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Ionospheric Radio Propagation
- an overview of the HF propagation and the different ways in which radio signals can travel or propagate using the ionosphere.
Radio signals can travel over vast distances. On the short wave of HF bands, signals can be regularly heard from the other side of the globe. HF propagation has long been of interest. It was used back in the earliest days of wireless to provide worldwide communications, and even today HF propagation is still used by many organisations from international broadcast stations and radio amateurs through to shipping services, weather stations and a variety of other two way radio communications systems and mobile radio communications systems.
The Atmosphere and HF radio propagation
The atmosphere plays an all important part in the HF propagation of radio waves. With its different layers extending to over 400 km above the surface of the earth there is a wide variety of different effects which can bend and reflect signals so that they can be heard over vast distances. Some layers have a significant effect on radio waves whilst others have none.
The troposphere is closest to the earth. This mainly affects signals above about 30 MHz, and being close to the earth's surface the weather conditions have a significant effect.
Much higher in altitude the ionosphere can be found. In this region there are a number of ionised layers or regions which can reflect or absorb radio waves. As they are in the upper reaches of the atmosphere it is hardly surprising to find that they are affected by conditions on the sun. In fact they will change between night and day. They will also vary according to the seasons of the year as well as the 11 year sun spot cycle.
There are three main layers or more correctly regions in the ionosphere and letters are used to distinguish one from another.
These regions vary over time, and the way in which this occurs is of great importance for broadcasters as well as operators of two way radio communications systems.
How HF propagation varies with frequency
To explain what happens and how the different frequencies are affected, it is best to use the example of a transmitter which is radiating a signal on the medium wave band. Then the frequency can be increased to see how the radio propagation changes with frequency.
On the medium wave band the signal from the transmitter will spread out in all directions. Some of it will travel out parallel to the earth and will propagate via the ground wave. However signal that travels upwards towards the sky, i.e. skywave, reaches the D region, and during the day it will be absorbed.
If the frequency of the transmitter is increased two effects occur. The first is that the ground wave signal is reduced or attenuated more quickly and it does not travel as far. Secondly, any signals which travel upwards towards the ionosphere will start to penetrate right through the D region. Having passed through the D region the radio signal will travel towards the E region. Here the signals will be bent or "refracted" back to earth. It may appear that they have been reflected, but this is not strictly true.
When the radio signals are bent back towards the earth's surface they will reach the ground a considerable distance away from the transmitter. The maximum distance for reflections from the E region is about 2000 km. At night when the D region disappears it will be found the medium wave stations can be heard over much greater distances. This is because their signals are being reflected by the E region.
The signal travelling away from the ground to the ionosphere is called the skywave and the distance it travels before reaching the earth again is called the skip distance. It will also be found that there is often an area between the limit of the ground wave coverage and where the skywave returns to earth where no signal can be received. This area is called the dead zone or skip zone.
As the frequency of the transmitter is increased still further it will be found that the E layer is not able to bend the signal as easily. Eventually a point is reached when it passes through the layer and travels on to the F1 layer. Here the signals will again be reflected giving maximum distances of around 4000 km. Eventually a point is again reached when the signals pass through the it or on to the F2 layer if both are present. Here the signals will be reflected at first and then as the frequency increases further the signal will pass through. Once the signals pass through all the layers of the ionosphere they will travel on into outer space.
Very often signals will be heard over distances which are larger than would be possible with just one reflection. For example it is quite common to hear stations from the other side of the globe and this is considerably in excess of the maximum skip distance even for the F2 layer.
The most common way for signals to travel over greater distances is by multiple reflections. It is found that when a signal is reflected back to earth by the ionosphere then the earth can reflect it back up to the ionosphere. In this way signals can travel to anywhere on the globe.
HF propagation summarySignal propagation on the HF bands enables stations to have world wide coverage using relatively small powers and using relatively low cost equipment, certainly when compared to other forms of radio systems using satellite. As a result HF propagation is still widely used, although the varying HF propagation conditions must be taken into account.