Tropospheric propagation

- overview, tutorial or article about tropospheric propagation and the way in which radio signals at VHF and above propagate over greater distances than strictly line of sight in the troposphere

On frequencies above 30 MHz, it is found that the troposphere has an increasing effect on radio signals and radio communications systems. The radio signals are able to travel over greater distances than would be suggested by line of sight calculations. At times conditions change and radio signals may be detected over distances of 500 or even 1000 km and more. This is normally by a form of tropospheric enhancement, often called "tropo" for short. At times signals may even be trapped in an elevated duct in a form of radio signal propagation known as tropospheric ducting. This can disrupt many radio communications links (including two way radio communications links) because interference may be encountered that is not normally there. As a result when designing a radio communications link or network, this form of interference must be recognised so that steps can be taken to minimise its effects.

The way in which signals travel at frequencies of VHF and above is of great importance for those looking at radio coverage of systems such as cellular telecommunications, mobile radio communications and other wireless systems as well as other users including radio hams.


Line of sight radio communications

It might be thought that most radio communications links at VHF and above follow a line of sight path. This is not strictly true and it is found that even under normal conditions radio signals are able to travel or propagate over distances that are greater than the line of sight.

The reason for the increase in distance travelled by the radio signals is that they are refracted by small changes that exist in the Earth's atmosphere close to the ground. It is found that the refractive index of the air close to the ground is very slightly higher than that higher up. As a result the radio signals are bent towards the area of higher refractive index, which is closer to the ground. It thereby extends the range of the radio signals.

The refractive index of the atmosphere varies according to a variety of factors. Temperature, atmospheric pressure and water vapour pressure all influence the value. Even small changes in these variables can make a significant difference because radio signals can be refracted over whole of the signal path and this may extend for many kilometres.


N units

It is found that the average value for the refractive index of air at ground level is around 1.0003, but it can easily vary from 1.00027 to 1.00035. In view of the very small changes that are seen, a system has been introduced that enables the small changes to be noted more easily. Units called "N" units are often used. These N-units are obtained by subtracting 1 from the refractive index and multiply the remainder by a million. In this way more manageable numbers are obtained.


N = (mu-1) x 10^6


Where mu is the refractive index

It is found that as a very rough guide under normal conditions in a temperature zone, the refractive index of the air falls by about 0.0004 for every kilometre increase in height, i.e. 400 N units / km. This causes the radio signals to tend to follow the earth's curvature and travel beyond the geometric horizon. The actual values extend the radio horizon by about a third. This factor is often used in most radio communications coverage calculations for applications such as broadcast radio transmitters, and other two way radio communications users such as mobile radio communications, cellular telecommunications and the like.


Enhanced conditions

Under certain conditions the radio propagation conditions provided by the troposphere are such that signals travel over even greater distances. This form of "lift" in conditions is less pronounced on the lower portions of the VHF spectrum, but is more apparent on some of the higher frequencies. Under some conditions radio signals may be heard over distances of 2000 or more kilometres with distances of 3000 kilometres being possible on rare occasions. This can give rise to significant levels of interference for periods of time.

These extended distances result from much greater changes in the values of refractive index over the signal path. This enables the signal to achieve a greater degree of bending and as a result follow the curvature of the Earth over greater distances.

Under some circumstances the change in refractive index may be sufficiently high to bend the signals back to the Earth's surface at which point they are reflected upwards again by the Earth's surface. In this way the signals may travel around the curvature of the Earth, being reflected by its surface. This is one form of "tropospheric duct" that can occur.

It is also possible for tropospheric ducts to occur above the Earth's surface. These elevated tropospheric ducts occur when a mass of air with a high refractive index has a mass of air with a lower refractive index underneath and above it as a result of the movement of air that can occur under some conditions. When these conditions occur the signals may be confined within the elevated area of air with the high refractive index and they cannot escape and return to earth. As a result they may travel for several hundred miles, and receive comparatively low levels of attenuation. They may also not audible to stations underneath the duct and in this way create a skip or dead zone similar to that experienced with HF ionospheric propagation.


Mechanism behind tropospheric propagation

Tropospheric propagation effects occur comparatively close to the surface of the Earth. The radio signals are affected by the region that is below an altitude of about 2 kilometres. As these regions are those that are greatly affected by the weather, there is a strong link between weather conditions and radio propagation conditions and coverage.

Under normal conditions a there is a steady gradient of the refractive index with height, the air being closest to the Earth's surface having the highest refractive index. This is caused by several factors. Air having a higher density and that containing a higher concentration of water vapour both lead to an increase in refractive index. As the air closest to the Earth's surface is both more dense (as a result of the pressure exerted by the gases above it) and has a higher concentration of water vapour than that higher up mean that the refractive index of the air closest to the earth's surface is the highest.

Normally the temperature of the air closest to the Earth's surface is higher than that at a greater altitude. This effect tends to reduce the air density gradient (and hence the refractive index gradient) as air with a higher temperature is less dense.

However, under some circumstances, what is termed a temperature inversion occurs. This happens when the hot air close to the earth rises allowing colder denser air to come in close to the Earth. When this occurs it gives rise to a greater change in refractive index with height and this results in a more significant change in refractive index.

Temperature inversions can arise in a number of ways. One of the most dramatic occurs when an area of high pressure is present. A high pressure area means that stable weather conditions will be present, and during the summer they are associated with warm weather. The conditions mean that air close to the ground heats up and rises. As this happens colder air flows in underneath it causing the temperature inversion. Additionally it is found that the greatest improvements tend to occur as the high-pressure area is moving away and the pressure is just starting to fall.

A temperature inversion may also occur during the passage of a cold front. A cold front occurs when an area of cold air meets an area of warm air. Under these conditions the warm air rises above the cold air creating a temperature inversion. Cold fronts tend to move relatively quickly and as a result the improvement in propagation conditions tends to be short lived.


Fading

When signals are propagated over extended distances as a result of enhanced tropospheric propagation conditions, the signals are normally subject to slow deep fading. This is caused by the fact that the signals are received via a number of different paths. As the winds in the atmosphere move the air around it means that the different paths will change over a period of time. Accordingly the signals appearing at the receiver will fall in and out of phase with each other as a result of the different and changing path lengths, and as a result the strength of the overall received signal will change.

Any terrestrial signals received at VHF and above will be subject to the prevailing propagation conditions caused by the troposphere. Under normal conditions it should be expected that signals will be able to be received beyond the normal line of sight distance. However under some circumstances these distances will be considerably increased and significant levels of interference may be experienced.

By Ian Poole


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