Question
Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?
Answer Options
- A) The F region absorbs signals at these frequencies during daylight hours
- B) The F region is unstable during daylight hours
- C) The D region absorbs signals at these frequencies during daylight hours
- D) The E region is unstable during daylight hours
Correct Answer: C
Explanation
The lowest layer of the ionosphere is the D region, which forms only during daylight hours when solar radiation is present. The D region, being composed of relatively thick and dense air, absorbs radio waves, rather than refracting them. This absorption is most pronounced on the lower HF bands (40m to 160m).
During the day, the D region absorbs signals at these frequencies, preventing them from reaching the higher, refracting F layers. This phenomenon raises the Lowest Usable Frequency (LUF), making long-distance communication on the lowest HF bands very difficult or impossible until sunset when the D region quickly dissipates.
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