Antennas & Waveguides

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Presentation Transcript

Slide 1: 

1 Antennas & Waveguides by Lotis P. Patunob, M.Eng., PECE

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2 Antennas Antenna It is a metallic conductor system capable of radiating and capturing electromagnetic energy. It is used to interface transmission lines to the atmosphere, the atmosphere to transmission lines, or both. How does antenna works?

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3 Antennas

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4 Antennas Antenna Characteristics Radiation Pattern – is a polar diagram or graph representing field strengths or power densities at various angular positions relative to an antenna. Absolute radiation pattern - the radiation pattern is plotted in terms of electric field strength (ξ) or power density (P). Ex. Variable distance, fixed power. Relative radiation pattern - plots field strength or power density with respect to the value at a reference point. Ex. Variable power, fixed distance.

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5

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6 Terms: Major lobe – the primary beam, also called the Front lobe because it propagates and receives the most energy. Secondary or minor lobe – the beam in a direction other than that of the major lobe, usually represents undesired radiation or reception. Side lobes – the lobes adjacent to the front lobe such as the minor lobe. Back lobes - the lobes in direction exactly opposite the front lobe Front-to-back ratio - the ratio of the front lobe power to the back lobe power Antennas

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7 Near field or Induction field – refers to the field pattern that is close to the antenna. It is the area within a distance D^2/λ from the antenna where D is the antenna diameter. Far field or Radiatio field – refers to the field pattern that is at great distance. Antennas Front-to-side ratio - the ratio of the front lobe to a side lobe Line of shoot or point of shoot - the line bisecting the major lobe Terms:

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8 Antennas Types of Radiation Pattern 1. Omnidirectional

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9 2. Bi-directional Antennas Types of Radiation Pattern 3. Uni-directional

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10 Antennas Antenna Characteristics 2. Antenna beamwidth – is simply the angular separation between the two half-power (-3 dB) points on the major lobe of an antenna’s plane radiation pattern. Sometimes called -3dB beamwidth or half-power beamwidth. - Antenna gain is inversely proportional to beamwidth. - Typical antennas have beamwidths between 30° to 60°; for high-gain microwave antennas as low as 1°

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11 Antennas Antenna Beamwidth

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12 Antennas PARABOLIC DISH

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13 Antennas Antenna Characteristics 3. Antenna bandwidth – defined as the frequency range over which antenna operation is “satisfactory”. - it is normally taken as the difference between the half-power frequencies, but sometimes refers to variations in the antenna’s input impedance. - often expressed as a percentage of the antenna’s optimum frequency of operation.

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14 Antenna input impedance – it is simply the ratio of the antenna’s input voltage to input current. - An ac load to the transmission line Antennas Antenna Characteristics Where: Zin = antenna input impedance (ohms) Ei = antenna input voltage (volts) Ii = antenna input current (ampere) Note: Feedpoint – the point on the antenna where transmission line is connected.

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15 Antennas Antenna Characteristics 4. Antenna polarization – polarization of an antenna refers simply to the orientation of the electric field radiated from it. Types of Polarization: 1. Linear polarization – the polarization is constant. It could be horizontal (if the electric field is propagating parallel to the earth’s surface) or vertical (perpendicular) polarization. 2. Circular polarization – if the polarization vector rotates 360° as the waves moves one λ through space and the field strength is equal at all angles. 3. Elliptical polarization – the field strength varies with changes in polarization.

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16 Antennas Antenna Polarization

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17 Antennas Antenna Characteristics 5. Antenna resistance and Antenna Efficiency Radiation resistance - is an ac antenna resistance. - when referenced to the current maximum point, sometimes called Loop radiation resistance because a current maximum is also called a Current loop. Where: Rr = radiation resistance (ohms) Prad = Power radiated by the antenna (watts) i = antenna current at the feedpoint (ampere)

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18 Antennas Antenna Characteristics Antenna efficiency, η – the ratio of the power radiated by an antenna to the sum of the power radiated and the power dissipated (the total input power). Where: Pin = input power (watts) Prad = radiated power (watts) Pd = power dissipated in antenna (watts) i = antenna current (ampere) Rr = radiation resistance (ohms) Re = effective antenna resistance (ohms)

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19 Antennas Antenna Characteristics 6. Antenna gain Directive gain, D – the ratio of the power density radiated in a particular direction to the power density radiated to the same point by a reference antenna. Where: P = power density at some point with a given antenna (watts per meter squared) Pref = power density at the same point with a reference antenna (watts per meter squared)

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20 Antennas Antenna Characteristics Power gain, Ap – the same as directive gain except that the total power fed to the antenna is used (efficiency, η is taken into account). Effective Isotropic Radiated Power (EIRP) or ERP (Effective Radiated Power) – defined as an equivalent power that an isotropic antenna would have to radiate to achieve the same power density in the chosen direction at a given point as another antenna.

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21 Antennas Antenna Characteristics Where: Prad = total radiated power (watts) Dt = transmit antenna directive gain (unitless) Pin = total antenna input power (watts) At = transmit antenna power gain (unitless) The power density at given point distance R from a transmit antenna:

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22 Antennas Antenna Characteristics 7. Captured Power Density, Antenna Capture Area, &Captured Power Captured power density – antennas are reciprocal devices, thus, they have the same radiation resistance, efficiency, power gain, and directivity when used to receive electromagnetic waves as they have when transmitting electromagnetic waves.

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23 Antennas Antenna Characteristics Captured Power, Pcap – is directly proportional to the received power density and the effective capture area of the receiving antenna. Capture area, Ac - of an antenna is an effective area, sometimes considerably larger than their physical area. Where: Ar = receive antenna power gain (unitless) P = power density (watts per meter squared)

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24 Antennas Antennas Antenna Characteristics 8. Front-to-back ratio – the ratio expressed in dB of the output in the most optimum direction to the output 180° away from the optimum direction.

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25 Antennas BasicAntenna Elementary Doublet / Short Dipole / Elementary Dipole / Hertzian Dipole – an electrically short dipole

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26 Antennas BasicAntenna Where:  = magnitude of field strength r = distance Le = antenna length I = current amplitude  = the angle of the axis of the wire and the point of maximum radiation P = power density

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27 Antennas Half – Wave Dipole/ Hertz Antenna – one of the most widely used antennas at frequencies above 2 MHz. It is a multiple of quarter-wavelengths long and open circuited at the far end.

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28 Ground Effects on Half – Wave Dipole: λ/2 above ground λ/4 above ground

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29 Antennas Ground Effects on Half – Wave Dipole: * The best way to eliminate or reduce the effect of ground reflected waves is to mount the antenna far enough above Earth’s surface to obtain free space conditions. * Ground reflections are sometimes desirable to get the desired elevation angle for the major lobe’s maximum response. * The height of an ungrounded antenna above Earth’s surface also affects the antenna’s radiation resistance. Depending on the phase of the ground-reflected wave, the antenna current can increase or decrease, causing a corresponding increase or decrease in the input impedance.

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30 Antennas Ground Effects on Half – Wave Dipole:

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31 Antennas Grounded Antenna Monopole / Marconi Antenna – a one-quarter wavelength long, mounted vertically with the lower end either connected to ground or grounded through the antenna coupling network. Advantage: ½ long Disadvantage: Close to ground

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32 Antennas Antenna Arrays – is formed when two or more antenna elements are combined to form a single antenna. the purpose of an array is to increase the directivity of an antenna system and concentrate the radiated power within a smaller geographic area. Antenna element - is an individual radiator, such as a half- or quarter-wave dipole. Types of Antenna Elements: Driven Parasitic (Nondriven)

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33 Types of Antenna Elements: 1. Driven Elements – are directly connected to the transmission line and receive power from or are driven by the source. 2. Parasitic (Nondriven) – are not connected to the transmission line; they receive energy only through mutual induction with a driven element or another parasitic element. Reflector –longer than the driven element from which it receives energy. - it effectively reduces the signal strength in its direction and increases it in the opposite direction. Director - shorter than its associated driven element. - increases the field strength in its direction and reduces it in the opposite direction.

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34 Note: Radiation directivity can be increased in either the horizontal or vertical plane, depending on the placement of the elements and whether they are driven. If not driven, the pattern depends on whether the elements are directors or reflectors. If driven, the pattern depends on the relative phase of the feeds. Types of Antenna Arrays: 1. Broadside Array – one of the simplest types of antenna arrays. It is made by simply placing several resonant dipoles of equal size (both length and diameter) in parallel with each other and in a straight line. - it radiates at right angles to the plane of the array and radiates very little in the direction of the plane. Antennas

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35 Broadside Antenna Array Directivity can be increased even further by increasing the length of the array by adding more elements Additive in a plane at right angles to the plane of the array. Antennas

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36 Types of Antenna Arrays: Antennas 2. End-Fire Array – has the same element configuration as the broadside array except that the transmission line is not crisscrossed between elements. - the fields are additive in line with the plane of the array.

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37

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38 Types of Antenna Arrays: Antennas 3. Nonresonant Array: The Rhombic Antenna – it is capable of operating satisfactorily over relatively wide bandwidth, making it ideally suited for HF transmission (3 MHz to 30 MHz). - it is made up of four nonresonant elements each several wavelengths long. The entire array is terminated in a resistor if unidirectional operation is desired. - has a maximum efficiency of 67%. - gains of over 40 (16 dB)

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39 Rhombic Antenna Radiation Pattern: Antennas The antenna is mounted horizontally and placed one-half wavelength or more above the ground. The exact height depends on the precise radiation pattern desired.

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40 Special Purpose Antennas: Antennas 1. Folded Dipole / Flat Dipole – essentially a single antenna made up of two elements (λ/2 long). One element is fed directly, whereas the other is conductively coupled at the ends. - input current is ½ that of basic half-wave dipole. - input impedance is equal to the half-wave impedance (72Ω) times the number of folded wires squared. - the bandwidth can be increased even further by making the dipole elements larger in diameter.

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41 Folded Dipole Antenna: 2. Yagi Uda Antenna / Yagi – a widely used antenna that commonly uses a folded dipole as the driven element. - a linear array consisting of a dipole and two or more parasitic elements: one reflector and one or more directors. - Commonly used for VHF television reception because its wide bandwidth extends from 54 MHz to 216 MHz.

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42 Yagi Uda Antenna The typical directivity is between 7 dB and 9 dB The bandwidth can be increased by using more than one folded dipole, each cut to a slightly different length.

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43

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44 Special Purpose Antennas: 3. Turnstile Antenna – is formed by placing two dipoles at right angles to each other, 90° out of phase. - the radiation pattern is the sum of the radiation patterns from the two dipoles, which produces a nearly omni-directional pattern. - antenna gains of 10 or more dB are common.

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45 Special Purpose Antennas: Antennas 4. Log-Periodic Antenna – a class of frequency- independent antennas. - its primary advantage of log-periodic antennas is the independence of their radiation resistance and radiation pattern to frequency. - have bandwidth ratios of 10:1 or greater, it the ratio of the highest to the lowest frequency over which an antenna will satisfactorily operate. - it can be uni-directional or bidirectional and have a low to moderate directive gain. High gains may also be achieved by using them as an element in a more complicated array.

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46 Special Purpose Antennas: Antennas 4. Log-Periodic Antenna – like rhombic antennas, are used mainly for HF and VHF communications, but it is more efficient. - it consists of several dipoles of different length and spacing that are fed from a single source at the small end. The lengths of the dipoles and their spacing are related in such a way that adjacent elements have a constant ratio to each other. - the ends of the dipoles lie along a straight line, and the angle where they meet is designated α. For typical design, τ = 0.7 and α = 30°.

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47 Where: R = dipole spacing (inches) L = dipole length (inches) τ = design ratio (number less than 1)

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48 Special Purpose Antennas: Antennas 5. Phased Array Antennas – is a group of antennas or a group of antenna arrays that, when connected together, function as a single antenna whose beamwidth and direction can be changed electronically without having to physically move any of the individual antennas or antenna elements within the array. - the primary advantage is that they eliminate the need for mechanically rotating antenna elements. - the primary application is in radar when radiation patterns must be capable of being rapidly changed to follow a moving object.

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49 It is based on interference among electromagnetic waves in free space. When electromagnetic energies from different sources occupy the same space at the same time, they combine, sometimes constructively and sometimes destructively.

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50 Special Purpose Antennas: 6. Helical Antenna – is a broadband VHF and UHF antenna that is ideally suited for applications for which radiating circular rather than horizontal or vertical polarized electromagnetic waves are required. - can be used as a single element antenna or stacked horizontally or vertically in an array to modify its radiation pattern by increasing the gain and decreasing the beamwidth of the primary lobe. - the driven element of the antenna consists of a loosely wound rigid helix with an axis length approximately equal to the product of the number of turns and the distance between turns (pitch).

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51 Power gain Beamwidth

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52 Waveguides Waveguides It is a conducting tube through which the energy is transmitted, in the form of electromagnetic waves. It is an alternative to cable for frequency of 1 Ghz and above.

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53 Electromagnetic Wave It is made up of magnetic and electric fields that are at right angles to each other and at right angles to the direction of propagation. It travels in a straight line at approximately the speed of light. Waveguides

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54 Modes of Propagation - the possible direction of distribution of energy 1. Transverse Electric (TE) – has the electric field transverse the direction of propagation, while the magnetic field is along the propagation direction 2. Transverse Magnetic (TM0) – has the magnetic field at right angles to the direction of propagation along the guide, and the electric field in the direction of propagation. Classification of Modes of Propagation: Waveguides

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55 Format: TEm,n where: n = indicates the no. of half wave variation of the electric field along the y or b (height) dimension. m = indicates the no. of half wave variation of the electric field along the x or a (width) dimension. Waveguides

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56 where: arrows = represent the E field perpendicular to the sides of the guide. x’s = represent the H field that is going into the waveguide. dots = represent the H field that is coming out of the waveguide. Waveguides

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57 Types of Waveguides: A. Rectangular – used when energy must be coupled from the source to a load and both are fixed in place since they are smaller than circular waveguides for a given wavelength. General formula for cut off wavelength, λc: Cut off wavelength for TEm,0: Waveguides

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58 Cut off wavelength for TE1,0: where: TE1,0 = called the dominant mode, the mode for the lowest frequency that can be propagated in a waveguide x = the width of the waveguide y = the height of the waveguide Note: x ≤ λ/2 for dominant mode means no propagation Waveguides

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59 B. Circular – used for rotating systems such as radar antenna where: K = 1.84 for dominant mode Example: What is the cut off wavelength that a 2.5 cm wide waveguide will support the dominant mode (m = 1)? How about for the next mode (m = 2)? Waveguides

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60 Key wavelength formula for rectangular/circular waveguide: Waveguides

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61 Group Velocity, Vg The actual speed at which a signal travels down the guide. Phase Velocity, Vp The rate at which the wave appears to move along the wall of the guide. -the velocity with which a wave changes phase in a direction parallel to a conducting surface, such as walls of a waveguide. Note: Vg·Vp = Vc2 Waveguides

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62 Waveguide Characteristic Impedance: TE mode: TM mode: Example: A 6 GHz signal is to be propagated in a waveguide whose width is 7.5 cm. Calculate the characteristic impedance for TE1,0 mode and TM1,1 mode if the thickness is 3.75 cm. Waveguides

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63 Microwave Communication & Waveguides Example: A 6 GHz signal is to be propagated in the dominant mode in a rectangular waveguide if its group velocity is to be 90% of the speed of light, what must be width of the guide? Wavelength of a guide, λg Example: Find the guide wavelength of a waveguide at a frequency of 5 GHz in a dominant mode if the cut off frequency is 3.75 GHz.

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64 THE END

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