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The Ultra High Frequency (UHF) band has long and widely been used for voice, video and data communication. Terrestrial Television (TV) broadcast used lower frequency band of the UHF which is 300 - 900 MHz. There is a various type of TV antenna. The ordinary UHF antennas for receiving TV signals are directional and quite large. Therefore, it would be better to have an omnidirectional and compact antenna. The dipole antenna is one of the omnidirectional that can be simply designed. This report is mainly a theoretical study on the how the UHF dipole receiving antenna used for Television works. For design purpose Microwave Office is used for designing the layout of antenna and simulation to evaluate the performance of the antenna that will be done in the Thesis II. From the study on the theoretical aspects, it is clearly understood that the unique properties of dipole antenna have been exploited in order to develop an antenna element designs that are multi-directional, low cost and compact in size.
The goal of this project is to design a low cost, small size, slim, light weight dipole antenna that put on the top of television. This project is the study and discusses on the concept of the dipole, review the progress in dipole antenna study and implementation, compare different types of dipole antenna elements and arrays and discuss the challenge and future of this type of antenna. Another pleasing property is that they are compact, meaning that they can occupy a portion of space more efficiently than other antenna types.
The objectives of this project are as follows:
The outcome of the project is a low cost, small size, slim, light weight dipole antenna that put on the top of television.
An easy and inexpensive solution that may work well when the receiver is near to the broadcasting transmitter and the building walls do not blocked the radio waves too much.
Can provide nice reception in the UHF band
Nice design that fits well into the room interior.
1.3 PROBLEM STATEMENT
The ordinary UHF antennas for receiving TV signals are quite large. The larger antenna is required for the larger wavelength portion of the electromagnetic spectrum.
The use of large outdoor UHF antenna should be replaced with a compact, low cost and easily manufactured dipole antenna that put on the top of television.
1.4 SCOPE OF PROJECT
The scope of project of this project are as follows:
Design the antenna based on the previous similar works in published journals using antenna design software.
Simulate the proposed antenna using antenna design software (Microwave Office).
Fabricate the antenna using the available materials.
1.5 GANTT CHART
An antenna or aerial is an electromagnetic device that emits or collects radio waves. In other words, an antenna is a transducer to switch from a high frequency electric current to radio waves (electromagnetic waves) and vice versa. It consists of material that conducts electricity arranged in such a way that it is in tune with the frequency of a radio signal. An antenna tuned to a specific frequency will resonate to a radio signal of the same frequency. There are many kinds of antenna ranging from very small size (such as a monopoly antenna in a cordless telephone) to very large antenna reflectors of 100 meters in diameter for radio wave astronomy. For examples, antennas are widely used in systems such as radio and television broadcasting, point-to-point radio communication, wireless LAN, radar, and astronomy. Common TV antennas are capable of transmitting as well as receiving.
2.2 Classification of antennas
2.2.1 Frequency and size
Antenna used for UHF and the one used for VHF is not the same, which is not the same from antennas for microwave. The wavelength are different at different frequencies, therefore the antennas must different in size to radiate signals at the correct wavelength.
Types of antennas can be omnidirectional, sectorial or directive. Omnidirectional antennas transmit in all directions in a complete 360 degrees pattern. The most preferred kind of antennas are the Dipole-Type and the Ground Plane antenna. Sectorial antennas radiate mainly for a specific area. The beam can be as broad as 180 degrees, or as narrow as 60 degrees. Directive antennas beamwidth is much smaller than in sectorial antennas. Directive antennas have been used for long distance links because it have the highest gain. Types of directive antennas are the horn, Yagi, the Parabolic Dish, the patch antenna, the biquad, the helicoidal and many others.
2.3 Antenna Properties
There are few important antenna characteristics that should be considered when deciding an antenna for application as follows:
The bandwidth of antenna is a measure of its capability to work over a specified range of frequencies. Bandwidth is depends on the operational requirement of the antenna. Typically, measured in hertz. For an antenna that has a frequency range, the bandwidth is usually expressed in ratio of the upper frequency to the lower frequency. The formula for calculating bandwidth is given in Equation (2.1).
2.3.2 Antenna radiation patterns
The radiation pattern of an antenna is a 3-D plot of its radiation far from the source means that it radiates equally in every x, y, z axis. Antenna radiation patterns usually consist of two forms, the azimuth pattern and the elevation pattern. The graph of the elevation pattern is the energy radiated from the antenna looking to it from the side as shown in Figure 2.1a. The graph of the azimuth pattern is the energy radiated from the antenna as if looking at it from directly above the antenna as shown in Figure 2.1b. The combination of the two graphs shows the 3-D representation of how energy is radiated from the antenna (Figure 2.1c).
Figure 2.1 Radiation pattern of an antenna
2.3.3 Power Gain
The power gain of antenna gain is the ratio of the power delivered by the aerial to a matched load connected to its terminals to the power delivered by the reference aerial to a matched load, the field strengths at the locations of the aerials being the same. The power gain is measured in dBi (dB referenced to an isotropic antenna) or dBd (dB referenced to a half wavelength dipole).
The Directive gain of an antenna is the ratio of the power radiated by an antenna in its direction of maximum radiation to the power radiated by a source antenna in the similar direction. It also be regarded the degree to which the output power or sensitivity of an aerial structure is concentrated into a small range of directions.
The definition of polarization is the orientation of the electric field vector. The plane wave is shown in Figure 2.2 is vertically polarized. The linear polarization of the wave which is vertical or horizontal polarization does not change as it propagates. Another polarization is circular, where the magnetic field vector and the electric field vector rotate as the wave travels. The polarization of the transmitting antenna and the receiving antenna needs to match up for best performance.
Figure 2.2 The propagation of a plane electromagnetic wave
2.4 Dipole Antenna
A dipole is a simple antenna structure composed of a single radiating element split into two sections (two straight collinear wires), not necessarily of equal length. A current flowing in a wire that length is accordingly related to the rf creates an electromagnetic field. This field is radiated from the wire and is set free in atmosphere. The radiation of electromagnetic energy principles are consists of two laws:
1. A moving electric field produce a magnetic field (H).
2. A moving magnetic field produce an electric field (E).
Most of the time, these two fields will be in phase and perpendicular to one another. Even though a conductor is usually considered exist when a moving magnetic or electric field is mentioned, the laws that carry out these fields does not mention about a conductor. Thus, these laws is true with or without the conductor. The current and voltage distribution of a half-wave (Hertz) antenna is shown Figure 2.3. A piece of wire is cut in half and connected to the terminals of a high-frequency ac generator as shown in Figure 2.3a. The frequency of the generator is set so that each half of the wire is 1/4 wavelength of the output. This will result a common type of antenna known as a dipole. Formula for the current distribution is given in Equation 2.2.
Figure 2.3a Half-Wave Antenna
Figure 2.3b Current Distribution
Figure 2.3c Charge Distribution
Figure 2.3 Current and voltage distribution on an antenna
The right side of the generator is positive and the left side negative. Because of charges repel, electrons will be attracted to the positive terminal and go away from the negative terminal. Figure 2.3b shows the direction and distribution of electron flow. Most of the current flows in the center and none flows at the ends as shown in the distribution curve. No matter how much or how little current is flowing the current distribution over the antenna will always be the same. Nevertheless, at any given point the current on the antenna will vary directly with the amount of voltage developed by the generator. One-quarter cycle after electrons has begun to flow, the generator will develop its maximum voltage and the current will decrease to 0. The condition that exist at that time is shown in Figure 2.3c. There will be no current flowing; on the other hand a maximum number of electrons will be at the left end of the line and a minimum number at the right end. Figure 2.3c show the charge distribution along the wire will change as the voltage of the generator changes. Thus, the following conclusions can be made:
Amplitude is varies with the generator voltage when a current flows in the antenna.
A sinusoidal distribution of charge exists on the antenna. The charges reverse polarity every 1/2 cycle.
The sinusoidal variation in charge magnitude lags the sinusoidal variation in current by 1/4 cycle.
The dipole antennas characteristics to be considered:
The dipole antenna has a unity gain in almost all directions. This characteristic can be seen from the far electrical field of the radiated electromagnetic wave in Equation (2.3).
Figure 2.4 below shows the radiation pattern of the half wavelength dipole. The image at the upper left shows the horizontal section of the radiation pattern, while the image at the lower right shows the horizontal section of the radiation pattern. The image at the lower left shows the perspective view of the same emission pattern.
Figure 2.4 Radiation pattern of the dipole antenna
(ii) Low gain
One of the drawbacks of dipole antenna is its low gain. Its omnidirectional characteristic makes its power distributed evenly on all directions so the power given to it is divided to many directions instead of one particular direction.
(iii) Easy to build
The dipole can be made only by using two pieces of conductor. So it can be easily build using any conducting materials such as copper wires.
(iv) No need ground plane
Unlike the monopole antenna which needs ground plane as a reflector, the dipole antenna does not need ground plane. The dipole has two radiating arms for which each are used to transmit or receive ¼ wavelength.
3.1 Introduction of Project Methodology
The UHF dipole antenna used for television project is developed within two phases, which are Thesis 1 phase and Thesis 2 phase. Through Thesis 1 phase, project planning, research and data collection on UHF dipole antenna have been made. Plus, a project proposal and presentation regarding the outline of the project have been performed. While, through Thesis 2 phase, the project software and hardware part are constructed including the antenna design, optimization process, fabrication, testing and analyzing the results. The printed dipole array project methodology flow chart is illustrated in Figure 3.1.
RESEARCH AND DATA COLLECTION
Thesis 2 Yes
Figure 3.1 Flow Chart of project methodology
3.2 Project Planning and Data Collection
The first step in developing UHF dipole antenna used for television project is preparing a project planning regarding the outlines of project scheduling, methods, and finances part. Hence, a Gantt chart and proposal are constructed as a time line guidance and scheme with the purpose of accomplishing the project successfully within time frame.
A research and data collection is done through books and internet regarding antenna theory and design which concerned on basic characteristics, critical parameters, and structure of antennas. Besides that, a lot of research also been done on the previous study of antenna miniaturization and some literature reviews of previous projects that are useful to be referenced through developing this project are highlighted.
3.2 Design Process
The major process involve during the whole process of designing UHF dipole antenna used for television project, which is software. For the software process, Microwave office 2009 is used for designing and simulation and for the hardware, fabrication and testing is needed.
Before beginning with the software the length of the UHF dipole should be calculated, using this formula,
C ≈3.0-108m/s=300 000km/s (light Velocity)
F=frequency (frequency band for designed antenna)
3.3 Software - Microwave Office 2009
The AWR Design Environment comprises two effective tools that can be used together to create an integrated system and RF design environment which is Visual System Simulator (VSS) and Microwave Office (MWO). These effective tools are fully integrated in the AWR Design Environment and allow the user to incorporate circuit designs into system designs without leaving the AWR Design Environment.
Figure 3.2 The AWR design and operating environment
Microwave Office enables the user to design circuits consist of electromagnetic (EM) and schematics structures from an extensive electrical model database, and then generate layout representations of these designs. Users can perform simulations using one of Microwave Office's simulation engines - a linear simulator, an advanced harmonic balance simulator, a 3D-planar EM simulator (EMSight), or an optional HSPICE simulator, and display the output in a varoius graphical forms based on analysis needs. Users can tune or optimize the designs and changes are quickly and automatically reflected in the layout.
3.4 UHF Dipole Antenna Design Characteristics
Several critical design characteristics in developing UHF dipole antennas are deliberated including calculation of width and length for dipole antenna, array structure, antenna feeding method and matching technique.
3.5 Calculation of Width and Length for Dipole Antenna
Basically, width and length are two critical parameters in designing dipole antenna. In developing this UHF dipole antenna project, a single element UHF dipole antenna is designed with each side of the dipole arm width is λ/4, for a total width of a λ/2 from end to end. Generally, there is no specific equation of length calculation for dipole antenna. The length of dipole antenna design is done by estimating length dimension through the drawing process using Microwave Office.
In conclusion, this project provides a valuable learning experience and I gained a lot of knowledge. The overall objective of this Thesis 1 phase project has been to develop the theoretical foundation required for the analysis and design of UHF dipole antenna. In the next part of this research (Thesis 2 phase), the principles of the algorithms will be further explained and simulations will be done. The simulations will be done by using Microwave Office.