Study Of A Ultra Wideband Antenna Computer Science Essay

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Antennas are very vital component of wireless communications systems. The recent allocation of the 3.1-10.6 GHz spectrum by the Federal Communications Commission (FCC) for Ultra Wideband (UWB) radio applications has presented a myriad of exciting opportunities and challenges for design in the communications arena, including antenna design. The antennas should be low cost and low profile to be embedded with the portable mobile devices. The antennas should be able to transmit and receive the short ultra wideband pulses. The focus of this work is to understand the antennas for wireless communications systems. This work recognizes the Ultra wideband as an emerging next generation wireless communication technology that has wide range of future applications. The design challenges for the UWB antennas are also identified in this review.

PROJECT OBJECTIVE

It is required to design an Ultra-Wideband Antenna in the project. A planar monopole antenna will be designed, fabricated and tested. The antenna is required to meet the following requirements:

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3.1 - 10.6 GHz Impedance Bandwidth

Omni-directional Radiation Pattern

INTRODUCTION

The world of wireless communications has gone through a tremendous progress over the past two decades [1]. The first development of consumer products for low power, high performance wireless Personal Area Networks (PANs) was started in 1996 at Interval Research, California [2]. The basic idea behind these Personal Area Networks was to remove the cumbersome cords within an indoor environment. Products concepts considered included video links for personal media players, video links from cameras to recording units, etc. It was revealed from the research that 2.4 GHz Industrial Scientific and Medical (ISM) Bands and 5 GHz Unlicensed National Information Infrastructure (U-NII) bands are not feasible for these applications [2]. The Federal Communication Commission (FCC) allocated the 3.1-10.6 GHz Ultra Wideband (UWB) spectrums in February 2002 for unlicensed In-door communications [3]. The FCC first Report and Order defined the UWB transmitter as an intentional radiator that occupies a fractional bandwidth of 20% or a UWB bandwidth of 500 MHz or more. FCC also set the emission limit of -41.3 dbm/MHz for the In-door and Handheld devices [4].

Although this Ultra Wideband technology is very attractive for high-speed short range wireless PANs but there are many design challenges for the UWB communication systems [5]. Design and implementation of suitable Antennas for the UWB systems is the most challenging problem at the Physical layer. The portable handheld devices require small and cheap antennas [6]. The antennas must be able to transmit and receive efficiently the UWB frequencies. Many Antennas have been designed and proposed that meet these requirements.

CHAPTER - 1 ANTENNAS

1.1 History of Antennas

The history of Antennas dates back to 1864 when James Clerk Maxwell presented his theory of Electromagnetism [7]. A German physicist Heinrich Hertz verified this theory in 1887 when he was able to demonstrate the first wireless electromagnetic system in the laboratory. The Guglielmo Marconi was the first man who used the antennas system to make the trans-Atlantic communications possible in 1901. After his work, Antenna systems were employed in the commercial radio systems until the World War - II, when Radar systems were introduced for the military applications [8].

The size of the antennas went on decreasing to incorporate the antennas in limited space. But, the reduction in antenna size presented problems to system designers due to performance penalties in antenna bandwidth and efficiency [9].

The evolution of mobile communications increased the challenges for antenna designers [10-11]. The need of complex antennas arise from several aspects, one being the cautionary measure of limiting the absorption and absorption density in the user head, the other being the optimization of the communication quality including the ease of use.

As the communication systems continue to evolve, the convergence of mobile communications and Internet accelerated the growth in wireless traffic. Smart antennas provided an attractive mean of increasing the capacity of third generation networks [12]. Multiple - Input Multiple - Output (MIMO) wireless systems, which use arrays of antennas instead of single transmitting and receiving antennas, is the promising technology to fulfil the increased data rate and reduced latency targets of beyond 3G systems [13-14]. Future applications of MIMO technology will include both fixed and mobile uses for indoor as well as outdoor environments.

1.2 ANTENNAS THEORY

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Antennas are the fundamental components of wireless communication systems. They are designed to transmit and receive the electromagnetic waves.

A typical wireless communication system is shown below.

Transmitting

Antenna

Receiving

Antenna

T

R

`

Transmitter Receiver

Figure 1.2 Communication Circuit with Waves from Transmitting Antenna arriving at receiving Antenna [15]

A transmitting antenna is connected to the transmitter through a transmission line and transforms a guided wave in the transmission line to a space wave. While receiving, an antenna is connected to the receiver through a transmission line and transforms a space wave to a guided wave in the transmission line. Thus, an antenna is a transducer between a guided wave and a space wave, or vice-versa [8, 15-16].

1.2.1 ANTENNA PARAMETERS

This section will introduce the antenna parameters from both the circuit point of view and field point of view.

1.2.1.1 INPUT IMPEDANCE

The antenna presents a load impedance to the transmission line at its input terminals. This impedance is called the Input impedance of antenna.

This input impedance determines how large a voltage must be applied at the antenna input terminals to obtain the desired current flow.

Mathematically, it is the ratio of input voltage Ei to the input current Ii.

Zi = Ei / Ii

1.2.1.2 REFLECTION COEFFICIENT, RETURN LOSS, VOLTAGE STANDING WAVE RATIO (VSWR)

If the input impedance of antenna (Zi) is not matched to the transmission line impedance (Zl), the input voltage is partly reflected and the voltage amplitude varies along the line to produce a standing wave pattern.

The ratio of the reflected and incident voltages is called the reflection coefficient. Mathematically,

 = Zi - Zl / Zi + Zl

The reflection coefficient in decibels is called the Return Loss.

Mathematically,

LRT = -20log10 (  )

The ratio of the maximum to minimum voltage at the line is called the voltage standing wave ratio, VSWR, that is

VSWR = Vmax / Vmin = 1 +   / 1 -  

1.2.1.3 BANDWIDTH

It is an important parameter of an antenna. It is defined as the range of frequencies for which the antenna satisfies the VSWR < 2 and/or LRT > 10 dB requirements.

1.2.1.4 RADIATION PATTERN

One of the most common descriptors of an antenna is its radiation pattern. Radiation pattern can easily indicate an application for which an antenna will be used.

According to the IEEE Standard Definitions of Terms for Antennas [17], an antenna radiation pattern (or antenna pattern) is defined as follows:

"A mathematical function or a graphical representation of the radiation properties of the antenna as a function of space coordinates. In most cases, the radiation pattern is determined in the far-field region and is represented as a function of the directional coordinates. Radiation properties include power flux density, radiation intensity, field strength, directivity phase or polarization."

Three dimensional radiation patterns are measured on a spherical coordinate system indicating relative strength of radiation power in the far field sphere surrounding the antenna. A two-dimensional radiation pattern is plotted on a polar plot with varying ϕ or θ for a fixed value of θ or ϕ, respectively.

Figure 1.2.1.4 A half-wave dipole and its three dimensional radiation pattern (on left) and two dimensional radiation pattern (on right) [18]

1.2.1.5 DIRECTIVITY

The directivity of an antenna is defined as "the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. The average radiation intensity is equal to the total power radiated by the antenna divided by 4Ï€."

D = U / U0 = 4U / Prad

Where,

U = Radiation intensity in given direction

U0 = Average radiation intensity

Prad = Total Radiated power

1.2.1.6 HALF POWER BEAMWIDTH

This parameter is useful in order to describe the radiation pattern of an antenna and to indicate its directivity. Half power beamwidth (HPBW) is defined as the angular distance from the centre of the main beam to the point at which the radiation power is reduced by 3 dB.

1.2.1.7 EFFICIENCY

The antenna efficiency takes into consideration the ohmic losses of the antenna through the dielectric material and the reflective losses at the input terminals.

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Reflection efficiency, or impedance mismatch efficiency, is directly related to the reflection coefficient (Γ). Reflection efficiency is indicated by r, and is defined mathematically as follows:

r = (1-|Γ|2) = reflection efficiency

Radiation efficiency is determined by the ratio of the radiated power, Prad to the input power at the terminals of the antenna, Pin:

e = Prad / Pin = Rr / Rr + RL

Where,

Rr = Radiation Resistance = the equivalent resistance which would dissipate the same amount

Of power as the antenna radiates.

RL = Antenna Ohmic Resistance

The total efficiency of the antenna is the product of Reflection and Radiation efficiency.

T = e. r

1.2.1.8 GAIN

The Gain of an antenna is usually defined as the peak gain, and hence is the product of Directivity D and radiation efficiency e.

Mathematically,

G = D. e

CHAPTER - 2 ULTRA WIDEBAND SYSTEMS

2.1 INTRODUCTION TO ULTRA WIDEBAND SYSTEMS

Ultra Wideband Radio (UWB) is a potentially revolutionary approach to wireless communication in that it transmits and receives pulse based waveforms compressed in time rather than sinusoidal waveforms compressed in frequency. This is contrary to the traditional convention of transmitting over a very narrow bandwidth of frequency, typical of standard narrowband systems such as 802.11a, b, and Bluetooth. This enables transmission over a wide swath of frequencies such that a very low power spectral density can be successfully received.

Figure 2.1a illustrating the equivalence of a pulse based waveform compressed in time to a signal of very wide bandwidth in the frequency domain [18]

Figure 2.1a illustrates the equivalence of a narrowband pulse in the time domain to a signal of very wide bandwidth in the frequency domain. Also, it shows the equivalence of a sinusoidal signal (essentially expanded in time) to a very narrow pulse in the frequency domain.

In February 2004, the FCC allocated the 3.1-10.6 GHz spectrum for unlicensed use [3].

This enabled the use and marketing of products which incorporate UWB technology.

Since the allocation of the UWB frequency band, a great deal of interest has generated in industry.

The UWB spectral mask, depicted in Figure 2.1b, was defined to allow a spectral density of -41.3 dBm/MHz throughout the UWB frequency band. Operation at such a wide bandwidth entails lower power that enables peaceful coexistence with narrowband systems. These specifications presented a myriad of opportunities and challenges to designers in a wide variety of fields including RF and circuit design, system design and antenna design.

Figure 2.1b FCC Ultra Wideband Spectral Mask [3]

Ultra Wideband is defined as any communication technology that occupies greater than 500 MHz of bandwidth, or greater than 25% of the operating centre frequency. Most narrowband systems occupy less than 10% of the centre frequency bandwidth, and are transmitted at far greater power levels.

2.2 UWB ANTENNAS

Designing a suitable antenna for Ultra Wideband applications is a challenging task. But, many good design techniques have been adopted for the better performance. Some popular designs that are common now a day will be discussed in this section. The main features of these designs for UWB applications will be discussed followed by an optimised design example.

2.2.1 MONOPOLE ANTENNAS

Monopole antennas are very popular for ultra wideband applications. They are usually constructed over the ground plane to behave like the dipoles. Many good designs of monopoles have been proposed and tested for ultra wideband applications.

The following are the important features of monopoles [19-24].

Monopoles offer wide bandwidth, simple structure, Omni-directional radiation pattern, and ease of construction.

Microstrip-fed monopole antennas are very suitable for integration with hand-held terminals owing to attractive features such as low profile, low cost, and light weight.

The impedance matched bandwidth (RL > 10dB) can be significantly increased by bevelling the bottom edge of the monopole. Similarly, by the introduction of notches at monopole's lower corner can improve the bandwidth of antenna.

The use of truncated or bevelled ground plane also improves the bandwidth of monopole.

The use of shorting pin at the edge of monopole reduces the lower-end frequencies. The size of the antenna can be significantly reduced below max/4, using the combination of shorting pins and bevelling ground plane.

[23] reported a modified design of a monopole antenna.

The following figure compares the antenna impedance bandwidth after the introduction of bevels. It clearly demonstrates the bandwidth improvement with the introduction of bevels.

Figure 2.2.1a Effect of introduction of bevel [23]

The following figure proves that the introduction of shorting pin reduced the lower frequency for a given antenna height. So, it was suggested that antenna height could be significantly reduced by the introduction of shorting pins.

Figure 2.2.1b Effect of shorting pin on impedance bandwidth [23]

2.2.2 VIVALDI ANTENNAS

Vivaldi antenna is a gradually tapered slotline flaring out in exponential form or linear form proposed by P.J.Gibson in 1979 [25]. In Dual Exponentially Tapered Slot Antennas (DETSAs), both the inner and outer edges are tapered. Vivaldi antennas are widely used in wireless and radar applications due to their broad bandwidth, low cross-polarization, and highly directive patterns [26].

Each Vivaldi antenna is composed of three sections: micro strip-line feed section, transition section and radiating section. The feed section connects the feed line and the antenna and a good match between them increase the efficiency of the antenna. Radiating section mainly radiates energy effectively and determines the frequency band of the antenna. Transition section is the most critical part in the antenna design, which gradually tapers from the feed section to the radiating section.

The special features of a Vivaldi antenna are as follow [25-28]:

Since higher frequency component is very sensitive to structure discontinuity, the transition from the feed section to the radiating section should be as smooth as possible to avoid sharp discontinuity, or, a serious distortion in the shape of radiating signal will be caused.

Vivaldi antenna radiates different frequency component in different part of the slot. The width of the slot mainly affects the performance at lower frequencies. A wider operating bandwidth can be obtained by rationally designing the dimension and the shape of the antenna.

Cross polarisation (perpendicular to the substrate plane) is relatively high in antipodal Vivaldi structures that radiate useless energy. So, a wideband balun is required that increases cost. However, the field can be balanced by the addition of another dielectric layer with metallization.

The careful design of the shape of the outer edge taper in DETSAs enhances the antenna performance.

Mostly the Vivaldi antennas are bulky, averaging over 15cm x 6cm. However, low-profile antennas have been designed that can be integrated with a circuit.

[26] presented an optimised design of a Vivaldi antenna. The proposed design is compact, covers the wideband, and has low cross-polarization levels as the following figures suggest.

Figure 2.2.2a Top and Bottom views of fabricated design on FR4 Substrate [26]

Figure 2.2.2b Antenna Return Loss indicating wideband operation [26]

Figure 2.2.2c Simulated Co- and Cross-polarisation radiation pattern at 4, 6, 8 and 10.5 GHz [26]

SLOT ANTENNAS

Microstrip-fed slot antennas especially printed wide-slot antennas have received much attention and are applicable in satellite and communications applications [29].

The following characteristics make it suitable for ultra wideband applications [29-33].

The wide slot antenna has the simple structure and broad bandwidth.

The radiation patterns of wide slot antennas are Omni-directional.

The use of wide slot, variation of slot shape and fork-shaped Microstrip feedline increases the bandwidth of slot antennas.

The use of parasitic strips and stubs introduces the band-notch characteristics to prevent the interference with existing communication systems.

[31] Reported that the introduction of conductor lines or stubs in the wide slot antennas can introduce the band-notch characteristics to avoid the interference with WLAN systems.

2.2.3a Optimised geometry (left) and Return-Loss (right) of the design

Figure 2.2.3b Simulated Radiation pattern of the design

2.2.4 UWB ANTENNAS COMPARISON

The above three types of antennas can be compared for Ultra Wideband applications. The following table compares the example of each type.

Antenna Type Size Band Width Main Features

Antipodal Vivaldi 40 x 42 mm 3 - 10 GHz Substrate FR4 (r = 4.4),

Low Cross-polarisation.

Planar Monopole 25 x 24 mm 2 - 10 GHz Bevelled lower corners

And shorting pin to

Increase the Band width.

Slot Antenna with 30 x 26 mm 3 - 11 GHz Conductor lines or stub

Band notch for Band notch.

2.3 Ultra Wideband Antennas - Design Challenges

Ultra wideband systems transmit and receive ultra short electromagnetic pulses, or in other words, they use ultra wide bandwidth signals with very low power transmissions [34]. Therefore, the traditional narrowband concepts and techniques often require revision in order to be applied in the UWB context [35].

The following challenges are present when the antennas are designed for the UWB systems.

Physical Compactness: Since the antennas are to be designed for personal mobile devices so, the antennas must be small. It is also highly desirable that the antenna be low cost and preferably constructed on a printed circuit board [36].

Impedance Matching: A clever matching network increases the system performance. Such matching networks become increasingly difficult to construct as the bandwidth increases. So, a good impedance match to an antenna is something that must be designed in from first principles, not added as an afterthought [35].

A Balun, when properly designed, is capable of enforcing a balance in current and/or voltage. The use of a balun for driving a UWB antenna places further limitations on performance such as loss and possibly dispersion [2].

Low Radiation Power: The regulatory limits for the UWB transmitters are defined in terms of Effective Isotropic Radiated Power (EIRP) that is given by the product of transmitted power PTX (f) and transmitted antenna gain GTX (f). i.e.

EIRP (f) = PTX(f) GTX(f)

A system designer must ensure the product PTX(f) GTX(f) to be constant and as close to the regulatory limit as a reasonable margin of safety (typically 3 db) will allow. Similarly, this power gain product must roll-off so as to fall within the skirts of the allowed spectral mask [35].

Additional Design Parameters: The development of UWB technology has shown that traditional antenna parameters such as gain, polarization, etc are applicable for narrowband antennas, but inadequate for UWB antennas. The additional parameters such as phase linearity, radiation pattern stability, etc are the major design issues for these systems [34, 37].

Time Domain Analysis: The radiation of short duration UWB signals from an antenna is significantly different from long duration narrowband signals [6]. The radiation of UWB signals involves fields that are time-delayed time derivatives of the signal currents from the transmitting antenna [3]. Conventional antennas are designed to radiate only over the relatively narrow range of frequencies used in conventional narrowband systems. If an impulse is fed to such an antenna, it tends to ring which severely distorts the pulse and spread it out in time. In UWB systems, the transmitting signal, the field signal and the received signal differ in shape. So, the UWB antenna design need to be considered in the time domain with due attention to their transient responses and to the time delay nature of radiation from the antenna [38].

Interference with Narrow-band Services: UWB systems share their frequency allocation with existing narrowband services, for example IEEE 802.11a systems operating from 5.15 to 5.825 GHz. The system designer must ensure not to interfere with these systems [2, 39]. A conventional filter in receiver front end or using the spread spectrum techniques can accomplish this. But, it is also possible to design antennas with band-notch characteristics to aid in narrowband signals rejection.

Propagation Environment: Propagation environments place fundamental limitations on the performance of wireless communications systems [40]. The existence of multiple propagation paths with different delays produces a complex transmission channel that limits the performance of wireless communications systems [41]. Although, UWB systems eliminate the significant multi path fading, but the system must be design keeping in view the particular environment.

Design Tools: Today, the Antennas are designed using the Computer aided tools. These computer tools are based on the numerical methods such as Method of moment, Finite element method, Finite difference time domain method, transmission line matrix method, etc. So, the choice of a certain tool for a certain application is also very critical in antenna design.

2.4 Ultra Wideband Technology - Applications and Future Directions

Ultra wideband technology is suited to enhance some of the most popular commercial products and applications with wireless connectivity or wireless personal area networks functionality. UWB comes with unique advantages that are appreciated for future applications [3, 10].

Enhanced capability to penetrate through obstacles.

Very high bandwidth enabling the high data rates.

Ultra high precision ranging at the centimetre level.

Very low spectral mask with much little electromagnetic radiation for environment and human body.

Potentially small size and low processing energy consumed.

These characteristics make the UWB suitable for the following applications [2,3,42-45].

Communications: UWB offers the wireless services for low data rate applications such as connecting peripherals as key board, mouse, printer, monitor etc with the computer. It also offers wireless services that require high data rates such as, wireless media players, wireless video downloading etc.

Sensor Networks: UWB sensors are being used to secure homes, automobiles and other property. UWB sensor networks are also being employed in medical monitoring to free the patient from wired sensors.

Position location and Tracking: UWB signalling is especially suitable for position location applications because it allows centimetre accuracy in ranging, as well as, low power and low cost implementation of communication systems. These features allow a new range of applications, including logistics (package tracking), security applications, search and rescue (communications with fire fighters etc), control of home appliances and military applications.