Ionospheric propagation tutorial includes . . . .
Ionospheric propagation Ionosphere Ionospheric layers Skywaves & skip Critical frequency, MUF, LUF & OWF How to use ionospheric propagation Multiple reflections & hops Ionospheric absorption Signal fading Solar indices Propagation software NVIS Transequatorial propagation Grey line propagation Sporadic E Spread F
HF radio communications use the ionosphere and ionospheric radio propagation to enable worldwide coverage to be gained.
The difficulty is that ionospheric radio propagation conditions are varying all the time. Some times it may be possible to hear stations from the other side of the globe, whereas at others radio communications may only be possible over relatively short distances.
Many factors cause these changes: different frequencies, time of day, time of the year, position on the globe, the state of the Sun and a variety of other issues.
While the radio propagation conditions themselves are beyond our control, there are ways in which the best can be made of the prevailing conditions, and the likelihood of achieving the results can be increased by simple choice is items like antennas, transmitting and receiving equipment, selecting the right frequencies, the right times of day and the year, etc. All these can make a large difference in achieving the required radio communications results.
Selecting the right frequencies
One of the major factors affecting radio communications via the ionosphere is the frequency of the incident signal. It is for this reason that a careful choice of frequency or band is required to enable the required radio communications to be established.
In order to see how the ionosphere reacts to signals at differing frequencies it is useful to consider a signal first at the bottom end of the spectrum and see the changes as the frequency is gradually increased.
It is necessary to remember this summary should only be seen as a very rough guide because the nature of the ionosphere is constantly changing. However, the frequency dependence is one of the key issues, and an explanation helps set down some fo the important concepts behind using ionospheric radio propagation.
It is helpful to take a signal at the bottom end of the radio spectrum that is affected by the ionosphere and then work upwards in frequency to see what happens.
A signal in the MF portion of the radio spectrum such as a medium wave broadcast transmission might be a good example. During the daytime the signal will be absorbed by the D region and no detectable levels of signal are able to reach the higher regions of the ionosphere and be returned to Earth. Coverage will typically be achieved using the ground wave signal.
At night as the level of ionisation in the D region reduces very significantly as the radiation that causes the ionisation is removed. This is because the air is not too thin at the altitude of the D region, the ions recombine to form stable molecules.
As a result, MF signals are able to reach the much reduced E region of the F region and be reflected back to Earth. It is worth noting that the E region and F region ionisation levels fall, but not to the same degree as the D region because the air is much less dense and recombination takes much longer.
Although radio signals still undergo some attenuation passing through the lower reaches of the E region they are still audible, and it for this reason that interference levels on the medium wave band increase dramatically at night as stations from further afield can be heard.
As the signal frequency is increased during the day, it is found that the level of attenuation introduced by the D region starts to fall. Typically around frequencies of 2 - 3 MHz and above signals start to penetrate the D region increasingly as the frequencies rise.
Ultimately the radio signals are able to pass right through and reach the E region where they are reflected back to Earth. When this happens, they are audible at much greater distances than are possible via the ground wave. It should be noted that signals are attenuated during each passage through the D region.
As the frequency is increased further, it is found that the level of attenuation introduced by the D region falls, and signal strengths of signals reflected by the E region start to rise. Additionally the signals will penetrate further into the E region and finally they will pass through, reaching the F1 region. Here again they will be reflected back to Earth, and then as the frequency rises still further they will reach the F2 region.
Is the frequency rises still further, the signal penetrated the F region even further, and ultimately passes through. The signals at these frequencies and above are unable to be reflected by the ionosphere and they travel on into outer space.
Angle of incidence and distances achievable
The angle at which a signal leaves the radiating antenna relative to the Earth and then reaches the ionosphere is of great importance. Signals reaching the ionospheric region almost parallel to the contours will need little refraction to return them to Earth. Those reaching the regions with almost vertical incidence will require a much greater degree of refraction.
It is also found that signals that are attenuated by the D region achieve a lower degree of attenuation if the path length within the D region is as short as possible. This means that signals entering the D region almost parallel to it will be attenuated more than those that enter with an angle nearer to the vertical.
The distances that can be achieved are also dependent upon the angles at which the signals travel. From basic trigonometry it can be seen that if a signal leaves the antenna at a low angle of radiation, i.e. almost parallel to the Earth's surface, then the distances achieved will be greater than signals leaving with a high angle of radiation, i.e. travelling at a much steeper angle upwards towards the ionosphere. Also the higher the ionospheric region that is used the greater the distances that will be achieved.
It is found that even relatively small changes in the angle at which the signal leaves the antenna can considerably reduce the distances that can be covered. The maximum distance that can be achieved using the E layer is generally considered to be 2000km (1250 miles), but this is reduced to just 400 km (250 miles) if the angle of radiation of the antenna is 20°. Similarly the maximum distance achievable using the F2 layer reduces from around 4000km (2500 miles) to just under 1000 km (600 miles) for the same angle of radiation. It should be noted that the angle of radiation of the antenna is taken as the angle between the earth and the direction of maximum radiation from the antenna.
In order to be able to place the maximum amount of energy where it is required, it is necessary to have a directional antenna.All antennas radiate more energy in some directions than others - the true isotropic source that radiates in all directions does not exist in reality. The radiation pattern of an antenna can be plotted out in what is termed a polar diagram.
To ensure the optimum performance the antenna should be orientated in the correct direction, and it should also have the correct angle of radiation, i.e. the angle between the direction of maximum radiation and the Earth's surface.
In many applications where the maximum distance is required, an antenna with a low angle of radiation is needed, although particularly for low frequencies that may be affected by the D region attenuation, this means that it travels through this region for longer and will suffer greater levels of loss. Not all applications require a low angle of radiation. By selecting frequencies and times of day, the D region attenuation can be minimised so that signals can travel the required distances.
Broadcast stations in particular will arrange for their antennas to have the correct angle of radiation so that the signal has the optimum elevation to reach the required target area.
By understanding the different effects that happen in the ionosphere over the day, the season and wit the activity of the Sun, etc, it is possible to gain a good idea of when radio communications from different regions of the globe may be possible. It is also possible to optimise any antennas to provide the required form of radiation angle, directivity, etc so that the radiated signal is used where it is most effective.