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  The modes are defined by the mechanics behind them. A Tropo DX mode is any condition that scatters, reflects or refracts signals in the Troposphere allowing DX to occur. Refraction occurs when the normal Index of Refraction has been altered. Vertical boundaries between different types of airmasses usually cause this..where a temperature inversion (warm air over cooler air) exists. However..the most important influencing factor is water vapour (humidity). Thus..a warm dry airmass on top of a cooler humid airmass produces the best conditions. Dry Mexican air flowing across the Gulf of Mexico or Dry Saharan air flowing across the Meditteranean are two examples of prime tropo-producing conditions. High pressure subsidence (the sinking and drying out of air)..if it occurs over the oceans..can produce reception across several thousands of km! Hawaii to California reception..both on UHF and not as uncommon as one might think. On the other hand..high mountains can physically block tropo DX..and deserts are generally too dry for tropo. Thus..tropo is rare in the very mountainous or dry regions of the world.

  As far as classifying DX..enhancement and ducting in particular form a grey line. As a rule of thumb..Enhancement is DX via inversions below 450 m (1500 ft) above ground...Ducting is DX via inversions above 450 m. (The layer of the troposphere below 450 m is called the "boundary layer" in meteorology).

Tropospheric V/UHF DX Modes

Line-of-Sight (GW) normal continuous reception where the receiving and transmitting antennas can see each other..taking into account the 4/3 Earth curvature of radio waves.

Tropospheric Scatter (TrS) ever-present under normal conditions. That's the mode that produces the distant fluttery signals that randomly fade in and out. These are your most distant regular stations that barely make it in. Depending on your location and equipment..tropo scatter can extend to 300..500..or even 700 km. The theoretical maximum limit for most TV/radio DXers is 800 km (500 mi) (Some semi-professional setups can extend furthur). Scatter is caused by small particles/droplets in the air such as haze, dust, volcanic ash, clouds, etc.

Tropospheric Enhancement (TrE)..(akaTropospheric Refraction) is common under normal conditions. On most clear nights with calm or light winds..the ground radiates and the air near the ground cools. Eventually an inversion is formed and signals begin to refract off the inversion. Stations that normally fade in and out via tropo scatter come in continuously..with increasing strength. Also..weaker tropo scatter stations that are normally not heard (because their signal strengths never cross the background noise threshold signal level) also begin to appear. When the sun comes up..the ground & air heats up..the inversion breaks down..and the enhancement disappears. The enhancement is subtle on some nights..and very obvious on other nights. Distances are no different than your tropo scatter's just that the signals are stronger. Tropo enhancement is greatly influenced by terrain..with valley and coastal paths favoured. ("Fog-prone" areas are also "DX-prone" areas!!). From a DXers point of view, multiple directions usually are enhanced at the same time.

Tropospheric Ducting (TrD) an abnormal condition. An inversion has formed at a much higher level above the ground...the vast majority of duct-producing inversions lie between 450 and 1500 m (1500 to 5000 ft)..with a few between 1500 and 3000 m (5000 to 10,000 ft). These inversions are not formed due to nighttime radiation/cooling..but rather because of some other weather phenomenon (high pressure subsidence aloft, warm frontal boundary, cold frontal boundary, oceanic or lake inversion, Chinooks, etc.). Because of this..ducting can occur day or night (though it strengthens at night) not usually influenced by terrain (the exception being large mountain chains like the Rockies)..and from a DXers point of view is usually either uni- or bi-directional. In fact..typical ducts are sharply directional. Signals refract off of and also travel along the inversion..thus the analogy of a duct. Distances are theoretically unlimited. One large area can have multiple ducts going on simutaneously..but they are usually parallel paths. It is possible in a very strong high pressure system to have large areas of ducting creating multi-directional openings. These are the rare "blockbuster" openings that make DXers' mouths water. They are most common over the oceanic areas in the tropics and sub-tropics.

Additional Characteristics of Ducting..
  Ducting may or may not occur simutaneously with enhancement (caused by nighttime cooling). Often there is both a low-level radiational inversion caused by nighttime cooling (producing enhancement)...and a mid-level "system-produced" inversion above that (producing ducting). However..just as often there is only the higher duct-producing inversion..especially if the skies are cloudy or if it is windy. not use your regular scatter/enhancement stations as propagation beacons for longer-distance DX acheived via ducting! Sometimes ducting can even display a "skip-like" character where distant stations on the same frequency and bearing can be received while closer-by stations are nowhere to be seen.

  Ducting is also very height selective..with maximum signal transmissions at and just below the altitude of the inversion. DXers normally receive ducting DX via ground-based inversions. Occasionally inversions can be based above ground, in which case a DXer beneath the duct could completely miss out on the DX opening. Meanwhile, DXers at a higher elevation could, at the same time, be in the midst of a strong opening. It is also possible that a DXer at too high an elevation (above the top of the duct) could also miss out.

  Ducting conditions usually vary over short time periods as opposed to enhancement which is more stable. Ducts located behind cold fronts ("post-frontal ducts") are notoriously unstable as paths can even be interrupted by things such as heavy rain showers associated with the cold front itself. Expect the unexpected from these types of ducts with sudden and rapid changes in signal strengths quite common (some post-frontal ducts last only 15 to 30 minutes). High-pressure and oceanic ducts are a bit more stable and can last for days..but again expect the unexpected as changes can occur quickly.

  Frequencies affected by ducting are determined by the vertical thickness of an inversion. Individual ducts will have a LUF (Lowest Usable Frequency) associated with them. Thin inversions (i.e.-thin ducts) will only propagate Microwaves. Thicker inversions will propagate UHF signals as well..while the thickest inversions will also propagate VHF signals. Unfortunately there is no reliable method known for forecasting inversion thicknesses. Also, in real-life tests, it has been found that reception at frequencies below the theoretical LUF is usually possible, although there is usually a sharp drop-off in signal strength at the LUF. See LUF  page.

Special Cases (Exotic DX Modes)

Rain Scatter (RS) a rare mode that sometimes occurs on the higher UHF-TV channels. A band of very heavy rain (or rain and hail) at a distance can scatter or even reflect signals. The effect is the one used for microwave Weather Radars. Distances are typically around 160 km..though up to 650 km (400 mi) is theoretically possible. (Note that heavy snow is not an useful reflector).

Ice Pellet Scatter (SS)..(called Sleet Scatter in the US) similar to Rain Scatter but is caused by bands of Ice Pellets in the wintertime.

Aircraft Scatter (AS)..(aka Tropospheric Reflection) is simply reflection off of aircraft..although reflections off of flocks of birds are also possible. A rare form of reflection is "Chaf Scatter". Chaf is strips of metal foil sent out by the military during training exercises. Chaf helps to confuse enemy radars..but also helps to produce DX. Maximum distances for all reflection modes are again up to 800 km (500 mi).

Lightning Scatter (LS) a mode that is sometimes discussed..but there is little documentation on it. The theory is that lightning strikes produce ionized trails. Reception is similar to other forms of scatter except that the DX is more burst-like similar to MS. LS is a mode that is very hard to distinguish and rarely reported.

Reflections off of hills and mountains..and Knife-Edge Diffraction are not considered true DX modes since they are omni-present..though they can help to extend DX via the other modes.

So these are the conditions in the troposphere that allow reception of VHF and UHF signals beyond their normal range. Basically..these are DX modes that are affected by the weather.

In the forecasts.....the reason that I stick with just Ducting is because it is a large-scale phenomena..can be forecasted in a reasonable amount of time..and produces the best tropo. Enhancement is forecastable..but it is so dependant on regional and local terrain and conditions that it would be a labour-intensive effort (not to mention a very lengthy one) to forecast for all of North America. The process that one would have to use to properly forecast Enhancement for a particular area is the same that a meteorologist would use to forecast overnight low tempertatures, chance of fog patches, etc.

The BBC has an excellent page on Tropo DX that describes the various types of DX-producing inversions.
here to check it out.

There are also conditions in the ionosphere that produce distant reception via a whole different set of modes. Ionospheric "skip" and scatter are not caused by the weather..but instead by the interaction between the Sun and the Earth's outer atmosphere..or by objects such as meteors. For information on these modes..consult the ARRL Handbook.

1999-2001 Special Thanks to William R. Hepburn

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