HF Ionospheric Radio Signal Propagation

- the basics of HF ionospheric radio propagation and how the ionosphere enables radio communications links to be established over large distances around the globe using what are termed sky waves or skywaves.

Radio propagation takes many forms at different frequencies. One area that aided with the development and usage of radio was the propagation conditions that exist on the HF radio bands.

Using the ionosphere, signals are refracted back to earth and are thus able to travel over very great distances.It was ionospheric propagation that enabled early long distance radio links to be formed and these helped establish the technology of radio as a medium via which global communications could be established.

The fact that the condition of the ionosphere varies with time means that ionospheric propagation is subject to major variations and is not always reliable. As a result, other means of communication are more widely used. Satellites provide another basis for establishing global communications, and nowadays there is a global telecommunications network that forms the basis for most communications.

HF skywaves

When using ionospheric propagation, signals leave the transmitting antenna and travel away from the antenna. Ultimately they will reach the ionosphere. Here they are reflected, or more correctly refracted back to earth being hear some distance away from the transmitter.

Obviously if the transmitted signals leave the transmitter travelling almost parallel to the earth they will travel a greater distance. They will leave the transmitter reaching the ionosphere near the horizon and then similarly be refracted back to earth. Signals may travel several thousand miles, especially if they leave the transmitter antenna at a low angle, or for higher angle radiation distances may be a few hundred miles.

Signals that leave the transmitter and travel towards the ionosphere are called skywaves as they travel towards the sky. The tem skywaves is widely used, and can be compared to a ground wave which is a specific type of wave that travels close to the ground.

Ionospheric regions

The ionosphere is a region extending to around 400km above the earth. In these regions radiation from the Sun causes the molecules to become ionised. The ionisation levels vary at different altitudes and as a result there are what are often termed different layers within the ionosphere. These layers are more correctly termed regions as there is ionisation extending across the ionosphere, the layers being peaks in levels of ionisation.

Within these regions it is primarily the free elections that vibrate with the incoming signals and cause them to be refracted or attenuated according to their frequency.

There are three main regions within the ionosphere:

  • D region:   The D region within the ionosphere is the first region above the Earth's surface where there is an appreciable level of ionisation that is sufficient to affect radio signals. This typically region attenuates the signals as they pass through as a result of the higher level of molecules that are present - more collisions occur as the free electrons vibrate and this results in signal attenuation. It is found that low frequencies are attenuated more than higher ones - the attenuation varying as the inverse square of the frequency, i.e. doubling the frequency reduces the level of attenuation by a factor of four. Often this means that low frequency signals do not reach the higher ionised regions within the ionosphere, except at night when the D region disappears.
  • E and F Regions   Once signals pass through the D region, they can then travel on to the higher regions. The first is referred to as the E region and above this is the F region. At the altitudes of these regions the air density is much less and when the electrons are excited there are far fewer collisions, but the signals are still affected by the level of free electrons being refracted away from areas of higher electron density. In the case of signals in the HF portion of the radio spectrum, the refraction is sufficient to bend the signals so that they return to Earth. In effect it appears that the region has "reflected" the signal. The level of refraction is dependent upon both frequency and the angle of incidence as well as the state of the ionosphere. As the frequency increases, so the level of refraction decreases until a frequency is reached where the signal passes through and on to the next layer or even into outer space.

Different frequencies

To gain a better idea of the characteristics of HF propagation using the ionosphere, it is worth viewing what happens to a radio communications signal if the frequency is increased across the frequency spectrum. First it starts with a signal in the medium wave broadcast band. During the day signals on these frequencies only propagate using the ground wave. Any signals that reach the D region are absorbed. However at night as the D region disappears signals reach the other regions and may be heard over much greater distances.

If the frequency of the signal is increased, a point is reached where the signal starts to penetrate the D region and signals reach the E region. Here it is reflected and will pass back through the D region and return to earth a considerable distance away from the transmitter.

As the frequency is increased further the signal is refracted less and less by the E region and eventually it passes right through. It then reaches the F1 region and here it may be reflected passing back through the D and E regions to reach the earth again. As the F1 region is higher than the E region the distance reached will be greater than that for an E region reflection.

Finally as the frequency of the radio communications signal rises still further the it will eventually pass through the F1 region and onto the F2 region. This is the highest of the regions in the ionosphere and the distances reached using this are the greatest. As a rough guide the maximum skip distance for the E region is around 2500 km and 5000 km for the F2 region.

Multiple hops

Whilst it is possible to reach considerable distances using the F region as already described, on its own this does not explain the fact that radio signals are regularly heard from opposite sides of the globe using HF propagation with the ionosphere. This occurs because the signals are able to undergo several "reflections". Once the signals are returned to earth from the ionosphere, they are reflected back upwards by the earth's surface, and again they are able to undergo another "reflection" by the ionosphere. Naturally the signal is reduced in strength at each "reflection", and it is also found that different areas of the Earth reflect radio signals differently. As might be anticipated the surface of the sea is a very good reflector, whereas desert areas are very poor. This means that signals that are "reflected" back to the ionosphere by the Pacific or Atlantic oceans will be stronger than those that use the Sahara desert or the red centre of Australia.

It is not just the Earth's surface that introduces losses into the signal path. In fact the major cause of loss is the D region, even for frequencies high up into the HF portion of the spectrum. One of the reasons for this is that the signal has to pass through the D region twice for every reflection by the ionosphere. This means that to get the best signal strengths it is necessary signal paths enable the minimum number of hops to be used. This is generally achieved using frequencies close to the maximum frequencies that can support communications using ionospheric propagation, and thereby using the highest regions in the ionosphere. In addition to this the level of attenuation introduced by the D region is also reduced. This means that a radio signal on 20 MHz for example will be stronger than one on 10 MHz if propagation can be supported at both frequencies.

HF propagation using the ionosphere is still a widely used as a form of radio communications. While not as reliable as satellite communications, it is not nearly as expensive, and can provide a useful back-up in case the satellite communications fail. It is also widely used as the primary form of radio communications by many organisations from radio broadcasters to radio amateurs, as well as ship to shore and many other forms of point to point communications. As a result HF propagation using the ionosphere is likely to remain in use indefinitely as a form of radio communications technology.

By Ian Poole

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