Ionospheric Absorption of Radio Signals

Signals are absorbed or attenuated when they undergo propagation using the ionosphere - there are several ways in which they are attenuated.

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The absorption and attenuation of radio signals by the ionosphere is obviously important. Understanding the ways in which signals are attenuated enables better use to be made of ionospheric propagation for radio communications purposes.

It is possible to use the best frequencies, best times of day and even use antennas with the required angle of radiation etc to minimise the losses and thereby enable the best radio communications to be made.

The behaviour of the ionosphere is one of the key areas to consider when planning a radio communications network or system, or when predicting HF propagation conditions.

Ways in which signals are attenuated by the ionosphere

There are several reasons for signals being attenuated when they are subject to propagation by the ionosphere.

Although it is not always possible to get round the issues, understanding what they are helps in being able to take action to have the best chance of establishing radio communications with the best signal strengths.

Some of the chief reasons for signals being attenuated include:

Distance
Attenuation by the ionosphere - chiefly the D region
Reflection by surfaces on the ground that have poor reflectivity - for multiple hops
Polarisation
Other minor losses

There are many different forms of loss encountered when using ionospheric propagation. It is necessary to assess them all when selecting a frequency, time of day, etc for planning a radio communications link, broadcasting schedule, etc.

Path distance

The level of a signal will fall as the distance between the receiver and transmitter increases.

When travelling in free space it is possible to calculate this with relative accuracy. However for ionospheric propagation this does not always hold true, but it is a good basis to start the calculations and then add in other losses and variables as needed.

Free space signal propagation showing signal reduction with distance — Signals reduce in intensity as they travel from the transmitter

The rate at which the signal intensity falls is proportional to the inverse of the square of the distance.

Signal level = \frac{k}{d^{2}}

Where:
k = constant
d = distance from the transmitter

As a simple example this means that the signal level of a radio transmission will be a quarter of the strength at 2 km distance compared to that at a distance of 1 km.

Where a radio signal comes under the influence of other factors, the basic formula can be altered to take account of this.

It is important to remember that under some circumstances propagation tunnels can exist that may act as a form of waveguide and they can result in path loss exponent values of less than 2.

While there is considerable variability to the actual loss, it will always increase with distance.

Ionospheric absorption / attenuation

The ionosphere is usually thought of as an area where radio waves on the short wave bands are refracted or reflected back to Earth. However it is also found that signals are reduced in strength or attenuated as they pass through this area. In fact ionospheric absorption can be one of the major contributors to the reduction in strength of signals.

Most of the attenuation occurs in the D region. There is some in the E and F regions, but the level is very much less than that experienced in the D region and it can generally be ignored.

When signals enter the D region they transfer energy to the electrons and set them in motion, vibrating in line with the radio signal. As the electrons vibrate in this manner they can collide with other molecule, ions, or electrons. Each time a collision occurs a small amount of energy is dissipated, and this is manifested as a loss in the strength of the signal.

The amount of energy that is lost depends primarily upon the number of collisions that take place. In turn this also is dependent upon a number of other factors. The first is the number of other molecules, electrons and ions that are present. In the D region the density of the air is relatively high, and so there are a large number of other molecules around and the number of collisions is high.

The second factor is the frequency of the signal. As the frequency is decreased, so the displacement of the vibrations increases and so does the number of collisions. In fact it is found that the amount of ionospheric absorption that occurs varies inversely as the square of the frequency. In other words if the frequency is doubled, then the attenuation will fall by a factor of four.

This is one of the major reasons why when a number of bands or frequencies will support HF propagation between two radio stations, then the highest one will yield the better results. It is also found that the level of attenuation is so high for signals on the medium wave radio broadcast band that during the day when the D region is present, no signals through it, and signals are only propagated via the ground wave. At night when the D region disappears, signals are heard from much further afield.

Effect of angle of radiation

The angle of radiation of a signal from the antenna can also have a major impact on the signal attenuation.

The angle of radiation is the angle between the signal path and the ground. A low angle of radiation means that the signal travels towards the horizon, and a higher one means that it travels upwards.

There are some advantages to having a low angle of radiation in terms of the ease of achieving longer distances. Geometry dictates that a low angle of radiation from the antenna will enable greater distances to be achieved.

The downside to having a low angle of radiation from the viewpoint of the signal loss is that the signal path will be within the D region for a greater distance and hence the loss will be greater when the D region is in existence.

Low angles of radiation have their advantages and disadvantages, but in general radio communications over greater distances is achieved using a lower angle of radiation.

It is interesting to note that broadcasters using the HF bands tailor the antenna to give the required angle of radiation and beam heading to target the required area. The D region losses will be part of the calculations and optimisations used, balancing D region losses with time of day, main area of ionosphere for reflections and the like.

Other radio communications applications often want the maximum distances and therefore antennas that provide a low angle of radiation are often used. That said, situations where near vertical incidence skywave are used to provide relatively local coverage will often used dipole antennas that are relatively ow to give a very high angle of radiation. Here the D region losses will be lower than those for many other applications.

Loss from Earth reflections

It might be expected that there is a loss when signals from the ionosphere are reflected back upwards again when a signal undergoes multiple reflections.

For each reflection by the Earth a certain degree of loss is introduced, but the actual amount of loss will depend upon a variety of factors.

The Earth is an imperfect reflector but different areas will reflect signals with different levels of loss: typically areas that are good conductors of electric current are better, as aree those that are flatter.

Sea water or flat wetland areas are the best. Dry sandy deserts are very poor, and mountainous regions where the terrain is very rough will tend to scatter the signal in many directions will also not be as good.

This means that signals that are reflected in the Atlantic or Pacific Oceans, etc are likely to be much stronger than those reflected by a desert region like the Sahara, etc.

Polarisation loss

There is a certain amount of loss or attenuation that is caused by the changing polarisation of the signals returned from the ionosphere. This results because it is found that the polarisation of the signal can be changed by the ionosphere.

The transmitted radio signals that enter the ionosphere from terrestrial antennas are normally linearly polarised. However the action of the ionosphere with the Earth's magnetic field results in the signal that emerges being elliptically polarised, rather than linearly polarised and in the same polarisation as that of the transmitter signal.

Often the polarisation loss is grouped together other forms of relatively low level losses. The degree of this loss varies according to a number of factors including the geomagnetic latitude, the season, time of day and the length of the signal path. Typically this loss may be around 9 dB, but it can be more or less.

Application of losses

An understanding about the losses and attenuation incurred when using ionospheric propagation for radio communications links needs to be able to be used to improve the performance.

Although many of the losses are fixed and there are many times when there is nothing that can be done, there are a few aspects of the operation that can be changed. Antennas, frequencies, time of day and the like.

As a signal passes through the D region every time it is reflected, one of the major causes of loss is dependent upon the number of reflections. This can be very important because the signal has to pass through the D layer twice each time it is reflected and with more than one hop, the signal passes through the D region several times.

As already mentioned the attenuation reduces with frequency. Apart from the fact that high frequency paths are more likely to use the F2 layer and have less reflections, the high frequency path will also suffer less loss from the D layer. This will mean that a signal on 30 MHz, for example, will be stronger than one on 15 MHz assuming that propagation can be supported at both frequencies.

It should also be remembered that the path length for a multiple reflection signal will be greater than the great circle distance around the globe, especially if high angles of radiation are used. This in itself will add to the signal loss because the loss is proportional to the path length.

Signal loss is an integral part of operation on the HF bands using ionospheric propagation. Understanding what the losses are and what cause the absorption, etc, helps provide a better understanding of what to expect. That said, there are sometimes steps that can be taken to help reduce the losses and improve the signal strength levels for the radio communications system.