Sky Wave Propagation

Ionosphere is the name we use to describe gasses in our atmosphere which can either refract (bend) or absorb RF energy depending on how excited these gasses are by radiation from the Sun, particularly ultraviolet and X-rays . The excitation is known as ionisation.

Ionisation changes with altitude, time of day, season and solar activity. Different regions of the ionosphere therefore affect radio signals in different ways, and the same band can behave very differently at different times, and different bands can behave differently at the same time of day.

For the radio operator, the important point is that these changes determine where a signal can go. By choosing an appropriate frequency, time of day and operating direction, it is possible to use the ionosphere to achieve anything from reliable regional coverage to intercontinental communication. 

Layers & Heights above Earths surface

D Layer: 60 to 90 km
E Layer: 105 to 160 km
F Layer: 160 to 180 km 

D Layer 60 to 90 km

Activity during the day:
The D layer forms and becomes strongly ionised when the Sun is illuminating the atmosphere. It is strongest around local midday and generally becomes weaker as the Sun gets lower.

Activity at night:
With the loss of solar radiation, ionisation falls rapidly and the D layer largely disappears. This greatly reduces its absorption of HF signals.

Relevant frequencies:
The D layer primarily affects the lower HF frequencies. Its absorption is particularly significant on 160, 80 and 40 metres, while its effect becomes progressively less important at higher frequencies.

Effect on propagation:
The D layer does not normally return HF signals to Earth. Instead, it primarily absorbs RF energy passing through it. This absorption is much greater at lower frequencies.

Practical significance:
During daylight, D-layer absorption can make the lower HF bands considerably less effective for long-distance communication. After sunset, the reduction in absorption is one of the main reasons 80 and 40 metres become much more effective for longer-distance communication.

E Layer 105 to 160 km

Activity during the day:
The E layer becomes ionised when exposed to solar radiation and is generally strongest during daylight hours. Its ionisation varies considerably with season, latitude and solar activity. Under normal conditions, the E layer is less persistent and less strongly ionised than the F layer, but it can still have a significant effect on HF propagation.

Activity at night:
The normal E layer weakens substantially after sunset as ionisation decreases. It does not usually provide the same persistent propagation mechanism at night that the F layer does.

Relevant frequencies:
The E layer can affect frequencies throughout the HF range, but its most significant practical effect occurs when sporadic E (Es) develops. Sporadic E can support propagation from roughly the lower HF bands through VHF, and is particularly important on frequencies from around 10 MHz upwards.

Effect on propagation:
Under normal conditions, the E layer can refract some HF signals back towards Earth, allowing contacts over distances typically greater than those achievable by ground-wave propagation but often shorter than typical F-layer DX paths.

The more significant phenomenon is sporadic E. This occurs when unusually dense patches of ionisation form within the E layer. These patches can strongly refract signals that would normally pass through the E layer, producing unusually strong propagation over distances of roughly 500–2,000 km. The exact distance depends on the height and position of the sporadic-E region and the angle at which the signal encounters it.

Practical significance:
The E layer is particularly important because it can produce propagation that is quite different from the normal behaviour expected from a particular band. Sporadic E can suddenly open a band that would otherwise be unsuitable for the desired distance, producing strong signals from stations that may normally be difficult or impossible to hear.

For the HF operator, sporadic E is especially significant on 10 metres, where it can produce strong regional and European openings even when normal F-layer propagation is poor. It can also affect 15 metres and, less frequently, 20 metres. Unlike ordinary F-layer propagation, sporadic-E openings are often relatively short-lived and can appear or disappear rapidly.

Sporadic E is particularly common during the late spring and summer months, making it an important additional propagation mechanism when operating on the higher HF bands.

F Layer 160 to 180 km

Activity during the day:
The F layer is the most important region of the ionosphere for long-distance HF communication. Solar radiation produces strong ionisation in this region, and during daylight the F region can support signals over very long distances. Its height and degree of ionisation change throughout the day, with ionisation generally increasing after sunrise and reaching higher levels during the daytime.

During daylight, the F region may separate into F1 and F2 layers, particularly during periods of higher solar activity. The F2 layer is the higher and generally more important of the two for long-distance HF propagation.

Activity at night:
After sunset, the F1 layer normally disappears as its ionisation decreases. The F2 layer, however, can remain ionised throughout the night and is the principal ionospheric mechanism supporting long-distance HF communication after dark.

The F2 layer also changes significantly overnight. Ionisation gradually decreases, but it can remain sufficient to support long-distance communication well into the night. This is one of the main reasons HF signals can continue to travel thousands of kilometres after the Sun has set.

Relevant frequencies:
The F region is relevant to virtually the entire HF spectrum, from the lower HF bands through to 10 metres when ionospheric conditions are favourable. The maximum frequency that can be returned towards Earth depends heavily on the level of ionisation, so higher-frequency bands such as 15, 12 and 10 metres are much more dependent on a sufficiently ionised F2 layer than 40 or 80 metres.

Effect on propagation:
Unlike the D layer, which primarily absorbs HF energy, the F region can refract HF signals back towards Earth. A signal entering the F region is progressively bent as it passes through the ionised plasma. If the frequency and ionisation conditions are suitable, the signal can be bent sufficiently to return to Earth many hundreds or thousands of kilometres from the transmitting station.

The height of the F region also produces a characteristic skip distance. Signals radiated at suitable angles can pass through the lower ionosphere, be refracted by the F region and return to Earth some distance away. A second reflection can then send the signal onwards again, allowing communication over very large distances through multiple hops.

Practical significance:
For the HF operator, the F2 layer is the principal mechanism for long-distance and intercontinental HF communication. It is responsible for much of the DX propagation experienced on 20, 17, 15, 12 and 10 metres, while also providing long-distance paths on the lower bands.

The higher the frequency, the greater the ionisation required for the F region to return the signal towards Earth. This is why 20 metres may remain usable when 10 metres is completely closed. Conversely, when solar activity produces strong F2 ionisation, the higher bands can open and provide exceptionally long-distance communication with relatively low power.

The F layer is heavily dependent on the state of the solar cycle. During periods of high solar activity, the F2 layer can support propagation at much higher frequencies, opening 15, 12 and 10 metres for worldwide communication. During periods of low solar activity, these bands may frequently close, while 20 metres and the lower HF bands remain considerably more reliable.