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1 Acknowledgements I would like to thank Mr. S L Ogaba for proposing and supervising the project and allowing me to use the facilities at the department. His invaluable guidance and insight accompanied by timelines added a great deal to the substance of this project. The Electrical Department Workshop cannot go unmentioned, especially Mr. Wangai, who accommodated all modifications to the antenna as soon as I requested them. i

2 Abstract The main aim of the project was to come up with a 10 element UHF aerial with optimal spacing that would give clear TV reception based on the frequency allocations given by Communication Commission of Kenya (CCK). A Yagi antenna was chosen for project due to its high directionality, broadband capability, high gain and the availability of materials locally. An attempt was made to simulate the entire antenna and test it on a SAMSUNG TV set. ii

3 Table of Contents Acknowledgements... i Abstract... ii Complete list of tables and figures... v List of figures... v List of tables... vi CHAPTER INTRODUCTION Objectives... 2 CHAPTER Introduction How an Antenna radiates Near and Far Field Regions Far field radiation from wires Antenna Performance Parameters Radiation Pattern Directivity Input Impedance Return Loss (RL) Antenna Efficiency Antenna Gain Polarization Bandwidth Types of Antennas Half Wave Dipole Monopole Antenna Loop Antennas Helical Antenna Horn Antennas Yagi-Uda Antennas CHAPTER TV Frequency allocations iii

4 3.2 Material for elements construction Electrolytic corrosion Yagi Uda dimension calculations Reflector lengths Dipole lengths Director lengths Spacing calculations CHAPTER DIPOLE FOLDING Making a Bending Jig Dipole Enclosure and Wiring Parabolic reflector construction Insulation Simulation Exercises Radiation Patterns SWR Chapter Conclusion Theoretical versus simulation results Testing on a SAMSUNG TV Set Improvements REFERENCES APPENDIX iv

5 Complete list of tables and figures List of figures 1.1 Geometry of the Yagi Uda Array Radiation from an antenna Field regions round an antenna Spherical coordinate system for a Hertzian dipole Radiation pattern of a general directional antenna Equivalent circuit of a transmitting antenna A linear (vertically) polarized wave Commonly used polarization schemes Measuring Band-width from plot of reflection coefficient Half-wave dipole Radiation pattern for half wave dipole Monopole antenna Radiation pattern for a half wave dipole Loop antenna Radiation pattern of small and large loop antenna Helix antenna Pattern radiation for a helix antenna Types of Horn antenna Yagi antenna with elements Aperture area capturing incoming radiation Metal groupings with varying corrosion Spacing calculations Element folded dipole Making a dipole bending jig 36 v

6 4.3 Wiring construction details Perforated parabolic reflector Plastic insulator for dipole and boom Full description of antenna details on EZNEC V Element length and spacing window element antenna simulation Azimuth plot pattern elevation plot type D Azimuth plot SWR plot for U.H.F band under test..44 List of tables 3.1 Status of TV frequency allocations Resistivity of different metals Skin depth, Radio frequency (R.f) resistance variations Loss resistance and radiation efficiencies Spacing calculations for different frequencies Azimuth plot parameters Elevation slice parameters vi

7 CHAPTER INTRODUCTION Yagi-Uda Antenna is a parasitic linear array of parallel dipoles; see Fig 1.1, one of which is energized directly by a feed transmission line while the other act as parasitic radiator whose currents are induced by mutual coupling. The basic antenna is composed of one reflector (in the rear), one driven element, and one or more directors (in the direction of transmission/reception).one driven element, and one or more directors (in the direction of transmission/reception).the Yagi-Uda antenna has received exhaustive analytical and experimental investigations in the open literature and elsewhere. The characteristics of a Yagi are affected by all of the geometric parameters of the array. Usually Yagi-Uda arrays have low input impedance and relatively narrow bandwidth. Improvements in both can be achieved at the expense of others. Usually a compromise is made, and it depends on the particular 1

8 1.1 Objectives Often one needs to improve reception of a particular radio or television station. One effective way to do this is to build a Yagi-Uda (or Yagi) antenna because of their simplicity and relatively high gain. The goal of the project is to design and optimize a 10 element Yagi antenna which covers all the UHF TV channels. We choose an upper frequency of 799 MHz, mid-band frequency of 643MHz and a lower band frequency of 487 this is because the UHF TV channel starts with 487MHz(channel 23) and ends with 799 MHz (channel 62). Antennas gains rise slowly up to the design frequency and fall off sharply thereafter [5]. It is therefore easier to make the design frequency a little higher than desired. 2

9 CHAPTER 2 In this chapter, the basic concept of an antenna is provided and its working is explained. Next, some critical performance parameters of antennas are discussed. Finally, some common types of antennas are introduced. 2.1 Introduction Antennas are metallic structures designed for radiating and receiving electromagnetic energy. An antenna acts as a transitional structure between the guiding device (e.g. waveguide, transmission line) and the free space. The official IEEE definition of an antenna as given by Stutzman and Thiele [4] follows the concept: That part of a transmitting or receiving system that is designed to radiate or receive electromagnetic waves. 2.2 How an Antenna radiates In order to know how an antenna radiates, let us first consider how radiation occurs. Aconducting wire radiates mainly because of time-varying current or an acceleration (or deceleration) of charge. If there is no motion of charges in a wire, no radiation takes place, since no flow of current occurs. Radiation will not occur even if charges are moving with uniform velocity along a straight wire. However, charges moving with uniform velocity along a curved or bent wire will produce radiation. If the charge is oscillating with time, then radiation occurs even along a straight wire as explained by Balanis [1]. The radiation from an antenna can be explained with the help of Figure 2.1 which shows a voltage source connected to a two conductor transmission line. When a sinusoidal voltage is applied across the transmission line, an electric field is created which is sinusoidal in nature and this results in the creation of electric lines of force which are tangential to the electric field. The magnitude of the electric field is indicated by the bunching of the electric lines of force. The free electrons on the conductors are forcibly displaced by the electric lines of force and the movement of these charges causes the flow of current which in turn leads to the creation of a magnetic field. 3

10 Due to the time varying electric and magnetic fields, electromagnetic waves are created and these travel between the conductors. As these waves approach open space, free space waves are formed by connecting the open ends of the electric lines. Since the sinusoidal source continuously creates the electric disturbance, electromagnetic waves are created continuously and these travel through the transmission line, through the antenna and are radiated into the free space. Inside the transmission line and the antenna, the electromagnetic waves are sustained due to the charges, but as soon as they enter the free space, they form closed loops and are radiated [1]. 4

11 2.3 Near and Far Field Regions The field patterns, associated with an antenna, change with distance and are associated with two types of energy: - radiating energy and reactive energy. Hence, the space surrounding an antenna can be divided into three regions. The three regions shown in Figure 2.2 are: Reactive near-field region: In this region, the reactive field dominates. The reactive energy oscillates towards and away from the antenna, thus appearing as reactance. In this region, energy is only stored and no energy is dissipated. The outermost boundary for this region is at a distance 0.62 where is the distance from the antenna surface, D is the largest dimension of the antenna and is the wavelength. Radiating near-field region (also called Fresnel region): This is the region which lies between the reactive near-field region and the far field region. Reactive fields are smaller in this field as compared to the reactive near-field region and the radiation fields dominate. In this region, the 5

12 angular field distribution is a function of the distance from the antenna. The outermost boundary for this region is at a distance 2 where is the distance from the antenna surface. Far-field region (also called Fraunhofer region): The region beyond 2 is the far field region. In this region, the reactive fields are absent and only the radiation fields exist. The angular field distribution is not dependent on the distance from the antenna in this region and the power density varies as the inverse square of the radial distance in this region. 2.4 Far field radiation from wires The far field radiation from a Hertzian dipole can be conveniently explained with the help of the spherical co-ordinate system shown in Figure 2.3. The z axis is taken to be the vertical direction and the xy plane is horizontal. θ denotes the elevation angle and Ø denotes the azimuthal angle. The xz plane is the elevation plane (θ = 0) or the E-plane which is the plane containing the electric field vector and the direction of maximum radiation. The xy plane is the azimuthal plane 6

13 (θ = π / 2 ) or the H-plane which is the plane containing the magnetic field vector and the direction of maximum radiation [1]. The far field radiation can be explained with the help of the Hertzian dipole or infinitesimal dipole which is a piece of straight wire whose length L and diameter are both very small compared to one wavelength. A uniform current I (0) is assumed to flow along its length. If this dipole is placed at the origin along the z axis, then as given by [1], we can write: sin 1 (2.1) 1 (2.2) 1 (2.3) 0 (2.4) 0 (2.5) 0 (2.6) For far field radiation, terms in and can be neglected; hence we can modify the above equations to write: sin (2.7) sin (2.8) 0 (2.9) Where η = intrinsic free space impedance k = 2π /λ = wave propagation constant r = radius for the spherical co-ordinate system. In all the above equations, the phase term has been dropped and it is assumed that all the fields are sinusoidally varying with time. It is seen from the above equations that the only nonzero fields are and, and that they are transverse to each other. The ratio, such that 7

14 the wave impedance is 120 and the fields are in phase and inversely proportional to r. The directions of E, H and r form a right handed set such that the Poynting vector is in the r direction and it indicates the direction of propagation of the electromagnetic wave. Hence the time average poynting vector given by [1] can be written as: (2.10) Where: E and H represent the peak values of the electric and magnetic fields respectively. The average power radiated by an antenna can be written as: s (watts) (2.11) Where: ds is the vector differential surface sin is the magnitude of the time average poynting vector ( ). The radiation intensity is defined as the power radiated from an antenna per unit solid angle and is given as: (2.12) Where: U is the radiation intensity in Watts per unit solid angle. 2.5 Antenna Performance Parameters The performance of an antenna can be gauged from a number of parameters. Certain critical parameters are discussed below. 8

15 2.5.1 Radiation Pattern The radiation pattern of an antenna is a plot of the far-field radiation properties of an antenna as a function of the spatial co-ordinates which are specified by the elevation angle θ and the azimuth angle Ø. More specifically it is a plot of the power radiated from an antenna per unit solid angle which is nothing but the radiation intensity [5]. Let us consider the case of an isotropic antenna. n isotropic antenna is one which radiates equally in all directions. If the total power radiated by the isotropic antenna is P, then the power is spread over a sphere of radius r, so that the power density S at this distance in any direction is given as: (2.13) Then the radiation intensity for this isotropic antenna can be written as: 4 An isotropic antenna is not possible to realize in practice and is useful only for comparison purposes. A more practical type is the directional antenna which radiates more power in some directions and less power in other directions. A special case of the directional antenna is the omnidirectional antenna whose radiation pattern may be constant in one plane (e.g. E-plane) and varies in an orthogonal plane (e.g. H-plane). The radiation pattern plot of a generic directional antenna is shown in Figure

16 Figure 2.4 shows the following: HPBW: The half power beamwidth (HPBW) can be defined as the angle subtended by the half power points of the main lobe. Main Lobe: This is the radiation lobe containing the direction of maximum radiation. Minor Lobe: All the lobes other then the main lobe are called the minor lobes. These lobes represent the radiation in undesired directions. The level of minor lobes is usually expressed as a ratio of the power density in the lobe in question to that of the major lobe. This ratio is called as the side lobe level (expressed in decibels). Back Lobe: This is the minor lobe diametrically opposite the main lobe. Side Lobes: These are the minor lobes adjacent to the main lobe and are separated by various nulls. Side lobes are generally the largest among the minor lobes. In most ireless systems, minor lobes are undesired. Hence a good antenna design should minimize the minor lobes Directivity The directivity of an antenna has been defined by [1] as the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. In other words, the directivity of a non-isotropic source is equal to the ratio of its radiation intensity in a given direction, over that of an isotropic source. Where: is the directivity of the antenna is the radiation intensity of the antenna is the radiation intensity of an isotropic source is the total power radiated (2.15) Sometimes, the direction of the directivity is not specified. In this case, the direction of the maximum radiation intensity is implied and the maximum directivity is given by [1] as: 10

17 (2.16) Where: is the maximum directivity is the maximum radiation intensity Directivity is a dimensionless quantity, since it is the ratio of two radiation intensities. Hence, it is generally expressed in dbi. The directivity of an antenna can be easily estimated from the radiation pattern of the antenna. An antenna that has a narrow main lobe would have better directivity, then the one which has a broad main lobe, hence it is more directive Input Impedance The input impedance of an antenna is defined by [1] as the impedance presented by an antenna at its terminals or the ratio of the voltage to the current at the pair of terminals or the ratio of the appropriate components of the electric to magnetic fields at a point. Hence the impedance of the antenna can be written as: (2.17) Where: is the antenna impedance at the terminals. is the antenna resistance at the terminals. is the antenna reactance at the terminals. The imaginary part, of the input impedance represents the power stored in the near field of the antenna. The resistive part, of the input impedance consists of two components, the radiation resistance and the loss resistance. The power associated with the radiation resistance is the power actually radiated by the antenna, while the power dissipated in the loss resistance is lost as heat in the antenna itself due to dielectric or conducting losses. 11

18 2.5.4 Voltage Standing Wave Ratio (VSWR) In order for the antenna to operate efficiently, maximum transfer of power must take place between the transmitter and the antenna. Maximum power transfer can take place only when the impedance of the antenna ( ) is matched to that of the transmitter ( ). According to the maximum power transfer theorem, maximum power can be transferred only if the impedance of the transmitter is a complex conjugate of the impedance of the antenna under consideration and vice-versa. Thus, the condition for matching is: Where (2.18) as shown in Figure 2.5. If the condition for matching is not satisfied, then some of the power maybe reflected back and this leads to the creation of standing waves, which can be characterized by a parameter called as the Voltage Standing Wave Ratio (VSWR). The VSWR is given by Milligan [8] as: Г Г (2.19) Г (2.20) 12

19 Where: Г is called the reflection coefficient is the amplitude of the reflected wave is the amplitude of the incident wave The VSWR is basically a measure of the impedance mismatch between the transmitter and the antenna. The higher the VSWR, the greater is the mismatch. The minimum VSWR which corresponds to a perfect match is unity. A practical antenna design should have an input impedance of either 50 Ω or 75 Ω since most radio equipment is built for this impedance Return Loss (RL) The Return Loss (RL) is a parameter which indicates the amount of power that is lost to the load and does not return as a reflection. As explained in the preceding section, waves are reflected leading to the formation of standing waves, when the transmitter and antenna impedance do not match. Hence the RL is a parameter similar to the VSWR to indicate how well the matching between the transmitter and antenna has taken place. The RL is given as by [8] as: 20 log Г (db) (2.21) For perfect matching between the transmitter and the antenna, Г = 0 and RL = which means no power would be reflected back, whereas a Γ = 1 has a RL = 0 db, which implies that all incident power is reflected. For practical applications, a VSWR of 2 is acceptable, since this corresponds to a RL of db Antenna Efficiency The antenna efficiency is a parameter which takes into account the amount of losses at the terminals of the antenna and within the structure of the antenna. These losses are given by [1] as: Reflections because of mismatch between the transmitter and the antenna Losses (conduction and dielectric) Hence the total antenna efficiency can be written as: 13

20 (2.22) Where = total antenna efficiency 1 Г = reflection (mismatch) efficiency = conduction efficiency = dielectric efficiency Since and are difficult to separate, they are lumped together to form the efficiency which is given as: (2.23) is called as the antenna radiation efficiency and is defined as the ratio of the power delivered to the radiation resistance, to the power delivered to and Antenna Gain Antenna gain is a parameter which is closely related to the directivity of the antenna. We know that the directivity is how much an antenna concentrates energy in one direction in preference to radiation in other directions. Hence, if the antenna is 100% efficient, then the directivity would be equal to the antenna gain and the antenna would be an isotropic radiator. Since all antennas will radiate more in some direction that in others, therefore the gain is the amount of power that can be achieved in one direction at the expense of the power lost in the others as explained by Ulaby [2]. The gain is always related to the main lobe and is specified in the direction of maximum radiation unless indicated. It is given as:,, (dbi) (2.24) Polarization Polarization of a radiated wave is defined by [1] as that property of an electromagnetic wave describing the time varying direction and relative magnitude of the electric field vector. The 14

21 polarization of an antenna refers to the polarization of the electric field vector of the radiated wave. In other words, the position and direction of the electric field with reference to the earths surface or ground determines the wave polarization. The most common types of polarization include the linear (horizontal or vertical) and circular (right hand polarization or the left hand polarization). If the path of the electric field vector is back and forth along a line, it is said to be linearly polarized. Figure 2.6 shows a linearly polarized wave. In a circularly polarized wave, the electric field vector remains constant in length but rotates around in a circular path. A left hand circular polarized wave is one in which the wave rotates counterclockwise whereas right hand circular polarized wave exhibits clockwise motion as shown in Figure

22 2.5.9 Bandwidth The bandwidth of an antenna is defined by [1] as the range of usable frequencies within which the performance of the antenna, with respect to some characteristic, conforms to a specified standard. The bandwidth can be the range of frequencies on either side of the center frequency where the antenna characteristics like input impedance, radiation pattern, beamwidth, polarization, side lobe level or gain, are close to those values which have been obtained at the center frequency. The bandwidth of a broadband antenna can be defined as the ratio of the upper to lower frequencies of acceptable operation. The bandwidth of a narrowband antenna can be defined as the percentage of the frequency difference over the center frequency [1]. According to [4] These definitions can be written in terms of equations as follows: Where =upper frequency = lower frequency = center frequency (2.25) % 100 (2.26) An antenna is said to be broadband if 2. One method of judging how efficiently an antenna is operating over the required range of frequencies is by measuring its VSWR. A VSWR 2 ( RL 9.5dB ) ensures good performance. 16

23 2.6 Types of Antennas Antennas come in different shapes and sizes to suit different types of wireless applications. The characteristics of an antenna are very much determined by its shape, size and the type of material that it is made of. Some of the commonly used antennas are briefly described below. 17

24 2.6.1 Half Wave Dipole The length of this antenna is equal to half of its wavelength as the name itself suggests. Dipoles can be shorter or longer than half the wavelength, but a tradeoff exists in the performance and hence the half wavelength dipole is widely used. The dipole antenna is fed by a two wire transmission line, where the two currents in the conductors are of sinusoidal distribution and equal in amplitude, but opposite in direction. Hence, due to canceling effects, no radiation occurs from the transmission line. As shown in Figure 2.9, the currents in the arms of the dipole are in the same direction and they produce radiation in the horizontal direction. Thus, for a vertical orientation, the dipole radiates in the horizontal direction. The typical gain of the dipole is 2dB and it has a bandwidth of about 10%. The half power beamwidth is about 78 degrees in the E plane and its directivity is 1.64 (2.15dB) with a radiation resistance of 73 Ω [4]. Figure 2.10 shows the radiation pattern for the half wave dipole. 18

25 2.6.2 Monopole Antenna The monopole antenna, shown in Figure 2.11, results from applying the image theory to the dipole. According to this theory, if a conducting plane is placed below a single element of length L / 2 carrying a current, then the combination of the element and its image acts identically to a ipole of length L except that the radiation occurs only in the space above the plane as discussed by Saunders [8]. 19

26 For this type of antenna, the directivity is doubled and the radiation resistance is halved when compared to the dipole. Thus, a half wave dipole can be approximated by a quarter wave monopole ( L / 2 = λ / 4 ). The monopole is very useful in mobile antennas where the conducting plane can be the car body or the handset case. The typical gain for the quarter wavelength monopole is 2-6dB and it has a bandwidth of about 10%. Its radiation resistance is 36.5 Ω and its directivity is 3.28 (5.16dB) [4]. The radiation pattern for the monopole is shown below in Figure Loop Antennas The loop antenna is a conductor bent into the shape of a closed curve such as a circle or a square with a gap in the conductor to form the terminals as shown in Figure There are two types of loop antennas-electrically small loop antennas and electrically large loop antennas. If the total loop circumference is very small as compared to the wavelength ( L <<< λ ), then the loop antenna is said to be electrically small. An electrically large loop antenna typically has its circumference close to a wavelength. The far-field radiation patterns of the small loop antenna are insensitive to shape [4]. 20

27 As shown in Figure 2.14, the radiation patterns are identical to that of a dipole despite the fact that the dipole is vertically polarized whereas the small circular loop is horizontally polarized. 21

28 The performance of the loop antenna can be increased by filling the core with ferrite. This helps in increasing the radiation resistance. When the perimeter or circumference of the loop antenna is close to a wavelength, then the antenna is said to be a large loop antenna. The radiation pattern of the large loop antenna is different then that of the small loop antenna. For a one wavelength square loop antenna, radiation is maximum normal to the plane of the loop (along the z axis). In the plane of the loop, there is a null in the direction parallel to the side containing the feed (along the x axis), and there is a lobe in a direction perpendicular to the side containing the feed (along the y axis). Loop antennas generally have a gain from -2dB to 3dB and a bandwidth of around 10%.. The small loop antenna is very popular as a receiving antenna [4]. Single turn loop antennas are used in pagers and multiturn loop antennas are used in AM broadcast receivers Helical Antenna A helical antenna or helix is one in which a conductor connected to a ground plane, is wound into a helical shape. Figure 2.15 illustrates a helix antenna. The antenna can operate in a number of modes, however the two principal modes are the normal mode (broadside radiation) and the axial mode (endfire radiation). When the helix diameter is very small as compared to the wavelength, then the antenna operates in the normal mode. However, when the circumference of the helix is of the order of a wavelength, then the helical antenna is said to be operating in the axial mode. 22

29 In the normal mode of operation, the antenna field is maximum in a plane normal to the helix axis and minimum along its axis. This mode provides low bandwidth and is generally used for hand-portable mobile applications [8]. In the axial mode of operation, the antenna radiates as an endfire radiator with a single beam along the helix axis. This mode provides better gain (up to 15dB) [4] and high bandwidth ratio (1.78:1) as compared to the normal mode of operation. For this mode of operation, the beam becomes narrower as the number of turns on the helix is increased. Due to its broadband nature of operation, the antenna in the axial mode is used mainly for satellite communications. Figure 2.16 above shows the radiation patterns for the normal mode as well as the axial mode of operations Horn Antennas Horn antennas are used typically in the microwave region (gigahertz range) where waveguides are the standard feed method, since horn antennas essentially consist of a waveguide whose end walls are flared outwards to form a megaphone like structure. 23

30 Horns provide high gain, low VSWR, relatively wide bandwidth, low weight, and are easy to construct [4]. The aperture of the horn can be rectangular, circular or elliptical. However, rectangular horns are widely used. The three basic types of horn antennas that utilize a rectangular geometry are shown in Figure These horns are fed by a rectangular waveguide which have a broad horizontal wall as shown in the figure. For dominant waveguide mode excitation, the E-plane is vertical and H-plane horizontal. If the broad wall dimension of the horn is flared with the narrow wall of the waveguide being left as it is, then it is called an H-plane sectoral horn antenna as shown in the figure. If the flaring occurs only in the E-plane dimension, it is called an E-plane sectoral horn antenna. A pyramidal horn antenna is obtained when flaring occurs along both the dimensions. The horn basically acts as a transition from the waveguide mode to the free-space mode and this transition reduces the reflected waves and emphasizes the traveling waves which lead to low VSWR and wide bandwidth [4]. The horn is widely used as a feed element for large radio astronomy, satellite tracking, and communication dishes. In the above sections, several antennas have been discussed. 24

31 2.6.6 Yagi-Uda Antennas A Yagi antenna has several elements arranged in echelon. They are connected together by a long element, called the boom. The boom carries no current. If the boom is an insulator, the antenna orks the same. The rear-most element is called the reflector. The next element is called the driven element. All the remaining elements are called directors. The directors are about 5% shorter than the driven element. The reflector is about 5% longer than the driven element. The driven element is usually a folded dipole or a loop. It is the only element connected to the cable. Yet the other elements carry almost as much current. The Yagi is the most magical of all antennas. No attempt will be made here to explain why it works. The more directors you add, the higher the gain becomes. Gains above 20 dbi are possible. But the Yagi is a narrowband antenna, often intended for a single frequency. As frequency increases above the design frequency, the gain declines abruptly. Below the design frequency, the gain falls off more gradually. When a Yagi is to cover a band of frequencies, it must be designed for the highest frequency of the band. An antenna has an aperture area, from which it captures all incoming radiation. The aperture of a Yagi is round and its area is proportional to the gain. As the leading elements absorb power, diffraction bends the adjacent rays in toward the antenna 25

32 26

33 CHAPTER TV Frequency allocations Following the objectives of the project, a survey of the channel frequency allocations was done with the help of Communications Commission of Kenya (CCK) Courtesy of CCK. Table 3.1 showing the status of TV frequency allocations STATUS OF TV FREQUENCIES Location: Identity TV channel Station ID Frequency allocated Status (MHz) 1 K.B.C 23 K.B.C channel On air Nairobi 2 K.B.C 29 K On air 3 K.B.C 31 Channel On air 4 Royal Media Services 5 Royal Media Services 6 Nation Media Group 7 Kitambo communications 34 Citizen 575 On air 39 Citizen 622 On air 42 N.T.V 639 On air 45 Aljazeera - Not detected 8 Capital Group 47 C.N.B.C 679 On air GOD TV 695 On air 10 Stellavision 56 Aljazeera 751 On air 11 K.T.N Baraza Limited 59 K.T.N 775 On air 12 Radio One IPP 62 E.A.T.V 799 On air 27

34 This table was important in establishing the upper and lower frequencies of the UHF band which would in turn provide the design frequencies at high, low and mid-band. Using the table, the following could be deduced: Upper frequency, = 799MHz (3.1) Lower frequency, = 487 MHz (3.2) Center (mid-band) frequency, 643 (3.3) 3.2 Material for elements construction The radiating efficiency of an aerial may be defined as the ratio of the power radiated, to the power input of the aerial. The difference between the radiated power and the input power, is the power lost in dissipation by the aerial itself,. When considering relative efficiencies of the various materials for aerial elements, it is the value of the effective loss resistance which is important.[3] Since most V.H.F and U.H.F aerials are made up of various elements of more or lee cross-sectional area, the D.C resistance may be calculated from, (3.4) Where L = length A = area And resistivity is the resistance per unit length or unit aerial of the material involved and is usually given in ohms per cube. For example taking L as a half wave length at 144MHz and A is the area of 1 8 " diameter rod then typical values of are given in table

35 Table 3.2 showing resistivity of different metals Metal Resistivity( ) Resistance, (Ω) Copper Aluminium Zinc Brass All these are negligible with respect to the radiation resistance of a half wave dipole, which for the chosen element diameter is about 65 ohms. This is however only part of the effective loss resistance. As the frequency of the current flowing in the material is increased, from zero frequency, another factor called skin effect modifies the current distribution in the cross section of the conductor, concentrating it more and more in the outer skin as the frequency is increased. This therefore reduces the working area of the conductor and increases its effective resistance. At V.H.F, the skin carrying most of the current becomes quite thin, about and is proportional to the square root of conductor resistivity. From this it is obvious that the skin will be thicker for metals with higher resistivities and since their radio frequency (r.f) resistance will be less relative to their d.c resistance, than for the better conductors such as copper, this makes their use more attractive than might be expected. The r.f resistance,, for a current which is constant on the length of a half wave element is approximately as shown in table

36 Table 3.3 showing skin depth, R.f resistance variations for different metals Metal Skin depth R.f resistance,,(ω) Ratio to Copper Aluminium Zinc Brass The radiation efficiency and loss in db for the metals are considered in table 3.4 Table 3.4 Showing loss resistance and radiation efficiency Metal (Ω) Radiation efficiency (%) Loss due to aerial elements(db) Copper Aluminium Zinc Brass These figures are for a 1 8 diameter, for elements of the more popular diameters,1 2, 1 4, the r.f resistance will be lower although for very thin wall tubes, the d.c resistance may be greater. 30

37 For solid rod elements the skin depth was calculated for resistance. (3.5) For thin walled tubes (3.6) Skin depth at high frequencies is given as Skin depth (inches) =2 (3.7) Where =resistivity in ohms per cube =permeability taken as 1 for non ferrous metals = frequency in hertz 3.3 Electrolytic corrosion The use of dissimilar metals in an aerial system is likely to cause considerable trouble due to electrolytic corrosion. Each metal has its electro-potential and unless metals of similar potential are used, the difference will cause corrosion at the point of contact even when dry and when moisture is present, the effects will even be more severe [3]. If for no reason dissimilar metals must be used care should be taken to exclude moisture, the corrosive effects which will vary with atmospheric pollution. The various metals can be arranged in groups as follows:- 31

38 Metals in each of the above groups can be used together with little corrosion action, but metals from different groups will suffer from this effect. Also since the above is arranged in order the greater the spacing in the list the greater the effect. [3] The lower of the metals in this list will corrode those in the upper portion, for example, brass or copper screws in aluminium will corrode the aluminium very considerably whereas cadmium plated brass or copper screws there will be very much less corrosion of the aluminium. 3.4 Yagi Uda dimension calculations Given that we have the upper, mid and lower frequencies as shown, we can calculate the dimensions. 32

39 3.4.1 Reflector lengths We know that c fλ (3.8) Where: c=speed of light=3 10 meters/second f=frequency in Hertz λ =antenna wavelength (m) f 487MHz (3.9) Reflector length= (3.10) Dipole lengths Using the same formulas for the reflector lengths, we can write 640 (3.11) (3.12) Dipole length= (3.13) 33

40 3.4.3 Director lengths Using the same formulas for the reflector lengths, we can write 799 (3.14) (3.15) Director length= (3.16) Spacing calculations For the experiment, one of the objectives was to have constant director lengths and with variable spacings. The following assumption was taken into account: Each element spacing was represented by a channel frequency to come up with 10 different pacings. The spacing formula was determined using information obtained from [10]. The calculations were tabulated as follows: Table 3.5 showing spacing calculations for different frequencies Element Frequency(MHz) Spacing formula(m) Wavelength (m) Spacing(mm) Reflector-Dipole λ Dipole-Director λ Director1-Director λ Director2-Director λ Director3-Director λ Director4-Director λ Director5-Director λ Director6-Director λ Director7-Director λ Director8-Director λ

41 CHAPTER4 Construction of the 10-element Yagi antenna 4.1 DIPOLE FOLDING Surprisingly, the only critical dimension seems to be the overall length (see figure 4.1). The second most important dimension is probably the tubing diameter, but both of these are less critical for a folded dipole than for a plain rod dipole or Yagi directors. The spacing between the two arms of the 'trombone' can vary between quite wide limits, which is a great comfort for the designer 4.2 Making a Bending Jig The key to good results is to invest a little time in building a bending jig. As in fig 4.2, this can be very simple and can be made out of scrap wood. The diameter of the round former needs to be about 5mm less than the inside diameter of the bend you're aiming for, to allow for some 'spring' when bending the tubing by hand. 35

42 A good way to make the round former would be to use a metal pulley (or a wheel with the rubber tyre removed) because the groove will help to locate the tubing as you bend it. However, the former could be nothing more elaborate than a short sawn-off length of antenna mast, secured to the wooden base of the jig by a few strong nails down the inside. The purpose of the back rail is to support the straight part of the tubing, and make sure that the bend starts with the tubing held tightly on to the former. If you're using a grooved former, you'll also have to provide a packing strip to make the tubing enter the groove at the right height. It's also useful to round-off the end of the base, in case you need to use a mallet to persuade the tubing to go round the former. 4.3 Dipole Enclosure and Wiring For the enclosure a plastic circular conduit was used to create housing for the dipole ends. The centre of a dipole is a relatively low-impedance area, so the grade of plastic is not critical. 36

43 Solder the centre core of the coaxial to one of the terminals and solder the braid to the other terminal. The dipoles were fastened to the circular conduit to make it rigid. Soldering was done as shown in figure 4.3. Care was taken to ensure that the coaxial braids and the cores were kept apart to avoid short circuiting the system. 4.3 Parabolic reflector construction Parabolic reflector diameter was 315mm. Two kinds of parabolic dishes were used: one which was plain (without drillings) and another which was perforated. The parabola was perforated to increase the front to back ratio of the Antenna system. There is no particular order in which the perforations were done nor are the hole diameters significant for the project. A comparison was to be done with a plain parabolic dish to compare the effects on the forward gain of the antenna. This is illustrated in Figure

44 4.6 Insulation Plastic insulations were used to prevent boom from coming into contact with the dipole and the directors. Without them there would be conductivity in the boom of the antenna which would make the polar diagrams very noisy and difficult to interpret. Figure 4.6 shows the insulation type that was tailor made to suit the antenna specifications. Some force had to be employed in fitting the directors and the boom into the insulator since they did not have a perfect finish. 38

45 4.7 Simulation Exercises Using the calculated director lengths and the table of different spacings, the following simulations were carried out using the program EZNEC Demo V 5.0. The parameters were loaded into the program and the simulation was done. The window giving the description of the yagi antenna and its various properties is shown in figure

46 The window required to input the element lengths and spacings is shown in figure

47 The following antenna was depicted from the data given Radiation Patterns The following plots were simulated from the given data Azimuth plot 41

48 Elevation angle Outer Ring 3D Max gain Slice Max gain Front/Back Beamwidth Sidelobe gain Front/Sidelobe Table 4.1 showing the various parameter of the azimuth plot 0.0 deg 9.97 dbi 9.97 dbi 9.97 Az angle = 0.0 deg db 43.1deg; 21.6 deg angle=215.0 deg db Elevation plot type: 42

49 Azimuth angle Outer Ring 3D Max gain Slice Max gain Front/Back Beamwidth Sidelobe gain Front/Sidelobe Table 4.2 Elevation Slice parameters 0.0 deg 9.97 db 9.97 db 9.97 angle=0.0 deg db deg; 24.8 deg 2.6 db angle=65.0 deg 7.37 db 3-Dimensional plot of the azimuth type: 43

50 4.7.2 SWR Using a 75 ohm coaxial line feed and a frequency range from 487 to 799MHz the following SWR plot was obtained in figure 4.12 Under normal analysis, the standing wave ratio is chosen for the centre frequency of the antenna in question. From the graph, we can deduce that at 640 MHz the SWR is approximately

51 Chapter 6 Conclusion 6.1 Theoretical versus simulation results Research from many books indicated that the gain of a 10 element Yagi antenna is approximately 13 dbi. From the simulations the antenna was found to be 10.18dBi. Using the simulator, the directors (wires) were given a resistivity of 2.82E-6 (ohm/cubic centimeters). Therefore losses were expected in the system should actual testing be carried out. The source power applied by the simulator was 160 watts which gave a SWR of 1.8(at centre frequency). In theory a 10 element Yagi gives a SWR of 1.2 (at centre frequency). For a 75 ohm coaxial cable, the input impedance of the antennae as seen by the simulator indicates a mismatch therefore a standing wave ratio is observed. This can be alleviated in two ways: 1. Vary the lengths of the director elements to bring the impedance to 75ohms or thereabouts. 2. We could perform stub matching using a (Balance to unbalance transformer) Balun. The first option could not be carried out since one of the objectives of the project was to keep the directors constant. The second option was considered but because I had not tested the antenna practically on a test pad to verify my simulations, therefore time was going to be a hindrance. 6.2 Testing on a SAMSUNG TV Set This project has involved the research, design and simulation of a 10 element Yagi antenna which was successful when tested on a Samsung TV set. All channels were found to give clear reception especially Citizen TV (channel ID = 34), this was because this is where the centre frequency of the antenna was located. 45

52 Surprisingly, there was one channel which appeared to be clear but was not designed for. This was the Family TV (channel ID=9).this channel belongs to the VHF High band (300 Mhz) 6.2. Improvements A number of improvements could be made on the design, construction and test procedure outlined in this report. These improvements would involve an increase in cost, and facilities. Some suggested areas of improvements are: 1. Simulations: An advanced three dimensional simulation package would be much more accurate and efficient for determining the outputs of the antenna designs. A package such as this should also allow for simulations other than straight wires. This would involve however, the purchase of a very expensive design package. 2. Equipment: The University could purchase its own computerized spectrum analyzer with a testing pad to accommodate all types of antennas. 3. Further design could be carried out for high frequencies in the gigahertz range that could be employed for Wi-Fi communications (since they have high gains and are highly directional). Most of these improvements would involve the acquisition of software and hardware that is very expensive and could not be justified for a small project such as this. The available facilities have been sufficient in designing, construction and testing an antenna that has met the required specifications. 46

53 REFERENCES [1]Balanis, C.A, Antenna theory: Analysis and Design, second edition, John Wiley and Sons inc [2]Ulaby, Fawwaz T, Fundamentals of Applied Electromagnetics, 1999 edition, Prentice Hall, [3]G.R Jessop, VHF/UHF Manual Radio Society of Great Britain, Hertfordshire, 4 th edition, [4]Richard C Johnson and Henry Jasik, Antenna Engineering Handbook, McGraw Hill, New York, 3 rd Edition, [5]Institute of Electrical and Electronic Engineers, IEEE Standard Test Procedures for Antennas, IEEE Inc, New York [6] [7] [8]Thomas A Milligan, Modern Antenna Design, second edition, John Wiley and Sons inc, 2004 [9] [10] 47

54 APPENDIX Some default data used by EZNEC V 5.0 EZNEC Demo ver. 5.0 Kizito 5/20/2009 8:10:16 AM SOURCE DATA Frequency = 643 MHz Source 1 Voltage = V at deg. Current = 1 A at 0.0 deg. Impedance = J ohms Power = watts SWR (50 ohm system) = (75 ohm system) = EZNEC Demo ver. 5.0 Kizito 5/20/2009 8:32:00 AM ATTENTION: This software CANNOT BE USED TO tell whether (1) the amount of electromagnetic energy being emitted from an antenna is unsafe to anyone; (2) an antenna subjects anyone to potentially hazardous electromagnetic exposure. LICENSOR DISCLAIMS ANY AND ALL WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Other disclaimers apply. Refer to the EZNEC on-line help for the complete text. DO NOT USE this software to determine whether an antenna is emitting an unsafe or hazardous level of energy NEAR-FIELD PATTERN DATA Frequency = 643 MHz Power = watts Max field = V/m RMS at X,Y,Z = 0, 0, 0 mm Electric (E) Field (V/m RMS) X (mm) Y (mm) Z (mm) Ex Mag Ey Mag Ez Mag Etot E

55 EZNEC Demo ver. 5.0 kizito 5/20/2009 8:32:39 AM CURRENT DATA Frequency = 643 MHz Wire No. 1: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open Wire No. 2: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open Wire No. 3: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open Wire No. 4: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open Wire No. 5: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open Wire No. 6: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open Wire No. 7: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open Wire No. 8: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open Wire No. 9: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open

56 Wire No. 10: Segment Conn Magnitude (A.) Phase (Deg.) 1 Open Open EZNEC Demo ver. 5.0 kizito 5/20/2009 8:33:29 AM FAR FIELD PATTERN DATA Frequency = 643 MHz Reference = 0 dbi Elevation Pattern Azimuth angle = 0 deg. Deg V db H db Tot db

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