Turn on an AM radio at night and stations from hundreds of kilometres away come in that were silent at noon. That is skywave: radio waves bent back to Earth by the ionosphere, the electrically charged layer of the upper atmosphere.

Ground wave, sky wave

A radio signal leaves the antenna in two ways. The ground wave hugs the Earth’s surface and fades within tens or a few hundred kilometres. The sky wave heads upward. If the ionosphere is charged enough for that frequency, it bends the signal back down, far beyond the horizon. The signal can bounce off the ground and up again, making several “hops” around the planet.

The layers

The ionosphere is charged by sunlight, so it changes through the day. Radio listeners talk about three regions:

  • D layer (about 60–90 km up): exists only in daylight. It doesn’t reflect signals; it absorbs them, especially lower frequencies. That is why medium-wave AM is mostly local by day.
  • E layer (about 90–150 km): reflects some signals by day and weakens at night.
  • F layer (about 150–500 km): the main reflector for shortwave. It stays charged after dark, splitting into F1 and F2 in daytime.

After sunset the D layer disappears within an hour or so. The absorbing filter is gone, the F layer is still there, and lower frequencies travel enormous distances. Around sunrise the D layer returns and the effect switches off.

Which frequency, when

Each path has a maximum usable frequency: above it, the signal punches straight through the ionosphere into space. In daylight the ionosphere is strongly charged, so higher frequencies work. At night the ceiling drops.

  • Daytime: try the higher shortwave bands, 13 to 25 metres (roughly 11–22 MHz).
  • Night: try the lower bands, 31 to 90 metres (roughly 3–10 MHz), and medium-wave AM.
  • Dawn and dusk: along the line between day and night, signals can travel unusually far. That is grey line DX.

The band guide lists every broadcast band. WWV’s operators give the same rule of thumb: its frequencies above 10 MHz work best by day, the lower ones by night.

The sun’s bigger cycle

The sun’s activity rises and falls over roughly 11 years. Near the peak, the ionosphere is more strongly charged and the higher bands open up more often. Solar flares can do the opposite for a while: a burst of X-rays strengthens the D layer and can black out shortwave on the daylit side of the Earth for minutes to hours.

The name Skywave DX comes from this: skywave for the bounce, DX for distance, the old radio shorthand for hearing far-away stations.