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G3C11

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

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Which ionospheric region is the most absorbent of signals below 10 MHz during daylight hours?

  1. AThe F2 region
  2. BThe F1 region
  3. CThe E region
  4. DThe D region
Why The D region is the lowest ionized layer, about 60 to 90 km up, where air density and therefore electron-neutral collision rates are highest. Electrons set in motion by a passing wave collide with neutral molecules and lose that energy as heat instead of re-radiating it, so the wave is attenuated rather than refracted. This absorption rises sharply as frequency falls (roughly as 1/f squared), which is why 160, 80 and 40 meters are short-range in daylight but open up at night when the D region quickly disappears.
Watch out The E and F regions are the ones that actually refract HF signals back to Earth; the F2 region is the main daytime long-haul reflector, not an absorber.
D is for Daytime Deadener: lowest region, kills the low bands until sunset.
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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