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G3C05

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

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Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?

  1. AThe F region absorbs signals at these frequencies during daylight hours
  2. BThe F region is unstable during daylight hours
  3. CThe D region absorbs signals at these frequencies during daylight hours
  4. DThe E region is unstable during daylight hours
Why The D region is the lowest ionospheric layer (about 50-90 km) and it only exists while the sun is up, because it requires constant solar ionization and its dense air lets ions recombine within minutes after sunset. Rather than refracting signals back to Earth, the D region absorbs them, and that absorption gets much stronger as frequency goes down (roughly proportional to 1/f squared). So the low bands, 160, 80, 60 and 40 meters, lose their energy on the way up during the day, and only come alive for DX after dark when the D region disappears.
Watch out The F region is what actually refracts HF signals back to Earth for long-haul work; it does not absorb the low bands, and daytime is when it is strongest, splitting into F1 and F2.
D is for Daytime and Death of the low bands. Lower frequency, more D-region absorption.
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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