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Tropospheric propagation describes electromagnetic propagation in relation to the troposphere. The service area from a VHF or UHF radio transmitter extends to just beyond the optical horizon, at which point signals start to rapidly reduce in strength. Viewers living in such a "deep fringe" reception area will notice that during certain conditions, weak signals normally masked by noise increase in signal strength to allow quality reception. Such conditions are related to the current state of the troposphere.
Tropospheric propagated signals travel in the part of the atmosphere adjacent to the surface and extending to some 25,000 feet (7,620 m). Such signals are thus directly affected by weather conditions extending over some hundreds of miles. During very settled, warm anticyclonic weather (i.e., high pressure), usually weak signals from distant transmitters improve in strength. Another symptom during such conditions may be interference to the local transmitter resulting in co-channel interference, usually horizontal lines or an extra floating picture with analog broadcasts and break-up with digital broadcasts. A settled high-pressure system gives the characteristic conditions for enhanced tropospheric propagation, in particular favouring signals which travel along the prevailing isobar pattern (rather than across it). Such weather conditions can occur at any time, but generally the summer and autumn months are the best periods. In certain favourable locations, enhanced tropospheric propagation may enable reception of ultra high frequency (UHF) TV signals up to 1,000 miles (1,600 km) or more.
The observable characteristics of such high-pressure systems are usually clear, cloudless days with little or no wind. At sunset the upper air cools, as does the surface temperature, but at different rates. This produces a boundary or temperature gradient, which allows an inversion level to form – a similar effect occurs at sunrise. The inversion is capable of allowing very high frequency (VHF) and UHF signal propagation well beyond the normal radio horizon distance.
The inversion effectively reduces sky wave radiation from a transmitter – normally VHF and UHF signals travel on into space when they reach the horizon, the refractive index of the ionosphere preventing signal return. With temperature inversion, however, the signal is to a large extent refracted over the horizon rather than continuing along a direct path into outer space.
Fog also produces good tropospheric results, again due to inversion effects. Fog occurs during high-pressure weather, and if such conditions result in a large belt of fog with clear sky above, there will be heating of the upper fog level and thus an inversion. This situation often arises towards night fall, continues overnight and clears with the sunrise over a period of around 4 – 5 hours.
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 radio and 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). A US listening post was built in Ethiopia to exploit a common ducting of signals from southern Russia.
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.
"DXing is the art and science of listening to distant stations (D=distance X=xmitter or transmitter)."
On June 13, 1989, Shel Remington, Keaau, Hawaii, received several 88-108 MHz FM signals from Tijuana, Mexico,at a distance of 2,536 miles (4,081 km).
- Throughout the 1990s, Fernando Garcia, located at what could be considered an ideal tropospheric DX location near Monterrey, Mexico, received numerous 1,000+ mile (1,600+ km) stations via tropospheric propagation, both over the Gulf of Mexico and past land. Among his receptions are WGNT-27 from Portsmouth, Virginia, at a distance of 1,608 miles (2,588 km) and low-power (LPTV) station W38BB from Raleigh, North Carolina, at a distance of 1,460 miles (2,350 km)
- On May 11, 2003, Jeff Kruszka, living in south Louisiana, received a few UHF DTV signals from 800+ miles. The longest of these was WNCN-DT, channel 55, Goldsboro, North Carolina, at a distance of 835 miles (1,344 km) (at the time, the record for UHF DTV).
- On December 9, 2004, Polish DXer Maciej Ługowski received "Five" TV station on UHF ch.37 from London-Croydon transmitter and BBC2 UHF ch.46 from Bluebell Hill transmitter in Góra Kalwaria, near Warsaw, Poland at 1466km and 1427km, respectively.
- On the late evening of June 19, 2007 and into the early morning hours of June 20, 2007, three DXers in eastern Massachusetts, Jeff Lehmann, Keith McGinnis, and Roy Barstow, received FM signals from southern Florida via tropo. All three logged WEAT 104.3 West Palm Beach, Florida, and WRMF 97.9 Palm Beach, Florida, at distances of around 1,200 miles (1,931 km), and Barstow logged WHDR 93.1 Miami, Florida, at a distance of 1,210 miles (1,947 km).
- On December 3, 2007 Bulgarian dxer "FMDXBG" received Radio Militsaysk, 105.5 MHz via tropo near Gurgulica chalet in eastern Rila, at a distance of 1,312 kilometres (815 mi).
- On December 17, 2007 Polish DXer Maciej Ługowski received BBC Radio Scotland on 93,7 MHz from Keelylang Hill (Orkney Islands) transmitter in Góra Kalwaria, Poland at 1,706 km (1,060 mi) distance. BBC Scotland reception continued for next two days, as extreme tropo ducting was built over Baltic and Northern Sea. Various DXers and radioamateurs in West and Central Europe experienced the exceptionally good tropo of December 2007.
On November 3, 2008 Swedish Radio Amateur Kjell Jarl SM7GVF contacted Russian Radio Amateur RA6HHTat a distance of 2,315 km (1,438 mi) on 144Mhz.
- On the late evening of August 24 into the afternoon of August 25, 2009, a DX'er in Burnt River, Ontario, Canada, received several FM radio stations via tropo from Arkansas, Illinois, Iowa, Kansas, Michigan, Missouri, Ohio, Oklahoma, Pennsylvania, and Wisconsin.
- Mediterranean Sea is notorious for ducting tropo spanning its coasts, especially stretching east-westwards. On August 9, 2012, Greek dxer Peter "p15able" (Pyrgos, Greece) received Alger Chaîne 2 on 97.5 MHz from Doukhane, Algeria at a distance of 1,228 kilometres (763 mi).
- On September 27 and 28, 2017 various DXers in northeastern Europe were observing extreme ducting distances in VHF broadcast band. Top distance was acheived by Łukasz K. in Tomaszów Mazowiecki, Poland, who reported signals from Kolari transmitter, northern Finland at 1,798 km (1,117 mi). 
- ARRL, association for amateur radio mantains the list of North American distance records, which includes tropo results. It can be accessed by a link
- Atmospheric duct
- MW DX
- Radio propagation
- Tropospheric scatter
- Velocity of propagation
- Thermal fade
- Clear-channel station
- Federal Standard 1037C
- Looming and similar refraction phenomena
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