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howto:hambasics:sections:propagation [2020/10/07 08:08] – ↷ Links adapted because of a move operation va7fihowto:hambasics:sections:propagation [2021/01/03 08:06] (current) va7fi
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-|  [[..:home|Ham Basics]]  |  [[..:test|About The Test]]  |  [[..:reference|References]]  ^  [[..:sections|Study Sections]]  | 
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 ====== Propagation ====== ====== Propagation ======
 Radio wave propagation describes the way in which radio waves travel from one point to another.  As we saw in the previous page, radio waves (like light waves) have polarization and are affected by the phenomena of reflection, refraction, diffraction, and scattering.  As we'll see next, these give rise to different ways that the signal can propagate through the atmosphere.  Radio wave propagation describes the way in which radio waves travel from one point to another.  As we saw in the previous page, radio waves (like light waves) have polarization and are affected by the phenomena of reflection, refraction, diffraction, and scattering.  As we'll see next, these give rise to different ways that the signal can propagate through the atmosphere. 
  
 {{ youtube>aPp4X-l0EYU }} {{ youtube>aPp4X-l0EYU }}
 +
 ====== Direct Waves (Line Of Sight) ====== ====== Direct Waves (Line Of Sight) ======
  
-{{ ..:lineofsight.png?400|}}+{{ lineofsight.png?400|}}
 VHF radio waves (above 50 MHz) travel more or less in a straight line, and so cannot go much beyond the horizon.  To increase the distance that an antenna can "see", we raise our antennas as high as possible.  The //radio horizon// is given roughly by: \$d = 4.12 \sqrt{h} \$ where \$d\$ is in kilometre and \$h\$ is in meters.((See [[wp>Line-of-sight_propagation#Geometric_distance_to_horizon |Wikipedia: Line Of Sight Propagation]] for more details)) VHF radio waves (above 50 MHz) travel more or less in a straight line, and so cannot go much beyond the horizon.  To increase the distance that an antenna can "see", we raise our antennas as high as possible.  The //radio horizon// is given roughly by: \$d = 4.12 \sqrt{h} \$ where \$d\$ is in kilometre and \$h\$ is in meters.((See [[wp>Line-of-sight_propagation#Geometric_distance_to_horizon |Wikipedia: Line Of Sight Propagation]] for more details))
  
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 ====== Ground Waves ====== ====== Ground Waves ======
  
-{{ ..:groundwave.png?400|}}+{{ groundwave.png?400|}}
 Ground waves occur when the signal curves with the Earth until it becomes too weak to be detected.  This phenomena happens because of diffraction for vertically polarized radio waves when the frequency is below 3 MHz.  The radio wave interacts with the ground where it loses some of its energy but also curves toward it.  Depending on the frequency, these waves can go beyond the horizon out to about 200 km. Ground waves occur when the signal curves with the Earth until it becomes too weak to be detected.  This phenomena happens because of diffraction for vertically polarized radio waves when the frequency is below 3 MHz.  The radio wave interacts with the ground where it loses some of its energy but also curves toward it.  Depending on the frequency, these waves can go beyond the horizon out to about 200 km.
  
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 ====== Skywaves ====== ====== Skywaves ======
  
-{{ ..:skywave.png?400|}}+{{ skywave.png?400|}}
 Depending on the frequency and atmospheric conditions, it's possible for radio waves going up to reflect back down to Earth.  From our location in British Columbia, we can very easily talk to people in Japan using Skywaves. Depending on the frequency and atmospheric conditions, it's possible for radio waves going up to reflect back down to Earth.  From our location in British Columbia, we can very easily talk to people in Japan using Skywaves.
  
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 ====== Ionosphere ====== ====== Ionosphere ======
  
-{{ ..:ionosphere_layers.png?400}}+{{ ionosphere_layers.png?400}}
 The region of our atmosphere between 50km and 400km altitude is called the ionosphere((Picture from [[wp>Ionosphere#The_ionospheric_layers |Wikipedia: Ionosphere]])), and to radio waves, it can act like: The region of our atmosphere between 50km and 400km altitude is called the ionosphere((Picture from [[wp>Ionosphere#The_ionospheric_layers |Wikipedia: Ionosphere]])), and to radio waves, it can act like:
   * a mirror that refracts and reflects a signal back to earth,   * a mirror that refracts and reflects a signal back to earth,
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   * The ionosphere layer is as high as possible.   * The ionosphere layer is as high as possible.
  
-{{  ..:takeoff.gif  }}+{{  takeoff.gif  }}
  
 The above animation is a gross oversimplification to illustrate the point that, all else being equal, signals sent near the horizon using the F layer will go further.  For example: The above animation is a gross oversimplification to illustrate the point that, all else being equal, signals sent near the horizon using the F layer will go further.  For example:
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 In reality, the ionosphere is a medium with a continuously varying index of refraction rather than a series of discrete "mirrors" As such, how much signals "curve" also depends on the takeoff angle, and just like what we saw in the previous [[waveinteraction#total_internal_reflection |section]], there's a critical angle that must be met for //Total Internal Reflection// to occur.  So the real picture is more like this one:((Picture is from Fig. 2-14 at [[https://www.globalsecurity.org/intell/library/policy/army/fm/24-18/fm24-18_3.htm]])) In reality, the ionosphere is a medium with a continuously varying index of refraction rather than a series of discrete "mirrors" As such, how much signals "curve" also depends on the takeoff angle, and just like what we saw in the previous [[waveinteraction#total_internal_reflection |section]], there's a critical angle that must be met for //Total Internal Reflection// to occur.  So the real picture is more like this one:((Picture is from Fig. 2-14 at [[https://www.globalsecurity.org/intell/library/policy/army/fm/24-18/fm24-18_3.htm]]))
    
-{{  ..:24180016.gif?800  }}+{{  24180016.gif?800  }}
  
 Note the following important terms on the above image: Note the following important terms on the above image:
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 And finally, real antennas do NOT transmit their signal at a single take off angle but over a range of them, which can vary depending on the antenna type and how high it is over the ground.  So in reality, many of these paths are used at the same time and even reflect off the ground and go back for a second or third hop.  Communications exceeding 5000 km uses //multihop// propagation, which looks like this:((Picture from [[http://www.ferzkopp.net/Personal/Thesis/node8.html]])) And finally, real antennas do NOT transmit their signal at a single take off angle but over a range of them, which can vary depending on the antenna type and how high it is over the ground.  So in reality, many of these paths are used at the same time and even reflect off the ground and go back for a second or third hop.  Communications exceeding 5000 km uses //multihop// propagation, which looks like this:((Picture from [[http://www.ferzkopp.net/Personal/Thesis/node8.html]]))
-{{  ..:multiphop.png  }}+{{  multiphop.png  }}
 ===== Frequency ===== ===== Frequency =====
  
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   * The F-Layer splits into two layers about half an hour before sunrise and recombines into one layer about half an hour after sunset.  It refracts higher frequency HF bands (40m, 20m, 10m), but VHF frequencies (above 50 MHz) go straight through it, acting either like a mirror or a clear window.   * The F-Layer splits into two layers about half an hour before sunrise and recombines into one layer about half an hour after sunset.  It refracts higher frequency HF bands (40m, 20m, 10m), but VHF frequencies (above 50 MHz) go straight through it, acting either like a mirror or a clear window.
  
-{{  ..:freq.gif  }}+{{  freq.gif  }}
  
  
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-{{  ..:freq2.gif  }}+{{  freq2.gif  }}
  
  
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 Finally, because the D-Layer disappears before the F-Layer recombines, and reappears after the F-Layer splits, the propagation can be interesting around sunrise and sunset.  This is called the //gray zone//. Finally, because the D-Layer disappears before the F-Layer recombines, and reappears after the F-Layer splits, the propagation can be interesting around sunrise and sunset.  This is called the //gray zone//.
  
-{{  ..:light.gif  }}+{{  light.gif  }} 
  
 +===== Animation =====
 +Here's the animation if you want to move the dials and experiment with it yourself.
  
 +{{ggb>/howto/hambasics/sections/earthpropagation.ggb 800,450}}
 ===== Meteor Scattering ===== ===== Meteor Scattering =====
 When meteors enter the ionosphere, they create intensely ionized columns of air that can scatter radio waves for very short periods of time (from a fraction of a second to a couple seconds per event).  This mode can be used on VHF frequencies between 30 MHz and 100 MHz but is most effective on the 6m band (50 MHz). When meteors enter the ionosphere, they create intensely ionized columns of air that can scatter radio waves for very short periods of time (from a fraction of a second to a couple seconds per event).  This mode can be used on VHF frequencies between 30 MHz and 100 MHz but is most effective on the 6m band (50 MHz).
howto/hambasics/sections/propagation.1602083330.txt.gz · Last modified: 2020/10/07 08:08 by va7fi