Understanding HF communications and what causes ‘fading’

An airliner is communicating with a station using HF radio. Gradually, communication quality declines until it has all but faded out. Why does this happen, and what actions are taken to restore communications?

Firstly, high frequency is used for communicating over long distances, from a ground station to an aircraft, or from an aircraft to a ground station, over the horizon, but not air to air. In the aviation industry, it is a requirement for trans-oceanic flights to be HF equipped due to the distances covered.

The HF or Decameter Band (10m to 100m wavelength), is defined as ranging from 3MHz to 30MHz. However, HF transmissions are between 2,850MHz – 24,890 MHz, taking note that 2,850MHz is less than 3Mhz, because this is the lowest single side band transmitted. In other words, HF communication uses a single-side band with a suppressed carrier.

How does HF work for aviation?
In aviation, HF (skip or skywave) propagation is horizontally polarised. The aircraft will either have a horizontal antenna built into the fuselage, or a visible horizontal antenna wire on the outside. In horizontal polarisation, the electric field is in the horizontal plane, and the magnetic field is in the vertical plane, providing lesser ground absorption.

With HF communications the frequencies are refracted (and eventually reflected down to earth) by the Ionosphere’s free electrons, allowing communications over great distances. The higher the density of the free electrons, the higher the frequencies that can be refracted.

Looking at the Ionosphere, during the day there may be 4 regions or layers present above ground level:
• D region 50 to 90 km;
• E region 90 to 140 km;
• F1 region 140 to 210 km;
• F2 region over 210 km.
F1 and F2 layer can merge in a single F layer during certain times of the Solar Cycle.

At night, the D, E, F1 regions become depleted of free electrons leaving only the F2 layer available for HF communications.
The D layer is not used for HF communications because it absorbs or attenuates rather that refracts the frequency.
The F2 region is best for HF communications because:
• It is present during day and night;
• It refracts the highest frequencies in the HF band;
• Its high altitude provides the greatest communication paths (hop distance);

The Ionosphere varies with the solar cycles, seasons, day and night, and therefore a single frequency will not provide optimal communications:
The solar cycle – at solar minimum and maximum, the lower and higher HF band frequencies respectively can only be used. Large solar flares occur more often at solar maximum, causing the D layer to be ionised, resulting in the absorption of the lower frequencies in the daytime – called a short wave fade-out. This can last several minutes to hours. If it is suspected that this is the cause of fading communications, then try using a higher frequency.

The seasons – during Winter, a lower frequency in the HF band should be used, and in Summer a higher frequency in the band can be used.

The Equinoxes (March and September) – frequencies can be higher.

Solar maximum – in Winter compared with solar maximum in Summer, sometimes a higher frequencies can be used – this is an anomaly, called the seasonal anomaly.
Night and day – the lower and higher HF band frequencies respectively can be used.
Latitudes – the higher the latitude, the lower the solar radiation (oblique rays), therefore only the lower frequencies can be used.

Of course, there are practical limits to the high and low frequencies in that:
Too high a frequency will pass straight through the Ionosphere and not be refracted, and hence not reflected back down to earth.
Too low a frequency, much of the energy will be absorbed by the D layer and it can also be screened by the E layer.

Again, it goes to show, the more solar radiation present, the higher density of free electrons, the higher the refractive index and thus the higher frequency that can be used.