Module 10

Card Set Information

Module 10
2012-01-03 18:33:38


Show Answers:

  1. Which two composite fields (composed of E and H fields) are associated with every antenna?
    Induction field and radiation field.
  2. What composite field (composed of E and H fields) is found stored in the antenna?
    Induction field.
  3. What composite field (composed of E and H fields) is propagated into free space?
    Radiation field.
  4. What is the term used to describe the basic frequency of a radio wave?
    Fundamental frequency
  5. What is the term used to describe a whole number multiple of the basic frequency of a radio wave?
    Harmonic frequency or harmonics.
  6. It is known that WWV operates on a frequency of 10 megahertz. What is the wavelength of WWV?
    30 meters.
  7. A station is known to operate at 60-meters. What is the frequency of the unknown station?
    5 megahertz.
  8. If a transmitting antenna is placed close to the ground, how should the antenna be polarized to give the greatest signal strength?
    Vertically polarized.
  9. In the right-hand rule for propagation, the thumb points in the direction of the E field and theforefinger points in the direction of the H field. In what direction does the middle finger point?
    Direction of wave propagation.
  10. What is one of the major reasons for the fading of radio waves which have been reflected from a surface?
    Shifting in the phase relationships of the wave.
  11. What are the three layers of the atmosphere?
    Troposphere, stratosphere, and ionosphere.
  12. Which layer of the atmosphere has relatively little effect on radio waves?
  13. What is the determining factor in classifying whether a radio wave is a ground wave or a space wave?
    Whether the component of the wave is travelling along the surface or over the surface of the earth.
  14. What is the best type of surface or terrain to use for radio wave transmission?
    Radio horizon is about 1/3 farther.
  15. What is the primary difference between the radio horizon and the natural horizon?
    Sea water.
  16. What three factors must be considered in the transmission of a surface wave to reduce
    (a) electrical properties of the terrain (b) frequency (c) polarization of the antenna
  17. What causes ionization to occur in the ionosphere?
    High energy ultraviolet light waves from the sun.
  18. How are the four distinct layers of the ionosphere designated?
    D, E, F1, and F2 layers.
  19. What is the height of the individual layers of the ionosphere?
    D layer is 30-55 miles, E layer 55-90 miles, and F layers are 90-240 miles.
  20. What factor determines whether a radio wave is reflected or refracted by the ionosphere?
    Thickness of ionized layer.
  21. There is a maximum frequency at which vertically transmitted radio waves can be refracted back
    to Earth. What is this maximum frequency called?
    Critical frequency.
  22. What three main factors determine the amount of refraction in the ionosphere?
    (a) density of ionization of the layer (b) frequency (c) angle at which it enters the layer
  23. What is the skip zone of a radio wave?
    A zone of silence between the ground wave and sky wave where there is no reception.
  24. Where does the greatest amount of ionospheric absorption occur in the ionosphere?
    Where ionization density is greatest.
  25. What is meant by the term "multipath"?
    A term used to describe the multiple pattern a radio wave may follow.
  26. When a wide band of frequencies is transmitted simultaneously, each frequency will vary in the amount of fading. What is this variable fading called?
    Selective fading.
  27. What are the two main sources of emi with which radio waves must compete?
    Natural and man-made interference.
  28. Thunderstorms, snowstorms, cosmic sources, the sun, etc., are a few examples of emi sources.
    What type of emi comes from these sources?
  29. Motors, switches, voltage regulators, generators, etc., are a few examples of emi sources. What
    type of emi comes from these sources?
  30. What are three ways of controlling the amount of transmitter-generated emi?
    • (a) filtering and shielding of the transmitter (b) limiting bandwidth (c) cutting the antenna to the
    • correct frequency
  31. What are three ways of controlling radiated emi during transmission?
    • (a) physical separation of the antenna (b) limiting bandwidth of the antenna (c) use of directional
    • antennas
  32. What are the two general types of variations in the ionosphere?
    Regular and irregular variations.
  33. What is the main difference between these two types of variations?
    Regular variations can be predicted but irregular variations are unpredictable.
  34. What are the four main classes of regular variation which affect the extent of ionization in the
    Daily, seasonal, 11-year, and 27-days variation.
  35. What are the three more common types of irregular variations in the ionosphere?
    Sporadic E, sudden disturbances, and ionospheric storms.
  36. What do the letters muf, luf, and fot stand for?
    Muf is maximum usable frequency. Luf is lowest usable frequency. Fot is commonly known as optimum working frequency.
  37. When is muf at its highest and why?
    Muf is highest around noon. Ultraviolet light waves from the sun are most intense.
  38. What happens to the radio wave if the luf is too low?
    When luf is too low it is absorbed and is too weak for reception.
  39. What are some disadvantages of operating transmitters at or near the luf?
    Signal-to-noise ratio is low and the probability of multipath propagation is greater.
  40. What are some disadvantages of operating a transmitter at or near the muf?
    Frequent signal fading and dropouts.
  41. What is fot?
    Fot is the most practical operating frequency that can be relied on to avoid problems of multipath,absorbtion, and noise.
  42. How do raindrops affect radio waves?
    They can cause attenuation by scattering.
  43. How does fog affect radio waves at frequencies above 2 gigahertz?
    It can cause attenuation by absorbtion.
  44. How is the term "temperature inversion" used when referring to radio waves?
    It is a condition where layers of warm air are formed above layers of cool air.
  45. How does temperature inversion affect radio transmission?
    It can cause vhf and uhf transmission to be propagated far beyond normal line-of-sight distances.
  46. In what layer of the atmosphere does virtually all weather phenomena occur?
  47. Which radio frequency bands use the tropospheric scattering principle for propagation of radio waves?
    Vhf and above.
  48. Where is the tropospheric region that contributes most strongly to tropospheric scatter propagation?
    Near the mid-point between the transmitting and receiving antennas, just above the radio horizon.
  49. What are the two basic classifications of antennas?
    Half-wave (Hertz) and quarter-wave (Marconi).
  50. What are the three parts of a complete antenna system?
    Coupling device, feeder, and antenna.
  51. What three factors determine the type, size, and shape of an antenna?
    • Frequency of operation of the transmitter, amount of power to be radiated, and general direction
    • of the receiving set.
  52. If a wave travels exactly the length of an antenna from one end to the other and back during the period of 1 cycle, what is the length of the antenna?
    One-half the wavelength.
  53. What is the term used to identify the points of high current and high voltage on an antenna?
    Current and voltage loops.
  54. What is the term used to identify the points of minimum current and minimum voltage on an
    Current and voltage nodes.
  55. The various properties of a transmitting antenna can apply equally to the same antenna when it is used as a receiving antenna. What term is used for this property?
    Reciprocity of antennas.
  56. The direction of what field is used to designate the polarization of a wave?
    Electric (E) field.
  57. If a wave's electric lines of force rotate through 360 degrees with every cycle of rf energy, what is the polarization of this wave?
    Circular polarization.
  58. What type of polarization should be used at medium and low frequencies?
    Vertical polarization.
  59. What is an advantage of using horizontal polarization at high frequencies?
    Less interference is experienced by man-made noise sources.
  60. What type of polarization should be used if an antenna is mounted on a moving vehicle at frequencies below 50 megahertz?
    Vertical polarization.
  61. What is the radiation resistance of a half-wave antenna in free space?
    73 ohms.
  62. A radiating source that radiates energy stronger in one direction than another is known as what type of radiator?
    Anisotropic radiator.
  63. A radiating source that radiates energy equally in all directions is known as what type of radiator?
    Isotropic radiator.
  64. A flashlight is an example of what type of radiator?
    Anisotropic radiator.
  65. What terms are often used to describe basic half-wave antennas?
    Dipole, doublet and Hertz.
  66. If a basic half-wave antenna is mounted vertically, what type of radiation pattern will be produced?
  67. In which plane will the half-wave antenna be operating if it is mounted horizontally?
    Vertical plane.
  68. Since the radiation pattern of a dipole is similar to that of a doublet, what will happen to the
    pattern if the length of the doublet is increased?
    The pattern would flatten.
  69. What is the simplest method of feeding power to the half-wave antenna?
    To connect one end through a capacitor to the final output stage of the transmitter.
  70. What is the radiation pattern of a quarter-wave antenna?
    A circular radiation pattern in the horizontal plane, or same as a half wave.
  71. Describe the physical arrangement of a ground screen.
  72. It is composed of a series of conductors arranged in a radial pattern and buried 1 to 2 feet below
    the ground.
  73. What is the difference in the amount of impedance between a three-wire dipole and a simple center-fed dipole?
    Nine times the feed-point impedance.
  74. Which has a wider frequency range, a simple dipole or a folded dipole?
    Folded dipole.
  75. What is the purpose of antenna stubs?
    To produce desired phase relationship between connected elements.
  76. What is the primary difference between the major and minor lobes of a radiation pattern?
    Major lobes have the greatest amount of radiation.
  77. What is the maximum number of elements ordinarily used in a collinear array?
  78. Why is the number of elements used in a collinear array limited?
    As more elements are added, an unbalanced condition in the system occurs which impairs efficiency.
  79. How can the frequency range of a collinear array be increased?
    By increasing the lengths of the elements of the array.
  80. How is directivity of a collinear array affected when the number of elements is increased?
    Directivity increases.
  81. What is the primary cause of broadside arrays losing efficiency when not operating at their
    designed frequency?
    Lower radiation resistance.
  82. When more than two elements are used in a broadside array, how are the elements arranged?
    Parallel and in the same plane.
  83. As the spacing between elements in a broadside array increases, what is the effect on the major lobes?
    They sharpen.
  84. What are some disadvantages of the end-fire array?
    • Extremely low radiation resistance, confined to one frequency, and affected by atmospheric
    • conditions.
  85. Where does the major lobe in the end-fire array occur?
    Along the major axis
  86. To maintain the required balance of phase relationships and critical feeding, how must the
    end-fire array be constructed?
  87. What two factors determine the directivity pattern of the parasitic array?
    Length of the parasitic element (tuning) and spacing between the parasitic and driven elements.
  88. What two main advantages of a parasitic array can be obtained by combining a reflector and a director with the driven element?
    Increased gain and directivity.
  89. The parasitic array can be rotated to receive or transmit in different directions. What is the name given to such an antenna?
    Rotary array.
  90. What are the disadvantages of the parasitic array?
    Their adjustment is critical and they do not operate over a wide frequency range.
  91. What is the advantage of adding parasitic elements to a Yagi array?
    Increased gain.
  92. The Yagi antenna is an example of what type of array?
    Multielement parasitic array.
  93. To radiate power efficiently, a long-wire antenna must have what minimum overall length?
    One-half wavelength.
  94. What is another name for the Beverage antenna?
    Wave antenna.
  95. What is the polarity of the currents that feed the V antenna?
  96. What is the main disadvantage of the rhombic antenna?
    It requires a large antenna site.
  97. What is the primary reason for the development of the turnstile antenna?
    For omnidirectional vhf communications.