Question
What happens to HF propagation when the LUF exceeds the MUF?
Answer Options
- A) Propagation via ordinary skywave communications is not possible over that path
- B) HF communications over the path are enhanced
- C) Double-hop propagation along the path is more common
- D) Propagation over the path on all HF frequencies is enhanced
Correct Answer: A
Explanation
HF skywave propagation is only possible for frequencies that fall within the ‘window’ defined by the Maximum Usable Frequency (MUF) and the Lowest Usable Frequency (LUF). The MUF is the highest frequency that can be refracted, and the LUF is the lowest frequency that avoids excessive absorption.
If the LUF rises higher than the MUF, it means that every frequency that is high enough to be refracted back to Earth is also high enough to be completely absorbed by the D-region. In this situation, propagation via ordinary skywave communications is not possible over that path. This scenario often occurs during poor conditions or severe geomagnetic disturbances.
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