The ionosphere is one of the main reasons amateur radio signals can travel far beyond the horizon. High above the Earth, sunlight ionises thin air and creates electrically active layers that can bend, absorb, or return HF radio signals back towards the ground.
Around 50 miles above the Earth and higher, the air becomes extremely thin. Radiation from the Sun can strip electrons away from oxygen and other atmospheric molecules. This process is called ionisation. The separated electrons and ionised molecules form electrically active regions that affect radio waves.
Ionisation is not constant. It changes with the time of day, the season, solar activity, and the roughly 11-year sunspot cycle. It can also change from hour to hour, which is why HF bands sometimes open suddenly and close just as quickly.
The ionosphere is normally described as three main regions: the D layer, the E layer, and the F layer. Each layer has a different effect on HF radio signals.
The D layer is the lowest of the main ionospheric layers. Because the air is denser here, free electrons recombine more easily, so the D layer needs constant sunlight to stay ionised.
The E layer sits above the D layer and is well known for short skip, especially on the higher HF bands and sometimes into VHF. Sporadic-E can appear quickly and produce very strong signals.
The F layer is the highest and most important region for long-distance HF communication. During the day it can split into F1 and F2. At night these regions usually merge into one F layer.
This simplified diagram shows the idea: D is the lower absorbing layer, E can create shorter skip, and F is usually the main long-distance DX layer. The real ionosphere is not a hard mirror; it is a changing region of plasma that bends radio waves gradually.
During the day, the D layer absorbs lower-frequency signals. After sunset, the D layer fades and lower bands can reach the higher layers. This is why AM broadcast stations, 160m, 80m and 40m can often travel much farther at night.
Lower frequencies suffer most from D-layer absorption in daylight. Higher HF signals are more able to pass through to the E and F layers.
Light travels in straight lines, so objects beyond the horizon cannot normally be seen. Radio waves also travel in straight lines through free space, but HF radio waves can be bent by the ionosphere and returned to Earth. This is called skywave propagation, and the distance covered by one return to Earth is often called skip.
A single hop travels from the transmitter to the ionosphere and back to Earth. A double hop returns to the ground, reflects upward again, and then comes back down farther away. Multiple hops can carry HF signals across continents and oceans.
Sunspots are linked with stronger solar ultraviolet and X-ray output, which increases ionisation in the ionosphere. In high sunspot years, the higher HF bands can support worldwide contacts with strong signals. In low sunspot years, the same bands may be quiet or completely closed for long periods.
10m, 12m, 15m and 20m can open for long-distance DX, often with low noise and strong signals.
10m and 15m may be closed on many days, while 20m may close earlier in the evening.
160m, 80m and 40m often become more useful for DX when daylight D-layer absorption disappears.
During a strong solar flare, the D layer can become heavily ionised. Instead of passing through to the higher layers, HF signals may be absorbed. This can cause a sudden shortwave fadeout or radio blackout, especially on sunlit paths.
