The ionosphere's F-layer is modelled as a flat reflecting mirror at adjustable height. A wave launched at elevation angle θ returns to the ground after a single-hop skip distance:
skip = 2 × F-layer height / tan(θ)
Whether that hop is possible depends on the F-layer's critical frequency (fo, roughly 9 MHz × density) and the frequency in use. The Maximum Usable Frequency is:
MUF = fo / sin(θ)
Signals above the MUF punch straight through the ionosphere instead of reflecting — this is why VHF and above rely on line-of-sight rather than sky-wave skip.