UNIVERSITY OF NAIROBI
FACULTY OF ENGINEERING
DEPARTMENT OF ELECTRICAL AND INFORMATION ENGINEERING
PROJECT INDEX: PRJ 18
PATCH ANTENNA ARRAY FOR THE 2.4 GHz ISM BAND
By
Waihenya Peter Ndung’u
F17/28805/2009
Supervisor: Dr. Wlifred N. Mwema
Examiner: Prof. H Ouma
A Project submitted in partial fulfillment of the requirements for the award of the degree
Of
Bachelor of Science in ELECTRICAL AND INFORMATION ENGINEERING of the Univeristy of Nairobi
Submitted on: April 24th, 2015
DECLARATION OF ORIGINALITY
Declaration Form for Students UNIVERSITY OF NAIROBI Declaration of Originality Form This form must be completed and signed for all works submitted to the University for examination.
Name of Student: WAIHENYA PETER NDUNG’U
Registration Number: F17/28805/2009
College: COLLEGE OF ARCHITECTURE AND ENGINEERING
Faculty/School/Institute: ENGINEERING
Department: ELECTRICAL AND INFORMATION ENGINEERING
Course Name: FINAL YEAR PROJECT
Title of the work: DESIGN OF A PATCH ANTENNA ARRAY FOR THE 2.4GHz ISM BAND
DECLARATION
- I understand what Plagiarism is and I am aware of the University’s policy in this regard 2. I declare that this project is my original work and has not been submitted elsewhere for examination, award of a degree or publication. Where other people’s work or my own work has been used, this has properly been acknowledged and referenced in accordance with the University of Nairobi’s requirements. 3. I have not sought or used the services of any professional agencies to produce this work 4. I have not allowed, and shall not allow anyone to copy my work with the intention of passing it off as his/her own work 5. I understand that any false claim in respect of this work shall result in disciplinary action, in accordance with University Plagiarism Policy.
Signature
Date 24/04/2015
DEDICATION
This project is dedicated to my loving parents Dr. and Mrs. Waihenya for their utmost support and encouragement throughout my education life and especially when I was undertaking my BSc. Electrical and Electronics Engineering degree.
AKNOWLEDGEMENT
I would like to express my heartfelt gratitude to Dr Wilfred Mwema of the department of Electrical and Information Engineering, University of Nairobi for his critical guidance as my project supervisor. I would also like to sincerely thank my family and classmates for their support. May God bless you.
ABSTRACT
In this study, a coaxial fed patch antenna array for application in the 2.4GHz ISM band was implemented using the Ansoft HFSS software. Standard formulas were used to calculate different parameters of the antenna. These were just used as a basis of design as some parameters varied considerably during simulation. A good extent of the antenna design was hence done through trial and error. The proposed antenna was designed to work at 2.44GHz frequency band. A fractional bandwidth of 2.62%, which was not close to the desired 10% and a reflection coefficient of -18.2131dB were attained. This may have been brought about by poor impedance matching and a high level of spurious feed radiation and surface waves. A way of improving the bandwidth would have been to use proximity coupling feeding method which offers the highest bandwidth (as high as 13%) and is somewhat easy to model and has low spurious radiation. However, its fabrication would have been more difficult. A directivity of 8.53dB was achieved. This was a fairly high though directivity increase could have been studied through use of different substrate material and thickness.
LIST OF FIGURES
Figure 2.1 Antenna measurement co-ordinate system………………………………………………………………….……...12
Figure 2.2 Microstrip antenna and coordinate system…………………………………………………………………..…….16
Figure 2.3 Microstrip line and its electric field lines, and effective dielectric constant…………………...……..17
Figure 2.4 Physical and effective lengths of rectangular microstrip patch…………………………………..…………18
Figure 2.5 Feed arrangements for microstrip patch arrays…………………………………………………………..……….20
Figure 3.1 Configuration for compensated right-angled bends……………………………………………………..……….23
Figure 3.2 Characteristics of the step width junction discontinuity………………………………………………….…...24
Figure 3.3 T-junction discontinuity compensation and minimization of the effect……………………………..….24
Figure 3.4 4 element patch antenna HFSS model………………………………………………………………………………....27
Figure 3.5 4-element patch antenna PCB layout with dimensions……………………………………………………...... 27
Figure 3.6 Implemented 4-element patch antenna array……………………………………………………………….……..28
Figure 3.7 ground plane of the patch array…………………………………………………………………………………….……..29
Figure 3.8 N male to sma female cable………………………………………………………………………………………….…...... 29
Figure 4.1 Return loss obtained for the patch array…………………………………………………………………….…..31
Figure 4.2 Simulated E-Plane (phi=, theta varying) ………………………………………………………………………....32
Figure 4.3 Simulated H-plane (theta=, phi varying)………………………………………………………………………....32
Figure 4.4 3D radiation pattern…………………………………………………………………………………………………………....33
Figure 4.5 E-Plane and H-Plane patterns in rectangular coordinates …………………………………………………....34
Figure 4.6 VSWR plot…………………………………………………………………………………………………………………………....35
Figure 4.7 Smith chart of the proposed patch antenna………………………………………………………………………....36
Figure A1.4 Rectangular microstrip patch and its equivalent circuit transmission-line model………………..40
Figure A1.5 Recessed microstrip- line feed……………………………………………………………………………………………43
LIST OF TABLES
Table 4.1 Variation of antenna parameters with changes in dimensions……………………………………………....30
Table 4.2 Variation of resonance frequency with changes in patch feed length………………………………….…34
Table 4.3 HFSS Antenna Parameters in HFSS…………………………………………………………………………………………37
TABLE OF CONTENTS
DECLARATION OF ORIGINALITY……………………………………………………………………………………………………………….2
DEDICATION…………………………………………………………………………………………………………………………………………....3
AKNOWLEDGEMENT………………………………………………………………………………………………………………………………..4
ABSTRACT…………………………………………………………………………………………………………………………………………..……5
LIST OF FIGURES………………………………………………………………………………………………………………………………………6
LIST OF TABLES………………………………………………………………………………………………………………………………………..7
CHAPTER 1: INTRODUCTION…………………………………………………………………………………………………………………10
CHAPTER 2: LITERATURE REVIEW…………………………………………………………………………………………………………11
Fundamental Specifications of Antennas ……………………………………………………………………………………………..11
Microstrip Antennas………………………………………………………………………………………………………………………………14
Basic Characteristics…………………………………………………………………………………………………………………..………….15
Transmission Line Model Analysis for a Rectangular Patch…………………………………………………………………….16
Arrays and Feed Networks………………………………………………………………………………………………………………....…19
CHAPTER 3: THE DESIGN METHODOLOGY………………………………………………………………………………………….…21
Design Procedure………………………………………………………………………………………………………………………………..…21
Ground Plane……………………………………………………………………………………………………………………………………..….22
Microstrip Discontinuities………………………………………………………………………………………………………………………23
Main Beam Direction…………………………………..…………………………………………………………………………………………25
Matching of Microstrip Lines to the Source……………………………………………………..…………………………………….25
Quarter Wave Transformer……………………………………………………………………………………………………………………25
Simulation…………………………………………………………………………………………….……………………………………………….27
Fabrication………………………………………………………………………………………….…………………………………………………28
CHAPTER 4: HFSS SIMULATION RESULTS AND ANALYSIS……………….……………………………………………………..30
Variation of Patch Length and Width……………………………………………………………………………………………………..30
Reflection Coefficient……………………………………………………………………………………………………………………………..31
Radiation Pattern…………………………………………………………………………………………………………………………………..32
Inset Feed Position…………………………………………………………………………………………………………………………………34
VSWR Plot………………………………………………………………………………………………………………………………………………35
Smith Chart……………………………………………………………………………………………………………………………………………36
Ground Plane………………………………………………………………………………………………………………………………………...36
H plane Inter-Element Separation….……………………………………………………………………………………………………...37
E Plane Inter-Element Separation………………………………..…………………………………………………………………………37
CHAPTER 5: CONCLUSION…………………………………………….……………………………………………………………………….39
APPENDICES…………………………………………………………………………………………………………….……………………………40
Appendix A……………………………………………………………………………………………………….……………………………………40
Conductance…………………………………………………………………………………………………….………….………………………..40
Resonant Input Resistance……………………………………………………………………………………………………..………………41
Appendix B…………………………………………………………………………………………………………………………………………….45
Matlab Code for calculation of the insed feed position where the input impedance is 50 Ohms…………….45
Matlab code for calculation for the width of the 50 ohm line……………………………………………………………..….45
REFERENCES………………………………………………………………………………………………………………………………………….47
CHAPTER 1: INTRODUCTION
An antenna is a transducer between a guided wave and a radiated wave, or vice versa. The structure that "guides" the energy to the antenna is most evident as a coaxial cable attached tothe antenna. A patch antenna is a type of radio antenna with a low profile, which can be mounted on a flat surface. It consists of a flat sheet of metal, usually copper, mounted on a larger sheet of metal called a ground plane.A patch array antenna is, in general, some arrangement of multiple patch antennas that are all driven by the same source. Frequently, this arrangement consists of patches arranged in orderly rows and columns (a rectangular array). The reason for these types of arrangements is higher gain. Higher gain commonly implies a narrower beamwidth and that is, indeed, the case with patch arrays.
This report presents the design and analysis of patch network antenna array for the 2.4GHz ISM bandwhich is largely license exempt and can be accessed freely for example bluetooth. The antenna will be designed with an aim of achieving high directivity and at least a 10% fractional bandwidth. The antenna will have a center frequency of 2.44 which is almost the same as the given ISM band center frequency. It was so chosen so as to have a bandwidth whose range is falls within the 2.4 Ghz band. The work presented here is the continuation or enhancement of the 2013final year patch antenna array project where a basic 4 element patch antenna array was designed without much emphasis on the gain, directivity or bandwidth.
The report consists of five chapters. After the introduction, the necessary theoretical background is presented in the second chapter. Then a chapter describing the design and all the steps and choices made for the patch antenna array follows.An Analysis of the simulated results together with discussions is done in chapter four. The conclusion, which includes a short summary of the design achievements, is presented in chapter five.
CHAPTER 2: LITERATURE REVIEW
An antenna is generally a bidirectional device, that is, the power through the antenna can flow in both directions, coupling electromagnetic energy from the transmitter to free space and from free space to the receiver, and hence it works as a transmitting as well as a receiving device. Transmission lines are used to transfer electromagnetic energy from one point to another within a circuit and this mode of energy transfer is generally known as guided wave propagation. An antenna can be thought of as a mode transformer which transforms a guided-wave field distribution into a radiated-wave field distribution. It can also be thought of as a mode transformer which transforms a radiated-wave field distribution into a guided-wave field distribution (since the two waves may have different impedances, it may also be thought of as an impedance transformer) [8].
Fundamental Specifications of Antennas
Lobes
Any given antenna pattern has portions of the pattern that are called lobes. A lobe can be a main lobe, a side lobe or a back lobe and these descriptions refer to that portion of the antenna pattern in which the lobe appears. In general, a lobe is any part of the pattern that is surrounded by regions of weaker radiation. So a lobe is any part of the pattern that sticks out [15].
Radiation Pattern
Radiation pattern is graphical representation of the relative field strength transmitted from or received by the antenna. It is measurement of radiation around the antenna. Antenna radiation patterns are taken at one frequency, one polarization and one plane cut. The patterns are usually presented in polar or rectilinear form with a dB strength scale. It is important to state that an antenna radiates energy in all directions, at least to some extent, so the antenna pattern is actually three-dimensional. It is common, however, to describe this 3D pattern with two planar patterns, called the principal plane patterns. These principal plane patterns can be obtained by making two slices through the 3D pattern through the maximum value of the pattern or by direct measurement. It is these principal plane patterns that are commonly referred to as the antenna patterns [14, 15].
Azimuth and Elevation Plane (E and H plane)
Characterizing an antenna's radiation properties with two principal plane patterns works quite wellfor antennas that have well-behaved patterns, that is, not much information is lost when only two planes are shown. Figure 2.1 shows a possible coordinate system used for making such antenna measurements [15].
Figure 2.1Antenna measurement co-ordinate system
The term azimuth is commonly found in reference to "the horizon" or "the horizontal" whereas the term elevation commonly refers to "thevertical". When used to describe antenna patterns, these terms assume that the antenna is mounted (or measured) in the orientation in which it will be used. In Figure 2.1, the -plane () is the azimuth plane (E-plane). The azimuth plane pattern is measured when the measurement is made traversing the entire -plane around the antenna under test. The elevation plane (H-plane) is then a plane orthogonal to the -plane, say the -plane (). The elevation plane pattern is made traversing the entire -plane around the antenna under test [15].
The Poynting vector describes both the direction of propagation and the power density of the electromagnetic wave. It is found from the vector cross product of the electric and magnetic fields and is denoted S:
Root mean square (RMS) values are used to express the magnitude of the fields. is the complex conjugate of the magnetic field phasor. The magnetic field is proportional to the electric field in the far field. The constant of proportion is η, the impedance of free space (η =376.73):
Because the Poynting vector is the vector product of the two fields, it is orthogonal to both fields and the triplet defines a right-handed coordinate system: (E, H, S) [6].
Return Loss
Return loss is a measure of the reflected energy from a transmitted signal. It is a logarithmic ratio measured in dB (decibel) that compares the power reflected by the antenna to the power that is fed into the antenna from the transmission line. The larger the value of return loss the less is the energy reflected. For good impedance matching resonant frequency must lie below [14] .
Bandwidth
Bandwidth is defined as the range between upper cut-off frequency at -10 dB and lower cut-off ( frequency at -10 dB.Bandwidth indicates range of frequency for which an antenna provides satisfactory operation [14].
3-dB Beamwidth
Also known as the Half Power Beamwidth (HPBW) is typically defined for each of the principle planes. The 3-dB beamwidth in each plane is defined as the angle between the points in the main lobe that are down from the maximum gain by 3dB.This is the point where the magnitude of the radiation pattern decreases by 50% (or -3 dB) from the peak of the main beam [14, 15].
VSWR
VSWR stands for Voltage Standing Wave Ratio. The parameter VSWR is a measure that numerically describes how well the antenna is impedance matched to the radio or transmission line it is connected to. The smaller the VSWR the better the antenna matched to the transmission line and the more the power delivered to the antenna. For the perfect matching VSWR = 1, there is no reflection and return loss. In the real system it is very hard to achieve a perfect match, so it is defined that having VSWR < 2 is still good matching system [14].
Directivity
Directivity of an antenna is a measure of the concentration of the radiated power in a particular direction [14]. If the antenna had 100% radiation efficiency, all directivity would be converted to gain. Typical half wave patches have efficiencies well above 90% [13].
Antenna Gain
Gain is a measure of the ability of the antenna to direct the input power into radiation in a particular direction and is measured at the peak radiation intensity [6].It is standard practice to use an isotropic radiator as the reference antenna in this definition. An isotropic radiator is a hypothetical lossless antenna that radiates its energy equally in all directions. This means that the gain of an isotropic radiator is G=1 (or 0 dB). It is customary to use the unit dBi (decibels relative to an isotropic radiator) for gain with respect to an isotropic radiator [15].
Polarization
The Polarization of an antenna is the polarization of the wave radiated by the antenna in the far field [8]. Polarization is a property of waves that can oscillate with more than one direction [16].The plane in which the electric field varies is also known as the polarization plane. For optimum system performance, transmit and receive antennas must have the same polarization [13].
Front-to-back ratio
The front-to-back (F/B) ratio is used a figure of merit that attempts to describe the level of radiation from the back of a directional antenna. Basically, it is the ratio of the peak gain in the forward direction to the gain 180-degrees behind the peak. On a dB scale, it is just the difference between the peak gain in the forward direction and the gain 180-degrees behind the peak [15].
Microstrip Antennas
Microstrip antennas are also referred to as patch antennas. They are low profile,conformable to planar and non-planar surfaces, simple and inexpensive to manufacture using modern printed-circuit technology, mechanically robust when mounted on rigid surfaces, compatible with MMIC designs and when the particular patch shape and mode are selected, they are very versatile in terms of resonant frequency, polarization, pattern and impedance [1].
Major operational disadvantages of microstrip antennas are their low efficiency, low power, high Q (sometimes in excess of 100), poor polarization purity, poor scan performance, spurious feed radiation and very narrow frequency bandwidth, which is typically only a fraction of a percent or at most a few percent. There are methods, however, such as increasing the height of the substrate that can be used to extend the efficiency (to as large as 90 percent if surface waves are not included) and bandwidth (up to about 35 percent). However, as the height increases, surface waves are introduced which usually are not desirable because they extract power from the total available for direct radiation (space waves). The surface waves travel within the substrate and they are scattered at bends and surface discontinuities, such as the truncation of the dielectric and ground plane, and degrade the antenna pattern and polarization characteristics [1].
Basic Characteristics
Microstrip antennas, as shown in Figure 2.2, consist of a very thin (t , where is the free-space wavelength) metallic strip (patch) placed a small fraction of a wavelength (h, usually 0.003 ≤ h≤0.05) above a ground plane. The microstrip patch is designed so its pattern maximum is normal to the patch (broadside radiator). This is accomplished by properly choosing the mode (field configuration) of excitation beneath the patch. End-fire radiation can also be accomplished by judicious mode selection. For a rectangular patch, the length L of the element is usually /3 <L</2. The strip (patch) and the ground plane are separated by a dielectric sheet (referred to as the substrate). There are numerous substrates that can be used for the design of microstrip antennas, and their dielectric constants are usually in the range of 2.2≤ ≤12. The ones that are most desirable for good antenna performance are thick substrates whose dielectric constant is in the lower end of the range because they provide better efficiency, larger bandwidth, loosely bound fields for radiation into space, but at the expense of larger element size [1].
The radiating elements and the feed lines are usually photo-etched on the dielectric substrate. The radiating patch may be square, rectangular, thin strip (dipole), circular, elliptical, triangular, or any other configuration. Square, rectangular, dipole (strip), and circular are the most common because of ease of analysis and fabrication, and their attractive radiation characteristics, especially low cross-polarization radiation [1].
Figure 2.2Microstrip antenna and coordinate system
There are many configurations that can be used to feed microstrip antennas. The four most popular methods are the microstrip line, coaxial probe, aperture coupling, and proximity coupling. The microstrip-line feed is easy to fabricate, simple to match by controlling the inset position and rather simple to model. However as the substrate thickness increases, surface waves and spurious feed radiation increase, which for practical designs limit the bandwidth [1].