Review: UWB System and Antennas - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/72903/13/12. chapter 3.pdf · Review: UWB system and Antennas Study and Development of Compact

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


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.



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.



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



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)



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.



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



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.



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



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).



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



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



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)%.



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].



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



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.



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)



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)



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).



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



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.

Documents

Review: UWB System and Antennas - INFLIBNETshodhganga.inflibnet.ac.in/bitstream/10603/72903/13/12. chapter 3.pdf · Review: UWB system and Antennas Study and Development of Compact