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howto:hambasics:propagation [2019/11/24 19:13] – [Meteor Scattering] ve7hzfhowto:hambasics:sections:propagation [2021/01/03 08:06] (current) va7fi
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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 }}
  
 ====== 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))
  
-For example, VE7HZF's antenna is 20m above the ground, at an elevation of 100m overlooking the water.  It means that his antenna can see about 45 km in that direction.+For example, VA7FI's antenna is 20m above the ground, at an elevation of 100m overlooking the water.  It means that his antenna can see about 45 km in that direction.
  
 For direct waves to occur, the height of the antenna needs to be many times greater than the wavelength of the radio wave so that the signal doesn't interact with the ground.  In this example, the antenna (120m above sea level) is 60 wavelengths high on the 2 meter band, and 170 wavelengths on the 70 cm band. For direct waves to occur, the height of the antenna needs to be many times greater than the wavelength of the radio wave so that the signal doesn't interact with the ground.  In this example, the antenna (120m above sea level) is 60 wavelengths high on the 2 meter band, and 170 wavelengths on the 70 cm band.
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 +===== 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 (50MHz)+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).
  
  
 ===== Auroral backscatter ===== ===== Auroral backscatter =====
-Auroral activity creates strong ionization of the E-region.  Radio HF (and sometimes VHF) radio waves can backscatter and be heard up to 2000 km in the east-west direction.  CW is the best mode to use to take advantage of this mode.+Auroral activity creates strong ionization of the E-region.  HF (and sometimes VHF) radio waves can backscatter and be heard up to 2000 km in the east-west direction.  CW is the best mode to use to take advantage of this mode.
  
  
 ===== Sporadic-E propagation ===== ===== Sporadic-E propagation =====
-Sporadic-E propagation (not to be confused with ordinary E-layer propagation) takes advantage of ionization patches in the E layer that drift westwards at speeds of a few hundreds of kilometres per hour.  You can think of it as invisible clouds of ionized gases that move in the layers. If your signal is lucky enough to enter one of these clouds, it can bounce between 1000 and 2000 km in a single hop.  Sporadic-E is most often observed on the 6m band.+Sporadic-E propagation (not to be confused with ordinary E-layer propagation) takes advantage of ionization patches in the E-layer that drift westwards at speeds of a few hundreds of kilometres per hour.  You can think of it as invisible clouds of ionized gases that move in the E-layer. If your signal is lucky enough to enter one of these clouds, it can bounce between 1000 and 2000 km in a single hop.  Sporadic-E is most often observed on the 6m band.
  
  
 ====== Troposphere ====== ====== Troposphere ======
  
-On the previous page, we discussed how the Ionosphere (the region of our atmosphere between 50km and 400km altitude) can, reflect and refract radio waves, let them pass straight through, or absorbed them completely mostly due to the sun's ionization of the gas in these layers.+In the previous section, we discussed how the Ionosphere (the region of our atmosphere between 50km and 400km altitude) can, reflect and refract radio waves, let them pass straight through, or absorbed them completely mostly due to the sun's ionization of the gas in these layers.
  
-Here we discuss how the [[wp>Troposphere |troposphere]] (the lowest region of our atmosphere below 20km altitude) can also affect radio waves.+Here we discuss how the [[wp>Troposphere |troposphere]] (the lowest region of our atmosphere below 20km altitude) can also affect radio waves because of variation in temperature, pressure, or water vapour content.  Normal VHF tropospheric propagation can have a range of roughly 800 km.
  
 +===== Tropospheric Ducting =====
  
 +The index of refraction of air is lower when the air is warmer.  So during a temperature inversion, the air on the ground is colder than the air above, which means that radio waves go from a high to a low index of refraction medium (that is, from a slow to a fast medium).  This causes the radio wave to refract back down toward the Earth.  Tropospheric ducting is when the radio wave follows the curve of that inversion layer until it exits back to the Earth after travelling several hundreds of kilometres (up to 2000 kilometers).
  
 +Unlike Ionospheric refraction, Tropospheric ducting is observed at VHF frequencies as opposed to HF frequencies.
  
  
 +===== Scattering =====
  
 +At VHF frequencies, small variations in the density of the troposphere (around 10 km) can scatter some of the radio waves back toward the ground to distances of 800 km.
  
-  * Tropospheric bending on 2m +Scattering can also allow HF signals from the skipzone to be heard.  Scatter is most likely involved when weak or distorted signals near or above the maximum usable frequency (ie, they should escape into space) are heard over unusual paths.
-  * Ducting +
-  * Sporatic E +
-  * Auroral propagation +
-  * Scatter +
-  * Meteor Scatter+
  
-See [[wp>Radio_propagation]] 
  
 +===== References =====
 +  * [[https://www.voacap.com/]]
 +  * [[http://prop.hfradio.org/]]
  
 ====== Questions ====== ====== Questions ======
   * B-007-001-001 -> B-007-004-002   * B-007-001-001 -> B-007-004-002
-  * B-007-005-001 -> B-007-007-001+  * B-007-005-001 -> B-007-008-001 
 +  * B-007-008-006 -> B-007-008-011
  
  
-[[sections |{{/back.png }}]] [[waveinterference |{{  /next.png}}]]+[[waveinteraction |{{/back.png }}]] [[stationassembly |{{  /next.png}}]]
  
howto/hambasics/sections/propagation.1574651626.txt.gz · Last modified: 2019/11/24 19:13 by ve7hzf