Tropospheric DX Conditions

Stable Air and Temperature Inversions = VHF / UHF Distance!

Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anticyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (1,300 km) (though some people have received "tropo" beyond 1,000 miles / 1,600 km), while with tropospheric-bending, stable signals with good signal strength from 500+ miles (800+ km) away are not uncommon when the refractive index of the atmosphere is fairly high.

Tropospheric ducting of UHF television signals is relatively common during the summer and autumn months, and is the result of change in the refractive index of the atmosphere at the boundary between air masses of different temperatures and humidities. Using an analogy, it can be said that the denser air at ground level slows the wave front a little more than does the rare upper air, imparting a downward curve to the wave travel.

Ducting can occur on a very large scale when a large mass of cold air is overrun by warm air. This is termed a temperature inversion, and the boundary between the two air masses may extend for 1,000 miles (1,600 km) or more along a stationary weather front.

Temperature inversions occur most frequently along coastal areas bordering large bodies of water. This is the result of natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers. The same action may take place in the morning when the rising sun warms the upper layers.

Even though tropospheric ducting has been occasionally observed down to 40 MHz, the signal levels are usually very weak. Higher frequencies above 90 MHz are generally more favourably propagated.

High mountainous areas and undulating terrain between the transmitter and receiver can form an effective barrier to tropospheric signals. Ideally, a relatively flat land path between the transmitter and receiver is ideal for tropospheric ducting. Sea paths also tend to produce superior results.

In certain parts of the world, notably the Mediterranean Sea and the Persian Gulf, tropospheric ducting conditions can become established for many months of the year to the extent that viewers regularly receive quality reception of signals over distances of 1,000 miles (1,600 km). Such conditions are normally optimum during very hot settled summer weather.

Tropospheric ducting over water, particularly between California and Hawaii, Brazil and Africa, Australia and New Zealand, Australia and Indonesia, Strait of Florida, and Bahrain and Pakistan, has produced VHF/UHF reception ranging from 1000 to 3,000 miles (1,600 – 4,800 km).

Tropospheric signals exhibit a slow cycle of fading and will occasionally produce signals sufficiently strong for noise-free stereo, reception of Radio Data System(RDS) data, and solid locks of HD Radio streams on FM or noise-free, color TV pictures.

Virtually all long-distance reception of digital television occurs by tropospheric ducting (due to most, but not all, DTV stations broadcasting in the UHF band).

From Unisys, a chart of the atmospheric K index. This index is a measure of stability and moisture in the air, and is a predictor of thunderstorm activity. It is useful for VHF and UHF DXers due to its ability to show areas of stable air and inverted lapse rates (temperature inversions). Best conditions along a path are when the K index is well below 20, to its lowest values around -5. When the values are above 25, think not of radio, but lightning safety and a dry place to watch the storms!!

From Unisys, here is a chart depicting the atmospheric Lifted Index. It is a measure of stability, and is best for tropospheric propagation when its value is greater than 2. This means that the air, if lifted from sea level to 18,0000 ft, would not be bouyant enough (warm enough) to continue bubbling up on its own. Positive values of the lifted index indicate stable air, and possibly temperature inversions if the values are well above 2. If you see values below -2, forget about radio, and expect thunderstorms. Thunderstorms and tropo DX don't mix.