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G3C04

RADIO WAVE PROPAGATION · - Ionospheric regions; critical angle and frequency; HF scatter; near vertical incidence skywave (NVIS)

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What does the term "critical angle" mean, as applied to radio wave propagation?

  1. AThe long path azimuth of a distant station
  2. BThe short path azimuth of a distant station
  3. CThe lowest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
  4. DThe highest takeoff angle that will return a radio wave to Earth under specific ionospheric conditions
Why Takeoff angle is measured up from the horizon, and the steeper a wave hits the ionosphere, the harder it is to bend it back down. For a given frequency and level of ionization there is a maximum takeoff angle at which the wave still refracts back to Earth; go any steeper and it punches through into space. That limit is the critical angle, and it is the angular counterpart of the critical frequency, which is the highest frequency returned when the wave is sent straight up.
Watch out The choice naming the lowest takeoff angle inverts the idea: low, shallow angles are the easiest to return and give the longest skip distances, so there is no lower cutoff. Long path and short path azimuths are bearings in the horizontal plane, nothing to do with elevation.
Too steep and you leak through: the critical angle is the HIGHEST angle that still comes back.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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The rest of G3C

  1. G3C01 Which ionospheric region is closest to the surface of Earth?
  2. G3C02 What is meant by the term "critical frequency" at a given incidence angle?
  3. G3C03 Why is skip propagation via the F2 region longer than that via the other ionospheric regions?
  4. G3C04 What does the term "critical angle" mean, as applied to radio wave propagation?
  5. G3C05 Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?
  6. G3C06 What is a characteristic of HF scatter?
  7. G3C07 What makes HF scatter signals often sound distorted?
  8. G3C08 Why are HF scatter signals in the skip zone usually weak?
  9. G3C09 What type of propagation allows signals to be heard in the transmitting station's skip zone?
  10. G3C10 What is near vertical incidence skywave (NVIS) propagation?
  11. G3C11 Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?
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