Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 28
Chapter 3
Review: UWB System and Antennas
3.1 Introduction
ltra wideband (UWB) is an emerging technology for future short-range
wireless communication with high data rates, radar imaging and
geolocation [1]-[4]. The word „ultra-wideband‟ commonly refers to signals or systems
that have a large bandwidth. The use of a large bandwidth offers multiple benefits such
as high date rates, robustness to propagation fading, accurate ranging, superior obstacle
penetration, interference rejection, and coexistence with narrow bandwidth systems. A
landmark patent in UWB communications was submitted by Ross in 1973. It was then
in 1989 that the term “Ultra Wideband” appeared in a publication of the Department of
Defence in the United States (U.S.) and the first patent with the exact phrase “UWB
antenna” was filed on behalf of Hughes in 1993[4]. The first UWB signals were
generated by Hertz, which radiated sparks via wideband loaded dipoles [65]. UWB
communications has drawn great attention since 2000.
Obstacles such as multiple access interference (MAI) and UWB emission over a
large frequency range were taken into account by the regulatory body for commercial
uses of UWB. In 2002, interest in UWB systems was greatly magnified by the decision
of the United States frequency regulating body, the FCC. They released a report
approving the use of UWB devices operating in several unlicensed frequency bands
such as (0–960) MHz, (3.1–10.6) GHz, and (22–29) GHz.
In April 2009, the Electronic Communications Committee (ECC) of Europe
proposed two sub-bands, the lower band ranging from 3.1 GHz to 4.8 GHz and the
higher band from 6 GHz to 8.5 GHz. Similarly, Japan published their proposed low and
high sub-bands from 3.4 GHz to 4.8 GHz and 7.25 GHz to 10.25 GHz respectively. The
upper limit for effective isotropic radiation power (EIRP) is common and is set to be
-41.3 dBm/MHz. Even though the authorized frequency bands are different for the
various world regions, the definition of UWB is universal.
U
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 29
3.2 UWB Wireless System and Standards
UWB describes wireless physical layer technology, which uses a bandwidth of at
least 500 MHz or a bandwidth which is at least 20% of the central frequency in use.
Thus those systems that have a relative bandwidth of larger than 20% are known as
ultrawideband. Four methods emerged to spread the signal over large relative
bandwidth, and are impulse radio (IR), direct-sequence code division multiple access
(DS-CDMA), orthogonal frequency division multiplexing (OFDM) and frequency
hopping [65]. There are two approaches for UWB systems: pulsed operation and
multiple narrow bands. The first approach is based on traditional impulse radio (IR)
method. Impulse Radio refers to the use of a series of very short duration pulses, which
are modulated in position or/and amplitude. As signals are carrierless (that is only
baseband signals exists) no intermediate frequency processing is needed. In IR systems,
the transmitting pulse occupies the entire or partial UWB spectrum (7.5 GHz
bandwidth). The second approach, the multiple narrow band is based on multiple carrier
orthogonal frequency division multiplexing (OFDM) and direct sequence code division
multiple access (DS-CDMA) methods. The other two competitive alternative schemes
of multiband approach are multi-band orthogonal frequency division multiplexing (MB-
OFDM) and multi-carrier code division multiple access (MC-CDMA). OFDM has
become popular for high data rate transmission in IEEE 802.11a/g wireless standards. In
MB-OFDM, the total UWB frequency band from 3.1GHz to 10.6 GHz is divided into
14 sub-bands, each of which has a bandwidth of 528 MHz and conforms to the FCC
definition of UWB as shown in Figure 3.1. Each 528 MHz band comprises of 128
carriers, modulated using QPSK on OFDM tones [3]. The main difference between
MB-OFDM and a traditional OFDM system is that the data transmission is not done
continually on all sub-bands. MB-OFDM provides flexibility to adopt the various
spectral regulations made by regulatory bodies, including multiple data rates as per the
need of the end user. Due to its multiband scheme, MB-OFDM permits adaptive
selection of the sub-bands so as to avoid interference with other systems at certain
frequency range. Within the sub-bands, the effect of non-linearity of the phase shift on
the reception performance can be ignored, because the phase varies very slowly with
frequency. In this thesis the antenna designed focuses on achieving frequency response
with respect to the return loss, VSWR, gain, and radiation pattern over the operating
band, which fully covers the UWB of 7.5 GHz.
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 30
Figure 3.1 Spectra of OFDM UWB systems compliant with the FCC’s emission limit
masks for indoor and outdoor UWB applications
Though the communication range may be within tens of meter, pulsed or OFDM
communication systems tend to use high data rates, typically in the range of 1 to 2 giga-
pulses per second. The use of high data rates can enable efficient transfer of data
amongst various handheld devices such as digital camcorders, cell phones, personal
digital audio, video players, laptops, etc.
In addition with the IEEE 802.11 standard based WLAN products (“Wi-Fi”) and
IEEE 802.15 standard Bluetooth-based WPAN products a variety of wireless
networking products are available with high data rate, to develop digital home and
commercial applications. Task Group TG3a has set out to develop a flexible standard,
which will enable high data rate WPAN (110 Mbps at 10m, 200 Mbps at 4m, and 480
Mbps at an unspecified distance). The task group TG3c (formed in March 2005)
developed a millimeter wave based alternative physical layer (PHY) for the existing
WPAN Standard 802.15.3-2003. This millimeter wave WPAN operates in band
including 57–64GHz unlicensed band defined by the FCC at 47 CFR 15.255. IEEE
802.15.3c-2009 was published on September 11, 2009. The millimetre wave WPAN
application allows a high data rate of over 2 Gbit/s. Presently, several UWB devices are
entering the market based on the 802.15.3a standards.
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 31
3.3 Definition, Advantages and Benefits of the UWB System
Definition of UWB
UWB signals can be defined as signals having a fractional bandwidth of at least
20% of the center frequency or has a bandwidth of at least 500 MHz, regardless of the
fractional bandwidth. The fractional bandwidth (FRB) is defined as:
(3.1)
Where: f2 = the upper -10 dB frequency point on the signal spectrum
f1 = the lower -10 dB frequency point on the signal spectrum
UWB is a wireless technology for transmitting digital data over a wide spectrum with
very high data rates and low power over short distance communication. UWB
technology has the ability to carry signals through doors and other obstacles. Improved
channel capacity is one of the major advantages of UWB. Information is transferred
through a RF spectrum channel. Shannon‟s capacity limit equation showed that capacity
increases as a function of bandwidth (BW), faster than as a function of SNR (signal to
noise ratio).
(3.2)
Where: C = Channel capacity (bits/sec)
BW =Channel bandwidth (Hz)
SNR= Signal to noise ratio
Where: P = Received Signal Power (Watts)
N0= Noise Power Spectral Density (Watts/Hz)
The Shannon‟s equation indicates that the UWB technology is capable of
transmitting very high data rates using very low power with an increase in channel
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 32
bandwidth. UWB antenna plays a very important role to increase channel capacity for
high data rate communication in an indoor environment.
Advantages
UWB offers many advantages over narrowband technology, such as: [3]
Coexistence with current narrowband and wideband radio services
Large channel capacity or huge data rate
Low transmit power
Ability to work with low SNRs
Resistance to jamming
High performance in multipath channel
Simple transceiver architecture
Benefits
Avoids expensive licensing fees
High bandwidth can support real-time high definition video streaming
Provides low probability of detection and interception
Reliable with hostile environments
Delivers higher signal strength in adverse conditions
However, the number of advantages in UWB systems also gives rise to a number
of challenges, such as:
Pulse shaped distortion
Channel estimation (difficult predicting the template signals)
High frequency synchronization (very fast analog to digital converters required)
Multi-access interference (hard to detect)
Low transmit power (information can travel only for a short distance)
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 33
3.4 UWB spectrum Issues
Many organizations and government entities around the world are grouped into
regional, national and international levels to lay down rules and recommendations for
UWB usage [2]. At the regional level, the Asia-Pacific Tele-community (APT) is an
international body that sets recommendations and guidelines for telecommunication in
the Asia-Pacific region. The European Conference of Postal and Telecommunications
Administrations (CEPT) had created a task group under the Electronic Communications
Committee (ECC) to draft a proposal regarding the use of UWB for Europe. At a
national level, USA was the first country to legalize UWB for commercial use. In the
UK, the regulatory body, called the Office of Communications (OfCom), opened
consultation on UWB matters in January 2005. All the regulatory bodies set rules for
protection of existing radio devices to keep the UWB out of their frequency range.
3.4.1 Frequency Regulations and Spectral Masks
UWB system minimizes the interference of existing wireless systems by
spreading the power over a very large bandwidth and follows the restrictions of the FCC
on the emitted power spectral density as shown in Figure 3.2 [65]. The FCC and other
regulatory groups have specified spectral masks for different applications and allowed
power output for specific frequencies. The frequency masking depends on the
applications as well as the environment in which the devices are operated. For indoor
communication, a power spectral density of -41.3 dBm/MHz is allowed in the
frequency band between 3.1 GHz–10.6 GHz. No intentional emissions are allowed
outside the 7.5 GHz band. The admissible power spectral density (PSD) for spurious
emission provides special protection for GPS and cellular services as shown in Figure
3.2. To avoid inadvertent jamming of existing systems such as GPS satellite signals, the
lowest band edge of UWB for communication is set at 3.1 GHz, and the highest is set at
10.6 GHz.
For outdoor communication such as wall imaging systems and ground penetrating
radar, the operation is admissible either in the (3.1–10.6) GHz range, or below 960
MHz. For the through-wall and surveillance systems, a number of military UWB
systems seem to operate in the frequency range from (1.99–10.6) GHz, and below 960
MHz. The frequency range from (24–29) GHz is allowed for vehicular radar systems.
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 34
The emissions mask protects various other government systems in the 1.61–3.1 GHz
band and satellite systems above 10.6 GHz. The FCC emission power limits for indoor
and hand-held systems is illustrated in Table 3.1. The PSD review of some common
wireless broadcast and communication systems is tabulated in Table 3.2. One of the
benefits of low PSD is the low probability of detection, which is of particular interest
for military applications, such as secret communications and radar.
Figure 3.2 FCC regulated spectral mask for various indoor and outdoor applications
Table 3.1 FCC emission power limits for various systems
Frequency range (MHz) Indoor emission mask
(dBm/MHz)
Outdoor emission mask
(dBm/MHz)
960-1610 -75.3 -75.3
1610-1900 -53.3 -63.3
1900-3100 -51.3 -61.3
3100-10600 -41.3 -41.3
above 10600 -51.3 -61.3
Table 3.2 PSD of some common wireless broadcast and communication systems
System Transmission
Power
Bandwidth PSD (W/MHz) Classification
Radio 50 KW 75 KHz 6,66,600 Narrowband
Television 100 KW 6 MHz 16,700 Narrowband
2G Cellular 500 mW 8.33 KHz 600 Narrowband
802.11a 1W 20 MHz 0.05 Wideband
UWB 0.5 mW 7.5 GHz 6.670 x 10-8
Ultra wideband
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 35
3.5 Spatial and Spectral Capacities
Another basic property of UWB systems is their high spatial capacity, measured
in bits per second per square meter [bps/m2] [3]. Spatial capacity can be calculated as
the maximum data rate of a system divided by the area over which that system can
transmit. The transmission area can be calculated from the circular area, assuming a
transmitter in the center. However, in practice the rule of thumb is to use the square of
the maximum transmission distance:
For narrowband systems the most popular measure of capacity is the spectral
capacity, measured in bits per second per hertz (bps/Hz), because the spectrum is the
most limited resource.
Comparison of spatial capacity of various indoor wireless systems is given in Table 3.3.
Table 3.3 Comparison of the spatial capacity of various indoor wireless systems
System Maximum data
rate [Mbps]
Transmission
distance [m]
Spatial
Capacity[kbps/m2]
Spectral
Capacity[bps/Hz]
UWB 100 10 318.3 0.013
IEEE
802.11a
54 50 6.9 2.7
Bluetooth 1 10 3.2 0.012
IEEE
802.11b
11 100 0.350 0.1317
The transmit data rate can be increased by increasing the bandwidth occupation or
transmission power, which will decrease the spectral capacity as expected, in the UWB
system. For UWB systems, which operate in other licensed spectra, the power has to be
kept very low. This is compensated for by the use of extremely large bandwidths. Using
the traditional measure of spectral capacity (bits/Hz), the UWB spectral capacity is low
compared with existing systems.
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 36
3.6 Speed of Data Transmission
The large bandwidth of UWB systems means extremely high data rates can be
achieved. As can be seen in Table 3.4 the data rates for present indoor wireless UWB
transmissions are between 110 Mbps and 480 Mbps. This is fast compared with the
existing wireless and wired standards. In fact, the transmission speed is presently being
standardized into three different speeds: 110 Mbps with a minimum transmission
distance of 10 m; 200 Mbps with a minimum transmission distance of 4 m; and 480
Mbps with no fixed minimum distance.
The reasons for these particular distances lie mostly with different applications.
For example, 10 m will cover an average room and may be suitable for wireless
connectivity for a home theatre. A distance of less than 4 m will cover the distance
between appliances, such as a home server and a television. A distance of less than 1 m
will cover the appliances around a personal computer.
Table 3.4 Comparison of UWB bit rate with other wired and wireless standards
Standard Speed [Mbps]
UWB, USB 2.0 480
UWB (4 m minimum), 1394a (4.5 m) 200
UWB (10 meter minimum) 110
Fast Ethernet 90
802.11a 54
802.11g 20
802.11b 11
Ethernet 10
Bluetooth 1
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 37
3.7 UWB Applications
1. High-rate WPANs
Wireless local area networks (WLANs) with a transmission range of about 100 m
and wireless personal area networks (WPANs), with a transmission range of about 10 m
or less, are rapidly being established as popular applications for wireless technology.
The typical applications suggested by IEEE 802.15.3a standard for high-rate WPANs
are digital home requirements, which include the following and are shown in Figure 3.3.
Wireless video projectors and home entertainment systems with wireless
connections between components.
High-speed cable replacement, including downloading pictures from digital
cameras to PCs and wireless connections between DVD players, PC, Camcorder
projectors and HDTV (high-definition television).
Coexistence and networking of audio, still video, and motion pictures for fixed
and portable low-power devices.
Wireless replacement for Universal Service Bus (USB) connections among
computers and peripherals such as printer, scanner, mass storage devices in a
home as well as the office indoor environment.
Home network of audio and video with internet gateway.
High speed data transfer for multimedia wireless distribution systems for dense
user environments, such as multi-tenant units/multi-dwelling units (MTU/MDU).
Office, home, auto, and wearable wireless peripheral devices.
Due to the high
data rate, UWB can be used as an alternative to other wireless technologies, such
as Bluetooth, Wi-Fi, and Personal Area Network (PAN) applications.
The UWB devices used to develop a smart digital home are illustrated in Figure
3.4 and potential UWB applications scenario is shown in Figure 3.5 [4].
2. The FCC outlined other possible applications of this UWB technology to include
radars for close range, which can be used for wall imaging systems and ground
penetrating radar (GPR) systems for landmine detection in the frequency range
3.1–10.6 GHz, through-wall imaging systems (1.61–10.6 GHz), surveillance and
urban warfare systems (1.99–10.6 GHz).
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 38
3. The commercial application of UWB is vehicular radar systems and
communication and measurement systems (22 GHz–29 GHz).
4. Another promising application is the wireless body area network (WBAN), geo-
location of nodes in a sensor network and medical systems (biological imaging)
for cancer detection in the frequency range of 3.1–1 0.6 GHz.
Figure 3.3 Modern digital home equipped with various UWB devices
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 39
Figure 3.4 UWB devices (a) USB storage device (b) USB hub (c) USB 2.0 networking
server (d) UWB HDMI Extender (e) UWB laptop (f) Audio video Extender
(g) Multimedia transmitter (h) Computer to TV Wireless Connection Kit
Figure 3.5 Potential applications of the UWB system
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 40
3.8 Ultrawideband (UWB) Antennas
The allocation of 7.5 GHz wide frequency spectrum with EIRP less than -41dBm /
MHz for UWB applications, presents numerous exciting opportunities and challenges
for antenna designers. Some of the important challenges are large operating bandwidth,
segmentation of the wide bandwidth, in built band-notched design to avoid narrow band
interference, and compact size.
Compact size and wide impedance bandwidth are desirable features of UWB
antennas for indoor applications. For practical UWB applications, planar antennas
printed on various substrate materials are the capable candidates. Such planar antennas
are low profile, cost less in manufacturing and can be easily integrated with MMICs of
the miniaturized wireless UWB device.
3.8.1 UWB Antenna Characteristics
An antenna does the important role of transmitting source signal, by converting
it to electromagnetic waves into free space for communication and vice versa. An
antenna is usually designed based on the need of the application in which band the
radiation energy is focused, and suppressed in others at certain frequencies. A good
design of the antenna can full fill system requirements and improve overall system
performance for communication. The performance of an antenna is described with
respect to its parameters, such as impedance bandwidth, VSWR, radiation pattern,
radiation efficiency and gain.
3.8.1.1 Radiation and bandwidth
The radiation pattern indicates the directions the signals will be transmitted over
the wide operating bandwidth. The radiation characteristic is expected to be constant.
Also across a large frequency spectrum the phase of the antenna is desired to be linear.
The -10 dB impedance bandwidth called as absolute bandwidth of the UWB antenna is
7.5 GHz [65]. Particularly, a UWB antenna is defined as an antenna having a fractional
bandwidth (FRB) greater than 20% and a minimum bandwidth of 500 MHz, which is
more when compared with the narrow (less than 1%) and wideband antenna (1–20)%.
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 41
3.8.1.2 Mechanical Characteristics
The mechanical requirements in antenna design are also important, such as small
and compact size, low profile and low cost. The increase in electrical length will
achieve miniaturization of the antenna, but the physical dimension of the antenna must
be suitable to integrate it with the MMIC of short range UWB devices.
3.8.1.3 Band-Notch Characteristics
The performance of an antenna designed with UWB of 7.5 GHz (3.1–10.6) GHz
may get degraded due to the interference occurring from various narrow band systems.
The interference of wireless systems, such as IEEE 802.11a wireless LAN in USA
(5.15–5.35, 5.725–5.825) GHz and HIPERLAN/2 in Europe (5.15–5.35, 5.47–5.725)
GHz, with the UWB spectrum is shown in Figure 3.6.
The use of an additional filter design to reject these interferences occurring in the
UWB will increase the complexity of UWB systems, whereas this task can be tackled
by special antenna designs with band-stop characteristics. Therefore, to obtain dual
benefits; firstly to avoid the existing band interference and secondly to achieve
multiband characteristics, the antenna must be designed with single or multiple band-
notch characteristics.
3.8.1.4 Group Delay
One important characteristic of the UWB antenna is its non-dispersive behaviour
over the operating region. This property is quantitatively evaluated by the group delay
parameter. Group delay is defined as the derivative of far field phase with respect to the
frequency [4]. If the phase is linear throughout the frequency range, the group delay will
be constant for the frequency range. Group delay is an important characteristic because
it indicates how well a UWB pulse will be transmitted and to what degree it may be
dispersed. In wideband technology, group delay is a more precise and useful measure of
phase linearity and of the phase response. In short, group delay quantifies the pulse
dispersion and far field phase linearity. The distortionless time domain performance of
the antenna can be confirmed by small variations in the group delay [29].
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 42
Figure 3.6 Interference of WLAN with the UWB spectrum
Figure 3.7 Simulation setup for group delay measurement in face-to-face orientation
Figure 3.8 Co-axial feed monopole antenna with various shaped radiators
Constant group delay is required in the signal bandwidth to maintain signal
integrity of the pulsed wideband signal. A flat (small variation) nature of group delay
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 43
indicates UWB antennas have good transient response and fairly good dispersion in the
working band. It gives an average time delay and the input signal suffers at each
frequency, thus it is related to the dispersive nature of the antenna. Moreover, it is
necessary to have good group delay performance, and is very important for impulse-
radio UWB systems. Simulation results are obtained for two identical antennas with a
distance of 300mm in face-to-face configuration. That is group delay is achieved by
exciting two identical antennas located in the far field. The simulation setup of the
antennas in face-to-face orientation for measurement of group delay is shown in Figure
3.7.
3.8.2 Planar Broadband Monopole Antennas
The planar monopoles or disk antennas show excellent radiation performance with
good impedance matching over a wide spectrum [11], [24]-[26]. Because of the
significantly small size, these antenna configurations are preferred for development of
compact printed UWB antennas. Planar monopole antennas are a good choice to
achieve wide impedance band when bandwidth enhancement techniques are applied.
The planar monopole antenna is a good candidate to replace the straight wire
configuration, in which the wire is replaced by a disc or by various polygon shapes.
Planar disc monopole antennas yield a very large impedance BW, which can be
explained in the following two ways:
1. A monopole antenna generally consists of a thin vertical wire mounted over the
ground plane, whose BW increases with an increase in its diameter [12]. A planar
monopole antenna can be equated to a cylindrical monopole antenna with a large
effective diameter.
2. The planar monopole antenna can be viewed as a microstrip antenna on a very
thick substrate with unity dielectric constant, and hence a large BW is expected.
In the radiating metallic patch, various higher order modes are excited. Since all
the modes will have a larger BW, these will undergo a smaller impedance
variation. The shape and size of these planar antennas can be optimized to bring
several modes within the VSWR = 2 circles on the Smith chart, leading to a very
large-impedance BW.
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 44
The monopole disc can assume various configurations such as rectangular,
triangle, circular, elliptical, square, trapezoid, pentagonal, hexagonal, and so on as
shown in Figure 3.8. The antenna‟s performance is determined by the shape and size of
the planar radiator as well as the feeding section. The size and shape of the radiator
mainly determine the frequency corresponding to the lower edge of the impedance
bandwidth. The feed gap, location of the feed point, and the shape of the bottom of the
radiator determine the impedance matching as shown in Figure 3.8(a-h). The impedance
matching is determined by the impedance transition between the probe and the radiator.
A broadband impedance transition will ensure impedance matching across a broad
bandwidth. The bandwidth of the rectangular planar antenna can be enhanced by
modifying the bottom part of radiator and the ground plane such as the trapezoidal
shaped [33]-[36], [51]-[52].
3.8.3 Lower Edge Frequency Determination
As the planar monopole antenna possesses a wide impedance bandwidth because
of excitation of higher order multi-modes and optimization of various polygon shape
radiators, it is cumbersome to determine the resonant frequency of the wideband
antenna [4], [12]. The lower edge frequency calculation for the monopole antennas are
discussed as follows.
3.8.3.1 Planar Rectangular Monopole Antenna
For rectangular planar monopole antenna of length L and width W, the lower
frequency corresponding to VSWR = 2 can be approximately calculated by equating its
area to that of an equivalent cylindrical monopole antenna of the same height L and
equivalent radius r [12], as described below:
(3.3)
which gives:
(3.4)
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 45
The input impedance of a /4 monopole antenna is half of that of the /2 dipole
antenna. Thus input impedance of an infinitesimally thin monopole antenna is
36.5 + j21.25 Ω [101]. The real input impedance of 37 Ω which will match well with 50
Ω standard transmission line (with VSWR =1.35 ≤ 2) is obtained with a slightly smaller
length of the monopole given by;
(3.5)
Where:
(3.6)
From the above two equations λ is obtained as:
(3.7)
Therefore, the lower edge frequency is given as:
(3.8)
Considering the probe length p , the above equation (3.8) is modified as:
(3.9)
From the above equation (3.9) the lower edge frequency of any monopole can be
obtained by the values of L and r of the effective cylindrical monopole.
3.8.3.2 Planar Hexagonal Monopole Antenna
The hexagonal monopole antenna feed in the middle of the side length l, the L and
r values of the equivalent cylindrical monopole antenna are obtained by equating their
areas as follows:
(3.10)
(3.11)
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 46
For the hexagonal monopole antenna of side length l, when the feed at the vertex,
the L and r values of the equivalent cylindrical monopole antennas are obtained by
equating their areas as follows:
(3.12)
(3.13)
Substituting the above value of L and r in equation (3.9), the lower edge
frequencies can be calculated for both cases.
3.8.3.3 Planar Circular and Elliptical Monopole Antenna
Similarly, for the planar circular monopole antenna of radius a, the values L and r
of the equivalent cylindrical monopole antenna are given by:
(3.14)
(3.15)
An elliptical monopole antenna is a generalized case of the circular monopole,
wherein the major axis is not equal to the minor axis. The dimensions of the elliptical
monopole (i.e., major axis length = 2a and minor axis length = 2b) are calculated,
keeping its area equal with that of the circular monopole.
For calculating f L of the elliptical monopole antenna, the L and r of the effective
cylindrical monopole are determined by equating its area as:
(3.16)
For elliptical monopole antenna fed at minor axis, L = 2b and r = a/4, and for
elliptical monopole antenna fed at the major axis, these parameters are L = 2a and r =
b/4. The flower is determined by equation (3.9).
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 47
3.9 Printed Planar UWB Antennas
An antenna with a very small dimension and wide impedance bandwidth is the
first priority when choosing an antenna for UWB wireless applications. Also, the
designed antennas on a printed circuit board with capability of integration with UWB
devices are attractive to system designers. These antennas are usually designed and
constructed by etching the radiator onto the dielectric substrate and a ground plane near
the radiator. The radiating patch and the ground plane can be printed separately on both
sides of the substrate or both can be printed on one side of substrate [20]-[64].
3.9.1 UWB Monopole Antenna
The antenna can be fed by a microstrip transmission line or a coplanar waveguide
(CPW) structure. The printed monopole structures are shown in Figure 3.9 in which the
radiating patch can be fed by a microstrip or a CPW feed. The radiating patch of any
shaped printed antenna is optimized to cover the UWB bandwidth. The radiator can be
slotted for good impedance matching over a wide bandwidth. The impedance bandwidth
and radiation performance can be enhanced by tuning the dimension of the ground plane
and the radiating patch. The printed monopole antennas can be used for indoor wireless
communication systems because of their wide impedance bandwidth, omnidirectional
radiation patterns, simple structure, and low cost.
Figure 3.9 Microstrip and CPW fed monopole and slot antennas
Review: UWB system and Antennas
Study and Development of Compact Ultrawideband (UWB) Antenna for Wireless Communication System 48
3.9.2 UWB Slot Antenna
UWB slot antennas are evolved from the microstrip slot antennas. The features of
slot antennas such as bidirectional radiation pattern, various types of slot geometry and
feeding techniques offer an additional degree of freedom in the design of the UWB slot
antennas [68], [73], [75]-[95]. Wide bandwidth slot antenna use the microstrip feed to
excite the wide slot printed in the ground plane. In the CPW feed slot antenna, the
ground plane and radiating patch are printed in one plane of the substrate. These slot
antennas are called as uniplanar as shown in Figure 3.9(c). The feed line is terminated in
a tubing stub. The desired 50 Ω impedance matching can be obtained by tuning the feed
line, ground plane and tuning stub. The tuning stub used to construct the slot antenna is
of various shapes such as rectangular, circular, ellipse, U-shaped, fork shaped and many
more [66]-[100]. Because of their structure, CPW feed slot antennas are also called as
monopole slot antennas. The slot antennas are capable of producing very wide
impedance bandwidth with various impedance matching techniques. The broad
bandwidth is achieved with good coupling between the slot, feed and tuning stub.
Researchers have demonstrated microstrip and CPW feed slot antennas of rectangular,
ellipse, and circular shapes. The desired slot antenna must have small size,
omnidirectional patterns, and simple structure that produces low dispersion, but can
provide large bandwidth. The size of the printed antennas can be made very small for
their use in wireless applications.