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Skywave, hops and the skip zone
How HF signals return to Earth hundreds or thousands of miles away.
How HF signals return to Earth hundreds or thousands of miles away.
Skywave is a signal that goes up, is bent back by the ionosphere, and returns to Earth far away. One trip up and down is a hop. The empty stretch between where your ground wave dies out and where the first hop lands is the skip zone. Skywave is how a few watts on HF can reach another continent.
Two things set how far the first hop lands:
The distance in the diagram comes from simple geometry: the signal is treated as reflecting from a mirror at a typical layer height over a round Earth. Real layers bend the wave gradually instead of reflecting it, so the numbers are indicative. The highest frequencies that return at all are covered in MUF, LUF and the best frequency.
When you are too close, the ground wave has already faded and the skywave is still passing overhead. That gap is silent, not because the signal is switched off but because the energy is somewhere else. Raise the frequency and the first-hop landing moves farther out, so the skip zone widens. Lower the frequency and it closes up; Near-vertical incidence skywave exploits this: on a low enough HF frequency, a signal sent almost straight up still returns and covers the region around you.
The zone is rarely perfectly silent. Small amounts of energy scatter into it by irregular paths, producing weak, fluttery signals.
To go farther than one hop, the signal returns to the ground, reflects, and goes up again. Each hop costs energy: the ground reflection (better over sea than dry land) and a second trip through the absorbing D layer, so multi-hop paths lose strength with each hop. Paths to the far side of the world take several hops, which is one reason they are usually weaker than shorter ones.