The troposphere is an important area for VHF and UHF radio propagation. Under normal conditions, the change in refractive index with height means that radio signals normally travel beyond the horizon extending coverage by about a third.
However under certain conditions, distances are extended much further and there can be talk of "lifts" "ducting" or "tropo."
Being able to understand the conditions that give rise to these phenomena can help in utilising them better, if as in the case of many radio amateurs it can give rise to the possibility of making contacts over much greater distances than normal.
For commercial organisations using these frequencies for two way radio communications, or for broadcasters using these frequencies, it can give rise to significant levels of interference as stations not normally heard become very strong.
Enhanced conditions: tropo lifts
Under certain conditions the radio propagation conditions provided by the troposphere are such that signals travel over much greater distances than normal.
This enhanced tropospheric refraction occurs when there is a significant increase above the normal value of the refractive index in the atmosphere.
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 1000 or more kilometres. This can give rise to significant levels of interference for broadcasters and two way radio communications services, etc 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.
As a result, this form of tropospheric propagation can be referred to as tropospheric enhanced bending or super-refraction.
Under some circumstances the signals reach an area of the troposphere where a sharp change in refractive index occurs and the signals can undergo a sharp refraction back to earth.
The distances achieved using enhanced refraction or bending can exceed 800 km or so (about 500 miles), although greater distances can be achieved on some occasions.
Another mechanism for tropospheric radio propagation is called tropospheric ducting.
As the name implies, for tropospheric ducting, the signal becomes trapped between two regions where reflections appear to take place - these tend to be areas where the signal is refracted, but are often viewed as reflections.
There are two mechanisms that can occur for the tropospheric ducting:
Surface tropospheric ducting: For this type of ducting, the signal travels away from the transmitter and upwards, reaching a sharp change in refractive index in the atmosphere and it is refracted back to Earth. It can then be reflected by the Earth's surface and reach another region where there is a sharp change in refractive index and it can again be refracted back to Earth.
Tropospheric elevated ducting: These ducts occur as the signals travel away from the transmitter and rise in altitude as a result of the earth's curvature. They become trapped between two regions of increased refractive index form an elevated duct. The signals bounce back and forth between the upper and lower boundaries of the duct until they ultimately are returned to Earth at the remote end.
Signals in the duct are unlikely to be heard by stations below it, and only by stations at the remote end. This is very similar to the skip zone experienced with HF ionospheric propagation.
As the signals are contained within an elevated segment of air where there is relatively little attenuation and they are not reflected by the Earth that also gives rise to attenuation, these signals suffer comparatively little reduction in strength, and they can be heard at good strength over much longer distances than might be expected.
Although both types of duct are relatively common, the surface ducting will undergo more attenuation because of the Earth reflections. Elevated ducting suffers far less attenuation.
The distances that can be achieved using ducting, and in particular the elevated ducting can exceed 1500km on some occasions.
It is easy to state that there are various forms of tropospheric propagation enhancements including lifts, ducting and the like, and to draw the mechanisms by which they occur.
in reality the situation is far less clear cut. It is unlikely that it is possible to detect exactly which form of mechanism is used at a particular time.
In fact, often the overall mechanism can be a mixture of types, with not just one mechanism being responsible for propagating the signal from the transmitter to the receiver.
in fact, the signal may even travel via several slightly different paths, which may utilise the different mechanisms to different degrees.
Although the mechanisms have been investigated over many years, it is difficult to assess which form is being used in a given instance.
Mechanism behind tropospheric propagation lifts & ducts
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.
This results in the expected increase in range of about a third caused by bending the radio signal around the Earth's curvature.
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.
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.
When to look for tropospheric enhanced conditions
As the troposphere is the region in which the weather we experience mainly occurs, it is hardly surprising that the weather directly affects the tropospheric radio conditions.
There are several conditions that can give rise to enhanced tropospheric radio propagation ,i.e. lifts and ducts.
Settled high pressure: The classic weather formation that causes a lift in conditions is the presence of a high pressure area. The particular enhancements occur along the isobar pattern, i.e. along lines of the equal pressure. It has also been found that the best enhancements occur when the pressure is falling slightly.
The best months for these lifts are often in the summer and autumn, although this does not exclude the other times of the year.
The characteristics of suitable high pressure periods are clear, cloudless days which are associated with little wind. During these periods, at sunset cooling occurs of both the surface and upper regions of the air, but as cooling occurs at different rates, an inversion layer can be formed. It is worth noting that a similar effect occurs at sunrise.
In more detail, radiation occurs of heat from the surface of the Earth and also warm air moves upwards. To replace the warm upward moving air, cooler air moves in. At higher altitudes the air tends to cool more slowly, and this gives rise to a temperature inversion. This process often continues all the way through the night until dawn. It sometimes produces inversion layers at altitudes of between 300 to 600 metres above the ground.
Fog: When fog is present, especially during a period of high pressure, this can give rise to excellent enhanced propagation conditions. A temperature inversion can occur because there is the fog with clear sky above it. As a result of the sun reaching the top level of the fog, this will be heated and not the lower regions because they are shielded from the Sun's warming rays. Accordingly, this causes a temperature inversion which can enhance the radio propagation.
Frosty mornings: It sometimes occurs that frosty mornings can give rise to a degree of enhancement. The cold Earth takes some while to warm, but the air above the earth will tend to warm faster as the Sun's rays affect it more quickly. This effect is more of a winter one and it is not as pronounced as the enhancements cause by summer high pressure areas.
For those interesting in looking for enhanced tropospheric radio propagation conditions, it is always good to keep a very good eye on the weather forecasts and the weather itself. This can give some very good indicators and predictions for the likelihood of good conditions and extended propagation.
In addition to this, there are many stations that can be used as beacons - distant VHF FM signals being heard, and the presence of many amateur radio beacons as well as repeaters within their frequency spectrum band allocations. Monitoring which ones are audible and also their signal strengths can give a good indication of the actual prevailing conditions.
Managing the Effects of Tropospheric Enhancements
Many people including radio amateurs, scanner enthusiasts and the like will relish the thought of a lift in propagation conditions. However, commercial operators of broadcast systems as well as two way radio communications systems and the like will find that it gives an unwanted increase in interference levels.
Accordingly it may be necessary to implement mitigation schemes to reduce the effects of the reception of distant co-channels stations.
There are a few ideas that can be implemented, although in many cases it is necessary to just live with the increased interference until it subsides. Some mitigation can be incorporated:
Alternative frequencies and bands: One solution that might be open to some organisations is to use any alternative frequencies or bands that might not be affected by the propagation conditions and where interference is less. Unfortunately for users where single channels or bands, this might not be an option. If available, it also places more pressure on the frequencies that are not affected by the interference.
Down-tilt antennas: Some systems are able to operate a system where the antenna can be titled downwards to reduce the range over which signals are received. However this can affect the wanted coverage and may not be acceptable.
A scheme called Remote Electrical Tilt, RET is incorporated on some radio communications systems to reduce the coverage areas when this might be needed. many systems will not have this luxury, even if it was appropriate in overcoming this interference.
Increase error correction on digital systems: Modern digital systems incorporate error correction schemes to detect and correct errors. Under conditions of high interference, the level of error correction can be increased - it often occurs automatically. The downside of this is that the increased error correction, resends, etc take more bandwidth and this will slow the data throughput.
Unfortunately the increased level of interference resulting from tropospheric lifts has no ideal solution, and in many cases the resulting interference just has to be lived with until the conditions subside and the system returns to normal.
Points to watch
There can be a number of points to watch associated with tropospheric radio propagation, and it is sometimes useful to be aware of various points as they can explain what may appear to be some anomalies.
It is possible to get above the tropo: While tropospheric ducting and inversion layers can serve to extend the distances travelled by radio signals, for those really high up they can also stop communication. In the same way that an inversion layer can return signals to Earth, they can also prevent them from reaching the earth.
This was made very obvious once when I climbed Mt Snowdon in Wales on a hot day with my amateur radio handheld. Although many repeaters were audible from the summit, some that were direct line of sight were not. One explanation could have been that an inversion layer had prevented the signals from travelling to the repeater and instead reflected them upwards and away.
The explanation that was given was that it was possible to be above the tropo and therefore direct line of sight paths that would normally be present, would be obstructed by the inversion layer.
Enhanced tropospheric radio propagation occurs from time to time and is a fact of life. Whether it brings the advantages of the possibility of hearing or making contact with more distant stations, or it gives rise to unwanted interference, there is nothing that can be done about its occurrence.
Understanding the modes of the enhanced propagation conditions can help predict when it is going to happen, and be aware of its likelihood and the advantages or disadvantages it brings.
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