Ultra Wide Band Antenna Design

Published Date: 02 Nov 2017

This project was initially proposed by PhD research students who needed to work on UWB frequency band between 3.1-5GHz for wireless localised network which raised the needs to design UWB antenna, printed on High frequency substrate with -10dB return loss. Due to its simple structure, low in cost of manufacturing and practical fabrication the rectangular microstrip patch antenna of a compact size and low cost was chosen to the application of a proposed simplified rectangular antenna structure. It is found wideband characteristics of the simplified rectangular antenna structures are similar to that of semi-circular based structures. Finally the antenna prototypes need to be fabricated and the patch antenna characteristics measured. The radiation patterns of the this antenna should be Omni-directional and S parameters of an antenna characteristics has been measured both magnitude and angle. The consistency between the simulated results and the measured results confirm the practicability of these techniques.

In this project, the compacted design and constructed microstrip Ultra Wide Band (UWB) antenna proposed has a capability of operating between 3.1 GHz to 5 GHz. The antenna parameters in frequency range assigned have been investigated to show its capability as an effective radiating element. As a result of the simulation output demonstrated realistic agreement with the theoretical calculation and also good ultra-wideband linear transmission performance has been achieved.

Contents page

Chapter 1

Introduction

The ultra-wideband (UWB) wireless communication has become an issue since the Federal Communication Commission (FCC) allocated unlicensed use of UWB band ranging from 3.1 GHz to 10.6 GHz for commercial use, the activities in the past few years in UWB technology has been rapidly advancing as an appealing high data rate wireless communication technology for various applications and received increasing attention in the wireless world [11]. Ultra-wideband (UWB) technology brings the convenience as well as mobility of wireless communications to the high-speed interconnections in communication devices throughout the digital home and office.

Antennas have become an attractive and challenging area in the research of the system design that needs to meet requirements of UWB systems sufficiently broad operating bandwidth for impedance Matching and high-gain radiation in desired directions. This project is focusing only modern UWB communication systems that are based upon 3.1 – 5 GHz antennas. The goal is to achieve the transmission of very low power pulses and high data rate transfer without disturbing other neighbouring wireless communications that share part of the Ultra Wide band. Furthermore, the phase transfer response must be linear with respect to frequency and ideally for Omni-directional emission, the dispersion that can be observed on radiated pulses reveals the antenna time behaviour Micro-strip antenna rectangular shape integration of an UWB antenna requires a minimum size, a low cost and a low dispersion, the key technical prerequisite of the project is to design a compact, low cost, wideband, Omnidirectional antenna capable to adopting electrical characteristics needed to employ wireless technology mostly sought for very high-data-rate and short-range wireless communication systems.

Aim and Objectives:

This project was chosen due to the needs from research students to have designer of a low cost UWB antenna with omnidirectional radiation pattern, smaller in size, printed on high frequency circuit board with ranging capability of 20/30 meters, to calculate both physical and electrical parameters of the microstrip patch antenna necessary to achieve its pre-requested performance.

The Project scope is to Model and design the simple omnidirectional antenna that introduces low distortions with large bandwidth which shows constant characteristics over the UWB frequency band (3.1GHz – 5GHz).

Omnidirectional

Low cost UWB antenna,

Smaller in size

Printed on FR4 board,

VSWR<=2

Gain in dB<5

Remain constant over the UWB frequency band (3.1 – 5GHz)

Centre frequency of 4GHz

Matched to 50Ὡ in its entire frequency band.

Ranging capability of 20/30 Meters

Radiation efficiency ≈85%

Very Low Transmitting power of -41.3dBm/MHz

Challenges

First and for most, to study and understand the ultra-Wide Band (UWB) technology and its use.

To study and understand the UWB antenna characteristics and design issues using appropriate literature.

Understanding the UWB antenna design and performance enhancements using available resources.

Deciding suitable software to design, Model and analyse, the characteristics and performance of the UWB antenna, such as: Gain, Loss(reflection coefficient) and S-parameters.

In order to understand UWB antenna characteristics and design issues, huge research was started to know more about the literature, history of UWB Technology, its use and concentrated design parameters of microstrip patch antenna

The modelling and then designing of an omnidirectional low cost UWB antenna that is small in size, printed on FR4 board. low cost, Voltage standing wave ratio of VSWR<2, with the gain less than 5dB and remain constant over the ultra-wide band frequency (3.1GHz – 5GHz).The antenna should be matched to 50Ὡ over the bandwidth between (3.1GHz - 5GHz). and to understand the concept of measurements of antenna Gain, Loss (reflection coefficient) and s parameters.

The antenna shall be designed, optimised and analysed using commercially available Design software Advanced Digital Design systems (ADS) from Agilent technology chosen due to its pioneering innovative digital designing, analysing and simulating the performance both physical and electrical of patch antenna parameters.

Chapter 2

Technical Background and context.

UWB background

The Ultra Wideband (UWB) is defined as the communication technology that occupies larger than 500 MHz of bandwidth, or larger 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. In February 2004 the FCC allocated the 3.1-10.6 GHz spectrum for unlicensed use [14] this enables the use of extremely low emission, short-range and because of short duration pulses gains significantly higher data rates transfer. The Ultra-wideband (UWB) technology offers a solution for the Necessary bandwidth required for the new electronics devices, As well as offering low cost and low power, with small physical Size and high transmission data rates

This Project was chosen to research and understand the concept and design requirements of ultra wide band technology and specifically antenna design, that leads me started to focus the best literature and understanding concepts of UWB technology and explore its fundamentals, including UWB definition, advantages, current regulations and standards Different parameters and characteristics of Microstrip patch antenna, physical size design and Electrical parameters commonly used in UWB applications, 10dB impedance bandwidth from 3.1 - 5 GHz and the. The antenna geometry is shown in Fig 1.

Ultra-Wide band Antenna transmits and receives digital data transfer over a wide spectrum of frequency band with very low power, data transfer at very high rates which is a key wireless local area network application. Within the power limit permissible under current FCC standards and regulations that allows Ultra Wideband to carry huge amounts of data over a short distance at very low power, [3][4]

Ultra-Wideband (UWB) is a technology which is used for transmitting and receiving information spread over a large bandwidth (>500 MHz), and able to share spectrum with other users with minimum interference. In February 2002, the FCC issued the FCC rulings that provided the first radiation limitations for UWB, and also allowed the technology commercially to use unlicensed Ultra Wide Band communication in the frequency range 3.1–10.6 GHz. That is planned to provide an efficient use of bandwidth while allowing high data rate in personal-area network (PAN) and wireless connectivity as well as longer-distance low data rate applications for radar imaging systems and positioning technology. "Deliberations in the International Telecommunication Union Radio communication" [3][4]

The fact that very low power density transmitted is simply means that the interference to other service users will be reduced to minimum that are not noticeable to traditional communication transmissions. Additionally the lowest frequencies for ultra-wideband (UWB) have been set above

3 GHz to ensure they do not interfere across bands currently operating for GPS, cellular and other services and channels. [7]

The performance of the antenna can be described for its basic characteristics like antenna’s impedance bandwidth and voltage standing wave ratio (VSWR), the amplitude radiation pattern, operating frequency, Antenna gain and directivity, all these parameters will be discussed individually through this project report.

Antenna impedance can be defined as the measure of total opposition of the AC current flow at the input of the antenna and is related voltage to the current and can be real or imaginary, in generally the real part of the impedance related to the radiated power while the imaginary part represents power stored in the near field of the antenna, which means non radiated power. The impedance of the antenna always varies with the frequency [8], the input impedance of the antenna need to match with the characteristics impedance of the transmission line so the maximum energy transfer is achieved, and reflection wave minimised and improve antenna efficiency.

The ratio between max voltage and minimum voltages on the transmission line is known as voltage standing wave ratio (VSWR) and is factor that can be observed how good is the antenna matching with the transmission line [9],

Technical approach

Microstrip Patch Antenna design

An antenna is a transducer designed to transmit or receive electromagnetic waves or converts alternating electrical currents to electromagnetic waves and radiates to free space and vice versa nad is an essential part of all radio equipment [13].

The main parameters needed to the design of rectangular Microstrip Patch antenna are:

• Operating Frequency ( fo ): The central frequency of the antenna selected According to project requirements. The ultra-wideband frequency range from 3.1-5GHz. Therefore, the antenna designed should be able to operate in this frequency range. And the resonant or central frequency selected for this design should be 4.05GHz.

• Dielectric constant of the substrate ( εr ): The dielectric material selected for this design is fiberglass substrate (also known as FR4). This has a dielectric constant of 4.6.

.• Height of dielectric substrate ( h ): Of the microstrip patch antenna to be used in

Localised UWB communication it is essential that the antenna is small in size. Hence, the substrate thickness selected as 1.575 mm.

Before the construction of the microstrip patch antenna, the physical parameters of the antenna has be calculated

Fig (1) Single Microstrip patch antenna

The microstrip patch antenna is built on one side of a dielectric substrate that has a ground plane on opposite side of the antenna. The Electro Magnetic waves fringes between the patch antenna into the substrate ground plane, reflecting off the ground plane and then radiates out into the free space. Microstrip Patch Radiation occurs mostly due to the fringing fields between the patch antenna and ground plane, the radiation efficiency of the Microstrip patch antenna depends mostly on the permittivity (εr) of the substrate dielectric constant. Example, a thick dielectric, low permittivity εr and increased efficiency, low insertion loss is preferable for broadband purposes. The advantages of microstrip antennas are its low-cost, conformable, bight weight and low profile [6]

The dielectric constant plays important role in the overall performance of the Patch Antenna due to the effect of the width, characteristic impedance and the length that results in changing resonant frequency and reduced transmission efficiency. The dielectric substrate proposed to use for the design of the project is FR-4, which is a composite material of woven fiberglass cloth with an epoxy resin binder that is flame resistant (self-extinguishing)[12].

FR-4 is very strong PCB and adaptable high-pressure thermoset plastic laminate graded with good strength to weight ratios material with almost zero water absorption and commonly used as an electrical insulator possessing substantial mechanical strength and electrical insulating both dry and humidity conditions. These attributes plus good fabrication characteristics, lead utility to this grade for a wider variety of electrical and mechanical applications [12][13].

BANDWIDTH

The initial operating frequency allocated for this project was the range between 3.1GHz - 5GHz and the bandwidth of this UWB microstrip patch antenna design is defined as the frequency band over which the antenna works and Matches with that of the feeding line, In other words, the frequency band over which the antenna achieves its minimum loss of energy. This means, the channels have usable large frequency range and thus results in improved transmission antenna bandwidth as usually defined by the standing wave ratio (SWR) value over the concerned frequency range. [6]

(1)

Where Fh is upper frequency band = 5GHz, and Fl is lower frequency band = 3.1GHz

BW= 1.9GHz.

Fractional Bandwidth

FBW = 2

Where Fh is highest frequency limit with signal 10dB below peak emission fh = 5GHz

Fl is the lowest frequency limit with signal 10dB below the peak emission fl = 3.1GHz

FBW= 0.47

Central frequency

Fc = 4.05GHz where Fc is central frequency.

Improved channel capacity is the major advantage of Ultra wideband, Hence the Shannon capacity equation is very useful as it shows the increasing channel capacity requires linear increase in bandwidth and exponential increase in power.

C = BW [log2(1+SNR)]

To calculate the patch Length

The length of the Microstrip patch antenna governs the central frequency thus it is a critical aspect, because its narrowband patch. The equation shown below was used to calculate the length of the patch. Since the fringing field cannot be accounted for accurately the results can’t be definite.

Leff = (2)

Where = 3.374, c = 3e8m/s, Fc = 4.05GHz, Leff = 20.2mm

In these expressions, the ∆ function is a correction for the effect of the fringing field

The In these expressions, the ∆ function is a correction for the effect of the fringing field

And can be calculated using the equation 3.

(3)

Where Patch Width, W = 22 mm, dielectric substrate height h = 1.474 mm, Effective dielectric constant, = 3.374,

The actual length of the Microstrip patch will be given equation (4)

Leff - 2 (4)

Where effective length of the of the patch, Leff = 20.2mm L = 18.74 mm

Width

The Microstrip patch antenna Width is calculated by using following equation

(5)

Where central frequency, Fc =4.04GHz, dielectric relative C = 3e8 m/s, permittivity, = 4.6. Patch antenna Width W = 22mm.

In this part we are comparing and analysing the microstrip antenna in infinite ground plane having dimension length, L = 18.736mm & width, W=22mm to the micro Strip antenna in finite ground plane having dimensions, length, L = 32.6mm & width, W=25.4mm. The proposed antenna designs have been analysed between 3.1GHz to 10 GHz. When the proposed antenna design on a 59mil GLASS PROXY Substrate of different dimension were examined for the same dielectric constant 4.6, loss Tangent of 0.02, at 5GHz and then verified result on Momentum simulating

The Ground plane dimensions can be calculated ( Lg and Wg):

This model of microstrip antenna design is based on transmission line with concept of indefinite Ground plane, which needs practical attention, to have finite ground plane.

As was long-established that the finite ground plane can be achieved if the size of the of the ground plane put greater than microstrip patch dimensions by approximately six time bigger than substrate thickness, therefore, the design parameters of the ground plane would be given.

Lg = 6h + L (6)

Where h is substrate thickness = 1.575mm, L is patch antenna length = 18.74 mm

Lg = 28.2 mm

Wg = 6h + W

Where h is substrate thickness = 1.575mm, W is patch width = 22mm

Wg = 31.43mm.

Results and discussions

The simulation results obtained from advanced design system (ADS) Momentum are shown below, The antenna characteristics such as return loss was indicated mismatch and Improvements can be made in order to match input impedance and patch antenna characteristic impedance match, which could Result in a better VSWR response over the frequency band 3.1 –5 GHz. The maximum

Simulated frequency band achieved was 3.1- 4.6GHz, theoretically the VSWR of a broadband antenna should be flat over the frequency band concerned, reflecting a VSWR of 1.

Fig 3, Single Microstrip Patch antenna (S11) Parameters on FR4 Dielectric Substrate

The above figure shows that the power distribution and absolute fields for Microstrip patch antenna

Figure 4 showing the 3D far field visualisation and antenna characteristics

. Conclusions and Recommendations for Further Work

From the work undertaken on designing microstrip patch antennas it become feasibility that

Microstrip patch antenna for use in UWB wireless indoor and outdoor systems operating the

3.1- 5 GHz frequency band, although the performance required to achieve become non conclusive due Matching between source and antenna characteristics impedance efficiency, it is however possible to overcome the difficulties and shape the antennas to suit the specifications required. The 3.1 GHz -5 GHz spans up to 3.1 to 4.8 GHz,

Offering a total active bandwidth of 1.7 GHz Instead of 1.9 GHz initially proposed.

The microstrip patch antennas were successfully implemented using ADS on FR4 substrate

And shows promising results in terms of performance and purposes of this project, the antenna

Was made from a narrowband rectangular patch antenna designed to resonate at 4.05 GHz,

. But this project performance of the antennas need to be improved considering the use of substrate and Patch antenna electrical parameters performance be further improved by using substrates and microstrip patch antenna physical parameters more adoptive each other.