Why is long-distance communication on the 40-, 60-, 80-, and 160-meter bands more difficult during the day?
The F region absorbs signals at these frequencies during daylight hours
The F region is unstable during daylight hours
The D region absorbs signals at these frequencies during daylight hours✓Correct answer
The E region is unstable during daylight hours
Explanation
The ionosphere's lowest layer is sustained by sunlight because rapid recombination clears its ionisation after sunset, and while present it absorbs rather than refracts HF signals. Because absorption rises sharply as frequency falls, the 40-, 60-, 80- and 160-meter bands lose much more energy during the day and recover after dark.