1 CHAPTER 1 INTRODUCTION This chapter discusses the latest trends in wireless systems, history of wireless communications, primitive antennas and fractal antennas desirable for communication. The chapter concludes with an outline of the dissertation. 1.1 LATEST TRENDS IN WIRELESS SYSTEMS The vision of the Wireless World Research Forum (WWRF) estimated that 7 trillion wireless devices will serve 7 billion people by 2017 (Jefferies 2008). Wireless technology has helped to simplify network which enables multiple users to share common resources available. Currently, Wireless Local Area Networks (WLAN) are incorporated widely in areas such as residence, educational institutions, and business centers. It focuses on many applications including wireless sensor networks, automated highways, palmtops, electronic gadgets, factories, and navigation aids. Wireless networking means the connectivity to have data transmission between multiple users. Wireless networking is used to access the common databases/resources concurrently without additional or interfering wiring in a host. The resources include a broadband internet connection, data transfer from one host to another network printing, streaming of audio and video files through wireless connectivity with directional antennas (Sedat Atmaca et al 2006). The demand for broadband grows across the globe. There is an urgent need to improve the capacity of these networks.
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1
CHAPTER 1
INTRODUCTION
This chapter discusses the latest trends in wireless systems, history
of wireless communications, primitive antennas and fractal antennas desirable
for communication. The chapter concludes with an outline of the dissertation.
1.1 LATEST TRENDS IN WIRELESS SYSTEMS
The vision of the Wireless World Research Forum (WWRF)
estimated that 7 trillion wireless devices will serve 7 billion people by 2017
(Jefferies 2008). Wireless technology has helped to simplify network which
enables multiple users to share common resources available. Currently,
Wireless Local Area Networks (WLAN) are incorporated widely in areas
such as residence, educational institutions, and business centers. It focuses on
many applications including wireless sensor networks, automated highways,
palmtops, electronic gadgets, factories, and navigation aids. Wireless
networking means the connectivity to have data transmission between
multiple users. Wireless networking is used to access the common
databases/resources concurrently without additional or interfering wiring in a
host. The resources include a broadband internet connection, data transfer
from one host to another network printing, streaming of audio and video files
through wireless connectivity with directional antennas (Sedat Atmaca et al
2006). The demand for broadband grows across the globe. There is an urgent
need to improve the capacity of these networks.
2
In last decades, there is an explosive growth in cellular system and
there is no chance that this growth will never slow down (Berridge et al
1998). Presently, three billion users in world utilize systems/devices to
establish wireless connectivity. The progressive increase in wireless
systems/devices which are connected, indicates a shining future for wireless
networks. The stand-alone systems with larger networking infrastructure
results in crammed wireless band.
Research in this area is driven by the need for larger capacity
networks with dual band, multiband, wideband, low cost, and compact
device/terminals which provide better mobility and interoperability. The
antenna plays a major role and it is considered as the heart for any
communication/wireless system. It serves in establishing a successful
wireless communication link between systems. It is crucial to consider the
size and the cost, which should deliver the need of wireless systems, markets
and the customers (Pozar 1996 and Rahmat-Samii et al 1998).
Patch antenna is a solution which has a tendency to occupy less
space on wireless boards. The antenna has many advantages. Inspite of this,
the antennas have a major disability. The disability is that the antenna exhibits
very narrow bandwidth for any design frequency. The antenna surmounts
various wireless applications with better bandwidth and return loss, which
allows prototype model to distribute a variety of wireless applications. A
competent design with the aid of fractal geometry on the antenna essentially
miniaturizes the size of antenna and can be made to resonate for multiband.
As a result, there is a progress in the overall performance of antennas.
A special attention towards the enhancement in modern antenna
technology is discussed in this chapter with the ancient times of history of
wireless communication. In general, this it includes a brief classification of
antennas, current developments in printed antenna technology and the
3
advantage of fractal geometry. This chapter concludes with the motivation of
the work along with the dissertation organization.
1.2 WIRELESS COMMUNICATION CHRONICLE -A CONCISE
THUMBNAIL
In 1873, James clerk discovered the objective reality of
electromagnetic waves, which resulted in wireless communication (Maxwell
1873). He claimed that electromagnetic radiation of other wavelengths
should be achieved when the light is electromagnetic in character. The
objective reality of these electromagnetic waves was logically proved by
Heinrich Hertz with the aid of first spark-gap generator in 1888. In 1896,
Guglielmo Marconi logically proved wireless telegraph by transmitting
message to English telegraph office (Beynon 1975). During the year 1894-
1900 processed research on electromagnetic waves with the first horn antenna
was investigated (Krauss 1985).
Guglielmo Marconi discovered the transmission of three dot morse
code for the letter �S� over a distance of three kilometer in modern wireless
communication (Garratt 1994). The first part of antenna era was expatiated
with an experiment on a transmitting antenna. This antenna has 50 vertical
wires. It resembles a fan. It is connected to the ground with a spark
transmitter. The receiving antenna was a 200m wire pulled and supported by
a kite. The radio transmission through wire antenna was made possible to the
other side of the world (1992).
In 1982, Global System for Mobile communication (GSM) group
was organized which laid as a backbone for the modern wireless mobile
networks. The release of first GSM specification and the experimentation of
�L� band digital radio were the key proceedings in wireless communication
history. In 1983, Edwin Armstrong marked the Frequency Modulation (FM)
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to David Sarnoff. In 1940, Daniel Noble, a professor of electrical engineering
at the university of Connecticut designed FM mobile radio for the state police
(George 1992). FM is a means of access to the transmission of digital
information carried over RF. The development in wireless technologies
creates an appeal for refined and non voice services such as Wireless Fidility
(WiFi), 3G, in addition to GSM, Code Division Multiple Access (CDMA),
and Universal Mobile Telecommunication System (UMTS) (Simon Haykin et
al 2005).
The first GSM call was made in Finland in 1991. Six years later
the IEEE 802.11 standard also known as WiFi was formed (Blake 1928).
Uninterrupted Bluetooth special interest group was formed. The first
Bluetooth product was introduced by Ericcson in the year 2000 (Bowers
1978). Wireless headset, and phone adapter was meant for cell phones. The
research in wireless communication is now rapidly increasing which enables
the communication easy. An overview of WiFi, GSM, CDMA, and UMTS
frequency band allocations for modern wireless communication standards are
summarized in Table 1.1.
1.3 ROLE OF ANTENNAS IN WIRELESS COMMUNICATION
SYSTEMS
Antenna is one of the deciding factors of components in wireless
communication systems. An antenna is regarded as an aerial according to
IEEE standard which is meant for radiating or receiving radio waves
(IEEE 1983). All antennas operate in accordance with electromagnetic
theory. The word antenna is derived from a Latin word antemna which
became, in Latin language antennae. Marconi was the first person to use the
term antenna. He used the term in a lecture in 1909 (Garratt 1994).
5
Table 1.1 Frequency band allocations
System Description Frequency Band (MHz)
GSM-900 Global System for Mobile communication
880-960
GPS Global Positioning systems 1208-1248 and1556-1595
DCS-1800 Digital Communications Service 1710-1795
PCS -1900 Personal Communication System 1850-1990
PHS Personal Handy-Phone 1905-1920
UMTS Universal Mobile Telecommunications Systems
1920-2170
Wi-Bro Wireless Broadband 2300-2390
ISM Industrial, Scientific and Medical 2400-2484 , 5150-5350 and 5275-5825
DVB-H Digital Video Broadcasting 470-890
RFID Radio Frequency Identification systems
30-2400
UWB Ultra Wide Band 3100-10600
The theories of James Clerk Maxwell in electricity and magnetism
paved way for the gradual development of antennas. In 1940, antenna
technology was generally related to a wire type of radiating elements. These
antennas operate in Ultra High Frequency (UHF) range frequencies. Modern
antenna technology with its associated elements namely waveguides,
apertures horn antenna, and reflectors, set forth a new era during the Second
World War (Sterling et al 2000). This new era of microwave communication
began by the discovery of microwave sources such as klystron and
magnetron.
6
Antenna technology was witnessed during the period of Second
World War. Ensuingly, there was a development in computer architecture,
and technology which plays a major role in the advancement of modern
antenna technology. The introduction of examining complex antenna were
dealt with numerical methods. It is burdensome to analyze the design of
antenna (Branko et al 2002).
A drastic change in the improvement of antenna technology was
witnessed in the middle years of nineteenth century. The improvement
technology of antennas impedance bandwidth is as great as 40:1 or more.
Instead of linear dimensions, these wideband antennas had the geometries
specified by angles. Therefore, antennas are stated as frequency independent.
Television reception, point to point communication and feed for
reflects, and lenses are considered as the major applications of these wideband
antennas. A new radiating element was introduced. Comparing to earlier
design patch antenna, many applications with much ease of fabrication was
found. These antennas provide coordination with active components. A
range of antenna characteristics namely gain, radiation pattern, and dimension
of main element can be controlled electronically.
In recent years, major advancement in millimeter wave antennas
has been made successful. In one compact unit, active and passive circuits
were combined with the radiating elements. Smart antennas which is other
wise called as adaptive arrays were also introduced. It incorporates signal
processing algorithms. The above said antennas pave way for easy integration
with the advanced digital systems (Tapan Sakar 2006).
In outline, antennas are the important components of any electric
system. They are the connecting links between a transmitter and free space,
then free space to a receiver. Between a guided wave and free space,
7
antennas acts as a transducer. To set up a successful communication between
any device, antenna serves as a stepping stone.
Courtesy: Antenna theory and design by Balanis.
1.3.1 Classification of Antennas
Antennas are broadly classified as:
i) Wire Antenna
ii) Aperture Antenna
iii) Corner Antenna
iv) Dipole Antenna
v) Printed Antenna
Antennas are commonly employed in automobiles, buildings, radio
receiver units warships, and aircrafts. This family covers classical antenna
types such as dipole, loop, helix, etc. In wire antennas, loop antennas are a
category. It can be realized in different shapes such as square, triangle,
rectangle and circular loops. Circular loop antenna is most commonly
employed in loop antenna. It has a simple construction. Aperture antennas
are the most appropriate candidate for aircraft and space applications, where
the antenna has to be mounted on the surface of large crafts.
To obtain improvement in radiation characteristics in the desired
direction, antenna arrays shall be incorporated by replacing single antenna
element. A few antennas are depicted in Figure 1.1. A recent development in
array antennas which includes adaptive array is capable of beam forming
(Kin-Lu Wong et al 2004, Chen et al 2007 and Keizer et al 2007).
8
As the name implies, printed antennas are simple and inexpensive
to fabricate using modern printed circuit technology. They are low profile and
conformable to planar and non planar surfaces. Planar antennas are
compatible with Monolithic Microwave Integrated Circuit (MMIC) designs.
One among the most popular printed antenna is Microstrip patch antenna.
The prenominal development in wireless communication systems
resulted in tremendous growth of compact handheld devices such as phones
and Personal Digital Assistants (PDA) (Row 2005 and Frigon et al 2007).
The antenna employed has to occupy less space on wireless boards. It helps
the antenna to be miniaturized in size and to meet the requirements. Planar
antennas are widely used in communication devices; especially in WLAN. It
is because they can be easily included on a board which reduces the
packaging cost.
Earlier, these antennas found application in microwave such as
microstrip, slot lines, coplanar lines, etc. Multiple resonances in the antenna
are obtained by introducing slots or various resonating patches that are
compact to bring down the lateral dimension (Pozar 1992 and James et al
2003).
Planar antennas with printed circuit technology on Printed Circuit
Board (PCB), tends to exhibit miniaturization. The various types of printed
antennas are presented in the following sessions.
The key features of printed antennas are:
Light weight and small volume (overall dimensional)
Easy to fabricate using printed circuit technology
Easy to integrate with electronic components
Easy to convert into array systems
These antennas suffer a major drawback of serving low efficiency
due to substrate dielectric loss.
10
1.4.1 Microstrip Antenna
Figure 1.2 depicts a simple microstrip patch antenna. These
antennas consists of a radiating element on one side and dielectric substrate
on the otherside, which is known as a ground plane. The signal is coupled
with the main radiating element through any one feeding techniques.
Figure 1.2 2D View of microstrip patch antenna
The patch is designed around 2
in wavelength to radiate effectively
and permits fringing of electromagnetic fields between the edge and the
ground plane.
Modern wireless systems widely employ microstrip patch antennas.
It is compact compared to conventional microwave antennas. Advances of
wireless communication system, and other wireless applications, antenna
design has become more significant in the recent years. The microstrip patch
antenna has attracted wide interest due to its fundamental characteristics.
Ground plane
Substrate
Radiating element
11
General Characteristics of microstrip antennas are:
Light weight and low volume
Low profile planar configuration, hence it can be easily
mounted on wireless boards
Fabrication cost is low, so bulk production becomes easy
Easily integrated with microwave integrated circuits
Operates for dual and triple bands with two orientations
Exhibits linear and circular polarizations with lucid feed
techniques
Microstrip patch antenna suffers major drawbacks when compared
to primitive antennas. The reasons are as follows.
Narrow bandwidth and its associated problems
Power handling capacity, efficiency and gain is low
Ohmic loss due to feed structure of arrays is more
Excitation of surface waves
Complex feed structures are required for high- performance
arrays
Inappropriate radiation from feed lines and junctions
12
Triangle Circle Ellipse
Square Rectangle Dipole Arc
Annular ring
Figure 1.3 Different geometries employed for microstrip antennas
Increase in quality factor (Q) of antenna, tends to exhibit narrow
bandwidth and low efficiency. If Q is reduced by increasing the substrate
thickness, power delivered by the source goes into a surface waves.
Therefore, it results in wastage of power loss. The surface waves
degrades the antenna characteristics due to scattering (Kin-Lu Wong et al
2002). Figure 1.3 displays a few commonly employed shapes. These patches
are not restricted to the shapes.
1
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14
of the resonators in a suitable form (Bhatti et al 2007). A planar monopole
antenna can be treated as a cylindrical monopole antenna with large effective
diameter.
In microstrip line feed technique, the feed line is directly connected
to the edge of the monopole radiator as shown in Figure 1.5. Here the patch
and the feed line are etched on the same substrate to provide a planar
structure. It provides easy integration with RF circuit boards but creates
spurious feed radiation. Various interesting designs involving PIFA, stacked
antennas and Electronic Band Gap (EBG) antennas are reported in the
literature of multiband and wideband applications (Virga and Rahmat-samii
1997, Mclean et al 1999, Skrivervik et al 2001, Raj Mittra 2005 and Sim et al
2006). The PIFA marches towards multiband characteristics with capacitive
loading which increases the overall size of the antenna (Rowell and Murch
1997, Garg et al 2001 and Park et al 2006).
1.4.3 Commonly Adapted Techniques for Exciting Printed Antennas
There are many configurations that can be used to feed microstrip/
printed antennas.
A few popular methods are listed below:
i) Coaxial probe feed (Probe feed)
ii) Microstrip line feed
iii) Aperture coupled feed
iv) Proximity coupled feed
v) Coplanar feed line.
15
In the case of a coaxial probe feed, the inner conductor of the
coaxial connector is soldered to the radiating monopole/ main radiating
element, while the outer conductor is grounded. In this folder, the entire
system is not planar because the radiating structure is perpendicular to the
ground plane. The coaxial probe feed is easy to fabricate, and match. It has
low spurious radiation. Nevertheless, coaxial feed has narrow bandwidth.
Microstrip feed is easy to model and fabricate. It is effortless to match the
impedance of antenna in inset position.
Figure 1.5 Feeding techniques of patch antennas
Transmission Line Feed Coplanar Wave Guide Feed
Coaxial Feed
16
(a) (b)
(d) (c)
Figure 1.6 Equivalent circuit of Microstrip feed line (a) Coaxial feed (b) Aperture coupling (c) Proximity coupling of patch antenna and (d) Transmission line feed
On the other hand, the substrate width, surface waves, and spurious
feed radiation increases, for practical designs which is frontier of the
bandwidth. The equivalent circuits for these feed techniques are given in
Figure 1.6 and the Table 1.2 illustrates the performance comparison between
the different feed techniques (Bahl and Bhartia 1980, Rainee Simons 2001,
Ramesh Garg 2001 and Balanis 2011).
17
Table 1.2 Comparison in different feed techniques of printed antennas
Uniqueness Coaxial probeInset feed
Proximity coupled
Aperture
Coupled CPW Feed
Spurious Feed radiation
More Less
Polarization Poor Excellent Good
Fabrication Soldering and
drilling Easy Alignment needed
Reliability Poor Better Good
Impedance matching
Easy
Bandwidth 2-5% 13% 21% 3%
1.4.4 Coplanar Waveguide and its Application in Antennas
Figure 1.7 shows a 2D View of Coplanar Waveguide. The Coplanar
Waveguide (CPW) was invented by Wen (1969). The main difference
between a CPW and a microstrip is that, if two microstrips are placed on a
same plane with spacing between them becomes CPW. This is the main
difference between a CPW and microstrip. The further improvements have
reached to Elevated CPW (ECPW).
A prevalent CPW consists of a centre strip conductor at the centre
and partial ground planes at both the sides of a dielectric substrate. CPW is
more advantageous when compared to a microstrip line.
18
Figure 1.7 2D View of Coplanar Waveguide
The features of CPW are easy to fabricate. In CPW, hosting of
active and passive devices is easy. In CPW, shunt as well as series surface
mounting of active and passive devices are possible. These techniques,
eliminate the need for wraparound through holes due to which the size reduction and radiation loss are achieved (Browne 1989 and Browne 1990). In
addition, the ground planes exist between any two adjacent lines. The effect
of cross talk is minimized between the line significantly (Browne 1987). Besides, ratio of distances between the line determines the characteristic
impedance. Hence, size reduction is possible. CPW circuits can be made
denser than conventional microstrip circuits. These, as well as several other
advantages, make CPW ideally suited for Microwave Integrated Circuits (MIC) and MMIC applications.
1.4.4.1 Categories of coplanar waveguides
Generally, CPW can be classified as:
Conventional CPW
Conductor backed CPW
Elevated CPW
Micro-machined CPW
Ground
Ground
Strip
19
In a conventional CPW, the ground planes are of semi-infinite
extent on either side. However, in a practical circuit the ground planes are
made of finite extent.
The micro machined CPWs are of two types namely:
The micro shield line
The CPW suspended by a silicon dioxide membrane above a