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Antennas wave pro frenzel


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Antennas wave pro frenzel

  1. 1. 1 Principles of ElectronicCommunication Systems Third Edition Louis E. Frenzel, Jr. © 2008 The McGraw-Hill Companies
  2. 2. 2 Chapter 14Antennas and Wave Propagation © 2008 The McGraw-Hill Companies
  3. 3. 3 Topics Covered in Chapter 14 14-1: Antenna Fundamentals 14-2: Common Antenna Types 14-3: Radio-Wave Propagation © 2008 The McGraw-Hill Companies
  4. 4. 4 14-1: Antenna Fundamentals The interface between the transmitter and free space and between free space and the receiver is the antenna. At the transmitting end the antenna converts the transmitter RF power into electromagnetic signals; at the receiving end the antenna picks up the electromagnetic signals and converts them into signals for the receiver. © 2008 The McGraw-Hill Companies
  5. 5. 5 14-1: Antenna FundamentalsRadio Waves  A radio signal is called an electromagnetic wave because it is made up of both electric and magnetic fields.  Whenever voltage is applied to the antenna, an electric field is set up.  This voltage causes current to flow in the antenna, producing a magnetic field.  These fields are emitted from the antenna and propagate through space at the speed of light. © 2008 The McGraw-Hill Companies
  6. 6. 6 14-1: Antenna FundamentalsRadio Waves: Magnetic Fields  A magnetic field is an invisible force field created by a magnet.  An antenna is a type of electromagnet.  A magnetic field is generated around a conductor when current flows through it.  The strength and direction of the magnetic field depend upon the magnitude and direction of the current flow.  The SI unit for magnetic field strength is ampere-turns per meter. © 2008 The McGraw-Hill Companies
  7. 7. 7 14-1: Antenna FundamentalsFigure 14-1: Magnetic field around a current-carrying conductor. Magnetic field strengthH in ampere-turns per meter = H = I I(2 πd). © 2008 The McGraw-Hill Companies
  8. 8. 8 14-1: Antenna FundamentalsRadio Waves: Electric Field  An electric field is an invisible force field produced by the presence of a potential difference between two conductors.  For example, an electric field is produced between the plates of a charged capacitor.  An electric field exists between any two points across which a potential difference exists.  The SI unit for electric field strength is volts per meter.  Permittivity is the dielectric constant of the material between the two conductors. © 2008 The McGraw-Hill Companies
  9. 9. 9 14-1: Antenna FundamentalsFigure 14-2: Electric field across the plates of a capacitor. © 2008 The McGraw-Hill Companies
  10. 10. 10 14-1: Antenna FundamentalsRadio Waves: Magnetic and Electric Fields in a Transmission Line  At any given time in a two-wire transmission line, the wires have opposite polarities.  During one-half cycle of the ac input, one wire is positive and the other is negative.  During the negative half-cycle, the polarity reverses.  The direction of the electric field between the wires reverses once per cycle.  The direction of current flow in one wire is always opposite that in the other wire. Therefore, the magnetic fields combine. © 2008 The McGraw-Hill Companies
  11. 11. 11 14-1: Antenna FundamentalsRadio Waves: Magnetic and Electric Fields in a Transmission Line  A transmission line is made up of a conductor or conductors.  Transmission lines do not radiate signals efficiently.  The closeness of the conductors keeps the electric field concentrated in the transmission line dielectric.  The magnetic fields mostly cancel one another.  The electric and magnetic fields do extend outward from the transmission line, but the small amount of radiation that does occur is extremely inefficient. © 2008 The McGraw-Hill Companies
  12. 12. 12 14-1: Antenna FundamentalsFigure 14-3: (a) Magnetic and electric fields around a transmission line. (b) Electricfield. (c) Magnetic fields. © 2008 The McGraw-Hill Companies
  13. 13. 13 14-1: Antenna FundamentalsAntenna Operation: The Nature of an Antenna  If a parallel-wire transmission line is left open, the electric and magnetic fields escape from the end of the line and radiate into space.  This radiation is inefficient and unsuitable for reliable transmission or reception.  The radiation from a transmission line can be greatly improved by bending the transmission-line conductors so they are at a right angle to the transmission line. © 2008 The McGraw-Hill Companies
  14. 14. 14 14-1: Antenna FundamentalsAntenna Operation: The Nature of an Antenna  The magnetic fields no longer cancel; they now aid one another.  The electric field spreads out from conductor to conductor.  Optimum radiation occurs if the segment of transmission wire converted into an antenna is one quarter wavelength long at the operating frequency.  This makes an antenna that is one-half wavelength long. © 2008 The McGraw-Hill Companies
  15. 15. 15 14-1: Antenna FundamentalsFigure 14-5: Converting a transmission line into an antenna. (a) An open transmissionline radiates a little. (b) Bending the open transmission line at right angles createsan efficient radiation pattern. © 2008 The McGraw-Hill Companies
  16. 16. 16 14-1: Antenna FundamentalsAntenna Operation  The ratio of the electric field strength of a radiated wave to the magnetic field strength is a constant and is called the impedance of space, or the wave impedance.  The electric and magnetic fields produced by the antenna are at right angles to one another, and are both perpendicular to the direction of propagation of the wave. © 2008 The McGraw-Hill Companies
  17. 17. 17 14-1: Antenna FundamentalsAntenna Operation  Antennas produce two sets of fields, the near field and the far field.  The near field describes the region directly around the antenna where the electric and magnetic fields are distinct.  The far field is approximately 10 wavelengths from the antenna. It is the radio wave with the composite electric and magnetic fields.  Polarization refers to the orientation of magnetic and electric fields with respect to the earth. © 2008 The McGraw-Hill Companies
  18. 18. 18 14-1: Antenna FundamentalsAntenna Reciprocity  Antenna reciprocity means that the characteristics and performance of an antenna are the same whether the antenna is radiating or intercepting an electromagnetic signal.  A transmitting antenna takes a voltage from the transmitter and converts it into an electromagnetic signal.  A receiving antenna has a voltage induced into it by the electromagnetic signal that passes across it. © 2008 The McGraw-Hill Companies
  19. 19. 19 14-1: Antenna FundamentalsThe Basic Antenna  An antenna can be a length of wire, a metal rod, or a piece of tubing.  Antennas radiate most effectively when their length is directly related to the wavelength of the transmitted signal.  Most antennas have a length that is some fraction of a wavelength.  One-half and one-quarter wavelengths are most common. © 2008 The McGraw-Hill Companies
  20. 20. 20 14-2: Common Antenna TypesThe Dipole Antenna  One of the most widely used antenna types is the half- wave dipole.  The half-wave dipole, also called a doublet, is formally known as the Hertz antenna.  A dipole antenna is two pieces of wire, rod, or tubing that are one-quarter wavelength long at the operating resonant frequency.  Wire dipoles are supported with glass, ceramic, or plastic insulators at the ends and middle. © 2008 The McGraw-Hill Companies
  21. 21. 21 14-2: Common Antenna TypesFigure 14-10: The dipole antenna. © 2008 The McGraw-Hill Companies
  22. 22. 22 14-2: Common Antenna TypesThe Dipole Antenna  The dipole has an impedance of 73 Ω at its center, which is the radiation resistance.  An antenna is a frequency-sensitive device.  To get the dipole to resonate at the frequency of operation, the physical length must be shorter than the one-half wavelength computed by λ = 492/f.  Actual length is related to the ratio of length to diameter, conductor shape, Q, the dielectric (when the material is other than air), and a condition known as end effect. © 2008 The McGraw-Hill Companies
  23. 23. 23 14-2: Common Antenna TypesThe Dipole Antenna  End effect is a phenomenon caused by any support insulators used at the ends of the wire antenna and has the effect of adding capacitance to the end of each wire.  The actual antenna length is only about 95 percent of the computed length.  If a dipole is used at a frequency different from its design frequency, the SWR rises and power is lost. © 2008 The McGraw-Hill Companies
  24. 24. 24 14-2: Common Antenna TypesThe Dipole Antenna: Antenna Q and Bandwidth  The bandwidth of an antenna is determined by the frequency of operation and the Q of the antenna according to the relationship BW = fr/Q.  The higher the Q, the narrower the bandwidth.  For an antenna, low Q and wider bandwidth are desirable so that the antenna can operate over a wider range of frequencies with reasonable SWR.  In general, any SWR below 2:1 is considered good in practical antenna work. © 2008 The McGraw-Hill Companies
  25. 25. 25 14-2: Common Antenna TypesThe Dipole Antenna: Antenna Q and Bandwidth  The Q and thus the bandwidth of an antenna are determined by the ratio of the length of the conductor to the diameter of the conductor.  Bandwidth is sometimes expressed as a percentage of the resonant frequency of the antenna.  A small percentage means a higher Q, and a narrower bandwidth means a lower percentage. © 2008 The McGraw-Hill Companies
  26. 26. 26 14-2: Common Antenna TypesThe Dipole Antenna: Conical Antennas  A common way to increase bandwidth is to use a version of the dipole antenna known as the conical antenna.  The center radiation resistance of a conical antenna is much higher than the 73 Ω usually found when straight- wire or tubing conductors are used.  The primary advantage of conical antennas is their tremendous bandwidth.  They can maintain a constant impedance and gain over a 4:1 frequency range. © 2008 The McGraw-Hill Companies
  27. 27. 27 14-2: Common Antenna TypesFigure 14-14: The conical dipole and its variation. (a) Conical antenna. (b) Broadsideview of conical dipole antenna (bow tie antenna) showing dimensions. (c) Open-grillbow tie antenna. © 2008 The McGraw-Hill Companies
  28. 28. 28 14-2: Common Antenna TypesThe Dipole Antenna: Dipole Polarization  Most half-wave dipole antennas are mounted horizontally to the earth.  This makes the electric field horizontal to the earth and the antenna is horizontally polarized.  Horizontal mounting is preferred at the lower frequencies because the physical construction, mounting, and support are easier.  This mounting makes it easier to attach the transmission line and route it to the transmitter or receiver. © 2008 The McGraw-Hill Companies
  29. 29. 29 14-2: Common Antenna TypesThe Dipole Antenna: Radiation Pattern and Directivity  The radiation pattern of any antenna is the shape of the electromagnetic energy radiated from or received by that antenna.  Most antennas have directional characteristics that cause them to radiate or receive energy in a specific direction.  The radiation is concentrated in a pattern that has a recognizable geometric shape.  The measure of an antenna’s directivity is beam width, the angle of the radiation pattern over which a transmitter’s energy is directed or received. © 2008 The McGraw-Hill Companies
  30. 30. 30 14-2: Common Antenna TypesFigure 14-15: Three-dimensional pattern of a half-wave dipole. © 2008 The McGraw-Hill Companies
  31. 31. 31 14-2: Common Antenna TypesThe Dipole Antenna: Antenna Gain  A directional antenna can radiate more power in a given direction than a nondirectional antenna. In this “favored” direction, it acts as if it had gain.  Antenna gain of this type is expressed as the ratio of the effective radiated output power Pout to the input power Pin. © 2008 The McGraw-Hill Companies
  32. 32. 32 14-2: Common Antenna TypesThe Dipole Antenna: Antenna Gain  Effective radiated power is the actual power that would have to be radiated by a reference antenna (usually a nondirectional or dipole antenna) to produce the same signal strength at the receiver as the actual antenna produces.  The power radiated by an antenna with directivity and therefore gain is called the effective radiated power (ERP). ERP = ApPt © 2008 The McGraw-Hill Companies
  33. 33. 33 14-2: Common Antenna TypesThe Dipole Antenna: Folded Dipole  A popular variation of the half-wave dipole is the folded dipole.  The folded dipole is also one-half wavelength long.  It consists of two parallel conductors connected at the ends with one side open at the center for connection to the transmission line.  The impedance of this antenna is 300 Ω.  Folded dipoles usually offer greater bandwidth than standard dipoles.  The folded dipole is an effective, low-cost antenna that can be used for transmitting and receiving. © 2008 The McGraw-Hill Companies
  34. 34. 34 14-2: Common Antenna TypesFigure 14-18: Folded dipole. (a) Basic configuration. (b) Construction with twin lead. © 2008 The McGraw-Hill Companies
  35. 35. 35 14-2: Common Antenna TypesMarconi or Ground-Plane Vertical Antenna  The one-quarter wavelength vertical antenna, also called a Marconi antenna, is widely used.  It is similar in operation to a vertically mounted dipole antenna.  The Marconi antenna offers major advantages because it is half the length of a dipole antenna. © 2008 The McGraw-Hill Companies
  36. 36. 36 14-2: Common Antenna TypesMarconi or Ground-Plane Vertical Antenna: Radiation Pattern  Vertical polarization and omnidirectional characteristics can be achieved using a one-quarter wavelength vertical radiator. This antenna is called a Marconi or ground-plane antenna.  It is usually fed with coaxial cable; the center conductor is connected to the vertical radiator and the shield is connected to earth ground.  The earth then acts as a type of electrical “mirror,” providing the other one-quarter wavelength making it equivalent to a vertical dipole. © 2008 The McGraw-Hill Companies
  37. 37. 37 14-2: Common Antenna TypesFigure 14-20: Ground-plane antenna. (a) One-quarter wavelength vertical antenna.(b) Using radials as a ground plane. © 2008 The McGraw-Hill Companies
  38. 38. 38 14-2: Common Antenna TypesMarconi or Ground-Plane Vertical Antenna: Ground Plane, Radials, and Counterpoise  When a good electrical connection to the earth has been made, the earth becomes what is known as a ground plane.  If a ground plane cannot be made to earth, an artificial ground can be constructed of several one- quarter wavelength wires laid horizontally on the ground or buried in the earth.  These horizontal wires at the base of the antenna are called radials, and the collection of radials is called a counterpoise. © 2008 The McGraw-Hill Companies
  39. 39. 39 14-2: Common Antenna TypesMarconi or Ground-Plane Vertical Antenna: Antenna Length  For many applications, e.g., with portable or mobile equipment, it is not possible to make the antenna a full one-quarter wavelength long.  To overcome this problem, shorter antennas are used, and lumped electrical components are added to compensate for the shortening. © 2008 The McGraw-Hill Companies
  40. 40. 40 14-2: Common Antenna TypesMarconi or Ground-Plane Vertical Antenna: Antenna Length  The practical effect of this design is a decreased inductance. The antenna no longer resonates at the desired operating frequency, but at a higher frequency.  To compensate for this, a series inductor, called a loading coil, is connected in series with the antenna coil.  The loading coil brings the antenna back into resonance at the desired frequency. © 2008 The McGraw-Hill Companies
  41. 41. 41 14-2: Common Antenna TypesFigure 14-22: Using a base leading coil to increase effective antenna length. © 2008 The McGraw-Hill Companies
  42. 42. 42 14-2: Common Antenna TypesDirectivity  Directivity refers to an antenna’s ability to send or receive signals over a narrow horizontal directional range.  The physical orientation of the antenna gives it a highly directional response or directivity curve.  A directional antenna eliminates interference from other signals being received from all directions other than the desired signal. © 2008 The McGraw-Hill Companies
  43. 43. 43 14-2: Common Antenna TypesDirectivity  A highly directional antenna acts as a type of filter to provide selectivity.  Directional antennas provide greater efficiency of power transmission.  Directivity, because it focuses the power, causes the antenna to exhibit gain, which is one form of amplification. © 2008 The McGraw-Hill Companies
  44. 44. 44 14-2: Common Antenna TypesFigure 14-25: Radiation pattern of a highly directional antenna with gain. (a) Horizontalradiation pattern. (b) Three-dimensional radiation pattern. © 2008 The McGraw-Hill Companies
  45. 45. 45 14-2: Common Antenna TypesDirectivity  To create an antenna with directivity and gain, two or more antenna elements are combined to form an array.  Two basic types of antenna arrays are used to achieve gain and directivity: 1. Parasitic arrays. 2. Driven arrays. © 2008 The McGraw-Hill Companies
  46. 46. 46 14-2: Common Antenna TypesParasitic Arrays  A parasitic array consists of a basic antenna connected to a transmission line plus one or more additional conductors that are not connected to the transmission line.  These extra conductors are referred to as parasitic elements and the antenna is called a driven element.  A Yagi antenna is made up of a driven element and one or more parasitic elements. © 2008 The McGraw-Hill Companies
  47. 47. 47 14-2: Common Antenna TypesFigure 14-26: A parasitic array known as a Yagi antenna. © 2008 The McGraw-Hill Companies
  48. 48. 48 14-2: Common Antenna TypesDriven Arrays  A driven array is an antenna that has two or more driven elements.  Each element receives RF energy from the transmission line.  Different arrangements of the elements produce different degrees of directivity and gain.  The three basic types of driven arrays are the collinear, the broadside, and the end-fire.  A fourth type is the wide-bandwidth log-periodic antenna. © 2008 The McGraw-Hill Companies
  49. 49. 49 14-2: Common Antenna TypesDriven Arrays: Collinear Antenna  Collinear antennas usually consist of two or more half- wave dipoles mounted end to end.  Collinear antennas typically use half-wave sections separated by shorted quarter-wave matching stubs which ensure that the signals radiated by each half- wave section are in phase.  Collinear antennas are generally used only on VHF and UHF bands because their length becomes prohibited at the lower frequencies. © 2008 The McGraw-Hill Companies
  50. 50. 50 14-2: Common Antenna TypesFigure 14-29: Radiation pattern of a four-element collinear antenna. © 2008 The McGraw-Hill Companies
  51. 51. 51 14-2: Common Antenna TypesDriven Arrays: Broadside Antenna  A broadside array is a stacked collinear antenna consisting of half-wave dipoles spaced from one another by one-half wavelengths.  This antenna produces a highly directional radiation pattern that is broadside or perpendicular to the plane of the array.  The broadside antenna is bidirectional in radiation, but the radiation pattern has a very narrow beam width and high gain. © 2008 The McGraw-Hill Companies
  52. 52. 52 14-2: Common Antenna TypesFigure 14-30: A broadside array. © 2008 The McGraw-Hill Companies
  53. 53. 53 14-2: Common Antenna TypesDriven Arrays: End-Fire Antenna  The end-fire array uses two half-wave dipoles spaced one-half wavelength apart.  The end-fire array has a bidirectional radiation pattern, but with narrower beam widths and lower gain.  The radiation is in the plane of the driven elements.  A highly unidirectional antenna can be created by careful selection of the optimal number of elements with the appropriately related spacing. © 2008 The McGraw-Hill Companies
  54. 54. 54 14-2: Common Antenna TypesFigure 14-31: End-fire antennas. (a) Bidirectional. (b) Unidirectional. © 2008 The McGraw-Hill Companies
  55. 55. 55 14-2: Common Antenna TypesDriven Arrays: Log-Periodic Antennas  A special type of driven array is the wide-bandwidth log-periodic antenna.  The lengths of the driven elements vary from long to short and are related logarithmically. The spacing is also variable.  The great advantage of the log-periodic antenna over a Yagi or other array is its very wide bandwidth.  The driving impedance is constant over this range.  Most TV antennas in use today are of the log-periodic variety so that they can provide high gain and directivity on both VHF and UHF TV channels. © 2008 The McGraw-Hill Companies
  56. 56. 56 14-2: Common Antenna TypesFigure 14-32: Log-periodic antenna. © 2008 The McGraw-Hill Companies
  57. 57. 57 14-2: Common Antenna TypesImpedance Matching  One of the most critical aspects of any antenna system is ensuring maximum power transfer from the transmitter to the antenna.  When the characteristic impedance of the transmission line matches the output impedance of the transmitter and the impedance of the antenna, the SWR will be 1:1.  When SWR is 1:1, maximum power transfer will take place. © 2008 The McGraw-Hill Companies
  58. 58. 58 14-2: Common Antenna TypesImpedance Matching  A Q section, or matching stub, is a one-quarter wavelength of coaxial or balanced transmission line of a specific impedance that is connected between a load and source and is used to match impedances.  A balun is a transformer used to match impedances.  An antenna tuner is a variable inductor, one or more variable capacitors, or a combination of these components connected in various configurations. © 2008 The McGraw-Hill Companies
  59. 59. 59 14-2: Common Antenna TypesFigure 14-33: A one-quarter wavelength matching stub or Q section. © 2008 The McGraw-Hill Companies
  60. 60. 60 14-2: Common Antenna TypesFigure 14-34: A bifilar toroidal balun for impedance matching. © 2008 The McGraw-Hill Companies
  61. 61. 61 14-2: Common Antenna TypesFigure 14-36: An antenna tuner. © 2008 The McGraw-Hill Companies
  62. 62. 62 14-3: Radio-Wave Propagation Once a radio signal has been radiated by an antenna, it travels or propagates through space and ultimately reaches the receiving antenna. The energy level of the signal decreases rapidly with distance from the transmitting antenna. The electromagnetic wave is affected by objects that it encounters along the way such as trees, buildings, and other large structures. The path that an electromagnetic signal takes to a receiving antenna depends upon many factors, including the frequency of the signal, atmospheric conditions, and time of day. © 2008 The McGraw-Hill Companies
  63. 63. 63 14-3: Radio-Wave PropagationOptical Characteristics of Radio Waves  Radio waves act much like light waves.  Light waves can be reflected, refracted, diffracted, and focused by other objects.  The focusing of waves by antennas to make them more concentrated in a desired direction is comparable to a lens focusing light waves into a narrower beam. © 2008 The McGraw-Hill Companies
  64. 64. 64 14-3: Radio-Wave PropagationOptical Characteristics of Radio Waves: Reflection  Any conducting surface looks like a mirror to a radio wave, and so radio waves are reflected by any conducting surface they encounter.  Radio-wave reflection follows the principles of light- wave reflection.  The angle of reflection is equal to the angle of incidence.  The direction of the electric field approaching the reflecting surface is reversed from that leaving the surface. This is equivalent to a 180° phase shift. © 2008 The McGraw-Hill Companies
  65. 65. 65 14-3: Radio-Wave PropagationFigure 14-37: How a conductive surface reflects a radio wave. © 2008 The McGraw-Hill Companies
  66. 66. 66 14-3: Radio-Wave PropagationOptical Characteristics of Radio Waves: Refraction  Refraction is the bending of a wave due to the physical makeup of the medium through which the wave passes.  Index of refraction is obtained by dividing the speed of a light (or radio) wave in a vacuum and the speed of a light (or radio) wave in the medium that causes the wave to be bent. © 2008 The McGraw-Hill Companies
  67. 67. 67 14-3: Radio-Wave PropagationOptical Characteristics of Radio Waves: Refraction  The relationship between the angles and the indices of refraction is given by a formula known as Snell’s law: n1 sin Θ1 = n2 sin Θ2 where n1 = index of refraction of initial medium n2 = index of refraction of medium into which wave passes Θ1 = angle of incidence Θ2 = angle of refraction © 2008 The McGraw-Hill Companies
  68. 68. 68 14-3: Radio-Wave PropagationFigure 14-38: How a change in the index of refraction causes bending of a radio wave. © 2008 The McGraw-Hill Companies
  69. 69. 69 14-3: Radio-Wave PropagationOptical Characteristics of Radio Waves: Diffraction  Diffraction is the bending of waves around an object.  Diffraction is explained by Huygen’s principle:  Assuming that all electromagnetic waves radiate as spherical waveforms from a source, each point on a wave front can be considered as a point source for additional spherical waves.  When the waves encounter an obstacle, they pass around it, above it, and on either side.  As the wave front passes the object, the point sources of waves at the edge of the obstacle create additional spherical waves that penetrate and fill in the shadow zone. © 2008 The McGraw-Hill Companies
  70. 70. 70 14-3: Radio-Wave PropagationFigure 14-39: Diffraction causes waves to bend around obstacles. © 2008 The McGraw-Hill Companies
  71. 71. 71 14-3: Radio-Wave PropagationRadio-Wave Propagation Through Space  The three basic paths that a radio signal can take through space are:  Ground wave  Sky wave  Space wave © 2008 The McGraw-Hill Companies
  72. 72. 72 14-3: Radio-Wave PropagationRadio-Wave Propagation Through Space: Ground Waves  Ground or surface waves leave an antenna and remain close to the earth.  Ground waves actually follow the curvature of the earth and can travel at distances beyond the horizon.  Ground waves must have vertical polarization to be propagated from an antenna.  Ground-wave propagation is strongest at the low- and medium-frequency ranges.  AM broadcast signals are propagated primarily by ground waves during the day and by sky waves at night. © 2008 The McGraw-Hill Companies
  73. 73. 73 14-3: Radio-Wave PropagationFigure 14-40: Ground or surface wave radiation from an antenna. © 2008 The McGraw-Hill Companies
  74. 74. 74 14-3: Radio-Wave PropagationRadio-Wave Propagation Through Space: Sky Waves  Sky-wave signals are radiated by the antenna into the upper atmosphere, where they are bent back to earth.  When a radio signal goes into the ionosphere, the different levels of ionization cause the radio waves to be gradually bent.  The smaller the angle with respect to the earth, the more likely it is that the waves will be refracted and sent back to earth.  The higher the frequency, the smaller the radiation angle required for refraction to occur. © 2008 The McGraw-Hill Companies
  75. 75. 75 14-3: Radio-Wave PropagationFigure 14-41: Sky wave propagation. © 2008 The McGraw-Hill Companies
  76. 76. 76 14-3: Radio-Wave PropagationRadio-Wave Propagation Through Space: Space Waves  A direct wave, or space wave, travels in a straight line directly from the transmitting antenna to the receiving antenna.  Direct-wave radio signaling is often referred to as line-of- sight communication.  Direct or space waves are not refracted, nor do they follow the curvature of the earth.  Line-of-sight communication is characteristic of most radio signals with a frequency above 30 MHz, particularly VHF, UHF, and microwave signals. © 2008 The McGraw-Hill Companies
  77. 77. 77 14-3: Radio-Wave PropagationFigure 14-42: Line-of-sight communication by direct or space waves. © 2008 The McGraw-Hill Companies
  78. 78. 78 14-3: Radio-Wave PropagationRadio-Wave Propagation Through Space: Space Waves  Repeater stations extend the communication distance at VHF, UHF, and microwave frequencies.  A repeater is a combination of a receiver and a transmitter operating on separate frequencies.  The receiver picks up a signal from a remote transmitter, amplifies it, and retransmits it (on another frequency) to a remote receiver.  Repeaters are widely used to increase the communication range for mobile and handheld radio units. © 2008 The McGraw-Hill Companies
  79. 79. 79 14-3: Radio-Wave PropagationRadio-Wave Propagation Through Space: Space Waves  In a trunked repeater system, multiple repeaters are under the control of a computer system that can transfer a user from an assigned but busy repeater to another, available repeater, thus spreading the communication load.  Communication satellites act as fixed repeater stations.  The receiver-transmitter combination within the satellite is known as a transponder. © 2008 The McGraw-Hill Companies
  80. 80. 80 14-3: Radio-Wave PropagationCommon Propagation Problems: Fading  Fading is the variation in signal amplitude at the receiver caused by the characteristics of the signal path and changes in it.  Fading typically makes the received signal smaller.  Fading is caused by four factors: 1. Variation in distance between transmitter and receiver. 2. Changes in the environmental characteristics of the signal path. 3. The presence of multiple signal paths. 4. Relative motion between the transmitter and receiver. © 2008 The McGraw-Hill Companies
  81. 81. 81 14-3: Radio-Wave PropagationCommon Propagation Problems: Diversity System  A diversity system uses multiple transmitters, receivers, or antennas to mitigate the problems caused by multipath signals.  With frequency diversity, two separate sets of transmitters and receivers operating on different frequencies are used to transmit the same information simultaneously.  Space or spatial diversity uses two receive antennas spaced as far apart as possible to receive the signals. © 2008 The McGraw-Hill Companies