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1Brief Technical Overview
Nowadays, in order to face the technological development, humankind needs to keep up with the evolution.Thisevolution lead to the development of cellular devices. This brought up manynew areas of investigation, the one with main interest for this project is the research of antennaswith fractal geometries.
The main problem of common antennas is that they only operate at one or two frequencies,restricting the number of bands that an equipment is capable of supporting. Another issue is thesize of a common antenna. Due to the very strict space that a handset has, setting up more thanone antenna is very difficult. To help these problems, the use of fractal shaped antennas is beingstudied.The USB applications require very low profile antennas capable to operate at different frequencies.
1.2Motivation
As previously mentioned, fractal antennas are being studied to integrate systems that requireoperation in different bands. These technologies need small size and high performance antennas.Example of such communication systems are mobile phones, wireless network cards, militarycommunications.
In our modern society people need to be in touch with the world, for that technology is beingdeveloped in such way that anyone can communicate or be informed about everything just by usinga small handset cellular device. This equipment needs to operate in a wide range of frequencies,allowing people to connect to the WEB (standards 802.11a, 802.11b or 802.11g), make phone calls(GSM), video conferences (UMTS) and other utilities. All these technologies operate in differentfrequencies demanding a high efficient multi-band antenna with a very compact size. This workwill show that fractal geometry antennas may help answer these requirements.
1.3 Objectives
The goal of this assignment is to study, analyse, design and describe fractal antennas capableof facing modern wireless communication transceivers. Various structures of fractals are going tobe tested in order to achieve a comparison between them. Return loss, radiation patterns, SWRcurves,input impedanceareused to compare the antennas.Main objective is to make an antenna capable to operate according to the IEEE 802.11 stan-dard, which means that the operating frequencies are:
• 802.11a -> 5,235 - 5,350 GHz and 5,725 - 5,875 GHz
• 802.11b -> 2,412 - 2,472 GHz
• 802.11g -> 2,412 - 2,472 GHz
1.4 Methodology
The methodology is as follow:
• Study the characteristics of an antenna (fractal) for instance, the return loss, VSWR, inputimpedance and radiation pattern.
• Prove that fractal shaped antennas have multi-band behaviour.
• Design a Sierpinski Carpet Fractal Antenna with ANSOFT HFSS and ADS
• Antenna implementation.
• Comparison between simulation and developed antennas.
1.5 Report Organization
This thesis is organized in 6 chapters.
Following this chapter, a review on antenna theory is presented. Microstrip, patch and monopoleantennas are also referred in chapter 2.
In chapter 3 fractal antennas are introduced and its geometries are described.The generation of the Sierpinski CarpetFractal Antenna for wireless Communication is also presented in this chapter.
Chapter4presents the simulation of The Rectangular patch Antenna. In this chapter wedescribe the simulation of two monopoles multiband antennas for wireless USB applications. An-tenna A was first designed and simulated.An optimization of the first antenna led to antenna Bwithimprovedperformance.Measurements and characterization of both antennas simulated in chapter 4, as well as thefabricationprocess inmicrostrip technology is described in chapter 5.Chapter 6 gives the final conclusion of this project.
Theory of Antennas
2.1Introduction
An antenna is a metallic structure that sends or receives electromagnetic waves, such as radiowaves. In other words, antennas convert radio frequency fields into electrical currents.This chapter presents a review of the theory of antennas. Antenna parameters (VSWR, inputimpedance,gain, radiation pattern, Half-power beam width (HPBW), directivity, polarization andbandwidth) are described with an overview on scattering parameters. Microstrip antennas tech-nology is then presented together with its advantages and disadvantages. Microstrip patch andmonopole antennas are briefly presented. Matching techniques are also described in this chapter.
2.2Antennas Background
There is a wide variety of antenna structures allowing operation on just one band, narrow-bandantennas, or several bands, known as multi-band or broadband antennas.
Narrow band antennas include not only single dipoles or verticals but also directive arrays.Such arrays have high gain and directivity to make the antennas more efficient to a certain di-rection. With these antennas signals coming from the back will be rejected due to its Front toBack ratio, this is the ratio of the maximum directivity of an antenna to its directivity in the op-posite direction. Yagi-Uda antenna, developed by Dr. Hidetsu Yagi and Dr. Shintaro Uda, is thecommonest directive antenna in the world.
The directivity of this antenna depends on the number of parasitic elements, usually known asdirectors that are placed in front of the driven element.
The most interesting multi-band antenna is the log-periodic, also knows as log-periodic dipolearray (LPDA). These antennas are broadband, multi-element, unidirectional with an impedanceand radiation characteristics that are continually repeated as a logarithmic function of the excita-tion frequency.
Antenna Parameters
• VSWR: Voltage Standing Wave Ratio is the ratio of maximum radio-frequency voltage tominimum radio-frequency voltage on a transmission line. It is given by:
VSWR=V_MAX/V_MIN
The VSWR can also be calculated from the return loss (S11) which meansthat it is also an indicator of an antennas efficiency. With the return loss we can determinethe mismatch between the characteristic impedance of the transmission line and the antennasterminalinputimpedance.If the the magnitude of the reflection coefficient is known the VSWR can be determined by:
vswr= (1+|S_11 |)/(1-|S_11 | )
The VSWR increases with the mismatch between the antenna and the transmission line anddecreases with a good matching. The minimum value of VSWR is 1:1 and most equipmentscan handle a VSWR of 2:1, the bandwidth of an antenna can be determined by the VSWRor the return loss. The best performance of an antenna is achieved when the VSWR under2:1 or the return loss is 10dB or lower.
• Input Impedance: Generally, an antenna is seen as a load to a transmission line with acertain impedance. This impedance is known as the input impedance of an antenna and itcan be determined by the following expressions:
Z_IN=R_l+R_r+jX_a
where Zin represents the input impedance, Rl is the loss resistance, Rr is the radiation resis-tance and Xa represents the reactance.
If the reflection coefficient is known:
Z_IN=Z_0 ((1+S_11)/(1-S_11 ))
where Zin represents the input impedance, Z0 is the characteristic impedance of the trans-mission line and S11 is a S-parameter also known as reflection coefficient, a parameter which is explained in section.
The input impedance can be used to determine the maximum power transfer between the transmission line and the antenna, this will only occur when both impedances are equal. If there is a mismatch between both impedances, power will be reflected back to the transmitter and this migth cause damage to the device.
•Gain: There are two types of gain, Absolute Gain and Relative Gain.
The Absolute Gain of an antenna is defined as the ratio between the antennas radiation intensity in a certain direction and the intensity that would be generated by an isotropic antenna fed by the same input power, therefore it can given by:
G(ϴ, φ )=U(ϴ, φ )/U_0
U_0 is given by:U_0=P_in/4π
where G(ϴ, φ )is the gain of the antenna in a certain direction, U(ϴ, φ ) is the radiation intensity in a certain direction and U0 is the radiation intensity of an isotropic antenna. Pin is the input power. The Absolute Gain is expressed in dBi as its reference is an isotropic antenna.
The Relative Gain of an antenna is defined as the ratio between the antenna radiation inten-sity in a certain direction and the intensity that would be generated by a reference antenna.
The Relative Gain is expressed according to reference antenna.
•Directivity: This is an important parameter that allows us to measure the concentration of radiated power in a certain direction. It is given by
D(ϴ, φ )=U(ϴ, φ )/U_0
where D(ϴ, φ ) is the directivity of the antenna in a certain direction, U(ϴ, φ )is the radiation intensity in a certain direction and U0 is the radiation intensity of an isotropic antenna and as given by Another way of measuring the directivity of an antenna is to calculate the HPBW .
• Efficiency: An antennas efficiency is defined as the ratio of the total radiated power to theinput power and it is given by:
e_cd=P_rad/P_in
•Radiation Pattern: The radiation pattern is a graphical representation of the characteristics of an antenna radiation in a certain direction. These characteristics include, radiation intensity, field intensity and polarization.
It is normally represented with rectangular or polar plots and it is expressed in dB. The radiation pattern is a plane cut and represents one frequency and one polarization.
•HPBW: The HPBW Half Power Beamwidth is a way of measuring the antenna directivity. This means that if the main lobe of an antenna is too narrow, the directivity is higher. It can be determined by taking out 3dB (half power) with respect to the main lobe power level. The HPBW can be determined in the polar plot of an antenna radiation pattern.
•Polarization: Represents the sense and orientation of the electromagnetic waves far from the source. There are three main types of polarization:
– Elliptical: Elliptical left hand, Elliptical right hand
– Circular: Circular left hand, Circulat right hand
– Linear: Vertical, Horizontal