What happens to HF propagation when the LUF exceeds the MUF?
Propagation via ordinary skywave communications is not possible over that path✓Correct answer
HF communications over the path are enhanced
Double-hop propagation along the path is more common
Propagation over the path on all HF frequencies is enhanced
Explanation
The usable window is the range of frequencies between the LUF and the MUF, so when absorption pushes the LUF above the MUF that window closes and no frequency is both low enough to be returned and high enough to survive absorption. Ordinary skywave communication over that path is then impossible.