plane.

For thick dielectric substrates, which provide broad bandwidth, the micro-strip

line feed and the coaxial feed suffer from numerous disadvantages. Both of them

possess inherent asymmetries and that leads to the generation of higher order modes

and cross polarization.

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Figure 1.3: Probe fed Rectangular Micro-strip Patch Antenna.

The non-contacting feed methods can solve the problems faced by contacting

feed methods.

1.3.3 Proximity Coupling

This feed technique employs electromagnetic coupling scheme. Two dielectric

substrates are used and the feed line is situated between the two substrates and the

radiating patch is on top surface of the upper substrate.

The main advantage is that it minimizes spurious feed radiation and provides

very high bandwidth (as high as 13%), due to overall increase in the thickness of the

antenna. This method provides choices between two different dielectric media, one

for the patch and one for the feed line; so it provides higher degree of freedom to

optimize the individual performances.

Figure 1.4: Proximity Coupled Feed

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The major disadvantage of this feeding method is that it is difficult to manufacture

because of the two dielectric layers which need proper alignment. Also, there is an

increase in the overall thickness of the microstrip antenna.

1.3.4 Aperture Coupling

The aperture coupling method is the most difficult one to fabricate. However, it is very

easier to model and has little spurious radiation problem. In this method the radiating

patch and the micro-strip feed line are separated by the common ground plane.

Coupling of energy between the patch and the feed line is made through a slot/

aperture in the ground plane.

Figure 1.5: Aperture Coupled Feed

The coupling aperture is usually made under the patch, leading to lower cross

polarization. The amount of energy coupling from the feed line to the patch depends

on the shape, size and location of the aperture. As the ground plane separates the patch

and the feed, spurious radiation is minimized. Generally, a thick, low dielectric

constant material is used for the top substrate and a high dielectric material is used for

bottom substrate to optimize radiation from the patch. The main disadvantage of this

feed method is that it is difficult to fabricate due to presence of multiple layers, which

also increases the antenna thickness. This feeding scheme also produces narrow

bandwidth.

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Table 1.1: [7] gives the comparison of the different feed techniques and their

characteristics

Characteristics Co-axial

Probe

Feed

Radia

ting

Edge

Coupl

ed

Non

radiating

Edge

Coupled

Gap

Coupl

ed

Insert

Feed

Proximi

ty

Coupled

Aperture

Coupled

Configuration Non Planar Copla

nar

Coplanar Copla

nar

Coplan

ar

Planar Planar

Spurious Feed

Radiation

More Less Less More More More More

Polarization Purity Poor Good Poor Poor Poor Poor excellent

Ease of fabrication Soldering

and drilling

needed

Easy Easy Easy Easy Alignme

nt

required

Alignment

required

Reliability Poor due to

soldering

Better Better Better Better Good good

Impedance Matching Easy Poor Easy Easy Easy Easy Easy

Bandwidth 2-5% 9-12% 2-5% 2-5% 2-5% 12% 21%

1.4 ADVANTAGES AND DISADVANTAGES OF SMA

Microstrip antennas (MSAs) have several advantages compared to the conventional

microwave. The main advantages of MSAs are listed as follows:

They are lightweight and have a small volume and a low-profile planar

configuration.

They can be made conformal to the host surface.

Their ease of mass production using printed-circuit technology leads to a low

Fabrication cost

They are easier to integrate with other MICs on the same substrate.

They allow both linear polarization and CP.

They can be made compact for use in personal mobile communication.

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Disadvantages

MSAs suffer from some disadvantages as compared to conventional microwave

antennas. They are the following:

Narrow bandwidth.

Spurious radiation (surface waves).

Need quality substrates (tanδ<0.002)

Difficult to achieve polarization purity.

Lower Power handling capability.

Large ohmic loss in the feed structures.

High Q-factor (sometimes more than 100).

Low Efficiency.

To overcome the problem of Microstrip antennas, we use CPW feeding

technique. In particular, CPW fed antennas have many salient features like less

radiation loss, less dispersion, easy integration with monolithic microwave integrated

circuits (MMIC). Therefore, CPW fed slot antennas are currently under consideration

for use of UWB systems for the numerous applications such as remote sensing, radar,

imaging, localization and medical applications.

1.5 COPLANAR WAVE GUIDE (CPW)

Coplanar Waveguide (CPW) is an alternative to Microstrip and Stripline that place

both, the signal and ground currents on the same layer. Cheng P. Wen is the inventor

of Coplanar Waveguide in 1969, when working at RCA's Sarnoff Laboratories. The

initial paper he published was: "Coplanar Waveguide: a surface strip transmission line

suitable for nonreciprocal gyro magnetic device applications".

The conductors formed a centre strip separated by a narrow gap from two

ground planes on either side. The dimensions of the centre strip, the gap, the thickness

and permittivity of the dielectric substrate determined the effective dielectric constant,

characteristic impedance and the attenuation of the line.

The gap in the coplanar waveguide is usually very small and supports electric

fields primarily concentrated in the dielectric. With little fringing field in the air

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space, the coplanar waveguide exhibits low dispersion. In order to concentrate the

fields in the substrate area and to minimize radiation, the dielectric substrate thickness

is usually set equal to about twice the gap width.

CPW has a zero cut-off frequency (suitable for wideband), but its low order

propagation mode is indicated with Quasi-TEM because it is not a real TEM

mode. At higher frequencies, the field becomes less-TEM, and more TE in nature.

The CPW magnetic field is elliptically polarized. CPW it is a printed circuit analogs

of the three-wire transmission lines.

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Figure 1.6: Coplanar Wave Guide

Figure 1.7: Field Distribution of CPW

Like stripline, CPW has two ground planes, which must be maintained at the

same potential to prevent unwanted modes of propagating. If the grounds are at

different potentials, the CPW mode will become uneven, with a higher field in one

gap than the other.

In the CPW two fundamental modes are supported: the coplanar mode, and the

parasitic slot line mode. Air bridges between ground planes have to be applied to

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suppress the undesired slot line mode. If bond wires are used to connect the ground

planes the wires should be spaced one quarter wavelength apart or less.

Figure 1.8: Bond Wire between CPW

In the CPW, the effective dielectric constant is approximately independent of

geometry, and simply equal to the average of dielectric constants of air and the

dielectric substrate. Frequency dispersion for CPW is generally small, but there is a

mild dependence on line dimensions, and narrow lines are less frequency dispersive

than wide lines.

1.5.1 Grounded Coplanar Waveguide

(GCPW) is used on printed circuit boards as an alternative to Microstrip line.

The gap s between the strip and ground is usually more than the thickness h of the

substrate, so the GCPW field is concentrated between the strip and the substrate

ground plane, and GCPW behaves like Microstrip. With vias connecting the ground

planes, GCPW is less prone to radiate and has higher isolation than Microstrip.

Since the number of the electric and magnetic field lines in the air is higher

than the number of the same lines in the Microstrip case, the effective dielectric

constant of CPW is typically 15% lower than the for Microstrip, so the

maximum reachable characteristic impedance values are higher than the Microstrip

values.

The effect of finite dielectric substrate is almost ignorable if h exceeds 2b =

W+2s.In addition, to avoid field radiation in the air, it is very important to use

substrates with a high dielectric constant, with recommended values greater than 10,

so that the electromagnetic field is mainly concentrated inside the dielectric.

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In CPW a ground plane exists between any two adjacent lines, hence cross talk

effects between adjacent lines are very week. As a result, CPW circuits can be made

denser than conventional Microstrip circuits.

1.5.2 Coplanar wave guide and its application in antennas

The coplanar waveguide was invented by C.P. Wen in 1969. Conventional

CPW consists of a centre strip conductor with semi-infinite ground planes on either

side on a dielectric substrate. Compared to a Microstrip line the CPW provides many

advantages like simplicity in fabrication, easy shunt as well as series surface

mounting of active and passive devices, eliminating the need for wraparound and via

holes, and reduction in radiation loss. In addition the ratio of a/b determines the

characteristic impedance so size reduction is possible. Moreover a ground plane exists

between any two adjacent lines; hence cross talk effects between adjacent lines are

very week. Therefore, CPW circuits can be made denser than conventional Microstrip

circuits. These, as well as several other advantages, make CPW ideally suited for MIC

as well as MMIC applications.

1.6 MOTIVATION

The UWB technology has experienced many significant developments in

recent years. However, there are still challengers in making this technology live up to

its full potential. One particular challenge is the UWB antenna.

Among the classical broadband antenna configurations that are under

consideration for use in UWB systems, a straight wire monopole features a simple

structure, but its bandwidth is only around 10%. A Vivaldi antenna is a directional

antenna [4] and hence unsuitable for indoor systems and portable devices. A biconical

antenna has a big size which limits its application. Log periodic and spiral antennas

tend to be dispersive and suffer severe ringing effect, part from big size [5]. There is a

growing demand for small and low cost UWB antennas that can provide satisfactory

performances in both frequency domain and time domain. In recent years, the circular

disc monopole antenna has attracted considerable research interest due to its simple

structure and UWB characteristics with nearly Omni-directional radiation patterns [6].

However, it is still not clear why this type of antenna can achieve ultra-wide

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bandwidth and how exactly it operates over the entire bandwidth. In this report, a

CPW fed octagonal antenna is investigated in detail in order to understand its

operation, find out the mechanism that leads to the UWB characteristic and also

obtain some quantitative guidelines for designing of this type of antenna.

1.7 OBJECTIVE

The objectives of the research work are,

To develop an inexpensive, light-weight, and portable new type of UWB

antennas for pulsed application

To develop UWB antennas to work without interference from the narrowband

services when they coexist with the UWB service

In order to achieve the above objectives, a comprehensive literature survey is

conducted to identify an appropriate antenna that satisfies the required design

considerations of the UWB antennas.

1.8 SCOPE OF THE PROJECT

The scopes defined for this project are as follows:

Understanding the antenna concept.

Perform numerical solutions using IE3D software

Analysed antenna properties.

1.9 ORGANIZATION OF THESIS

Chapter 1: In the first chapter basic overview of the project done is provided. This

chapter describes the history and background, feeding technique, objectives of the

research work, motivation and organization of the thesis. The remaining chapters of

the thesis are organized as follows:

Chapter 2: In this chapter, presents a relevant theory and literature survey of UWB

antenna and its geometric structures This chapter describes an overview of UWB

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concepts, fundamentals of UWB technology, primary requirements of UWB antennas,

the advantages and applications, and.

Chapter 3: This chapter presents the developed elemental hexagonal slot UWB

antenna using CPW feeding method. Performance and characteristics of the antennas

are investigated in both frequency and time domains. In addition, Design of UWB

antennas with notch and the important characteristics, which affect the antenna

performance, are investigated.

Chapter 4: This chapter presents simulation results of hexagonal slot containing

octagonal patched UWB antenna and its parametric analysis. Further, developed band

notch Antennas and discusses the important characteristics of single and dual notch

UWB antennas, which affect the antenna performance, are investigated both

numerically and experimentally. The experimentally measured result shows a

reasonably good agreement with simulated one.

Chapter 5: This includes conclusions and future scope to this project work