Tropospheric propagation

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 (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 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 . 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, noise-free, color TV pictures, or stable DTV reception, as well stable DAB Radio reception. With DVB-T it can also enable a wide SFN, so long as the two transmitters are within a guard interval and are almost equidistant from the receiver as well as synchronised. However, if they are not synchronised and are not equidistant they will interfere with each other. Virtually all long-distance reception of digital television occurs by tropospheric ducting (due to most, but not all, TV stations broadcasting in the UHF band). ==Notable and record distance tropospheric DX receptions==

"DXing is the art and science of listening to distant stations." The ARRL, association for amateur radio maintains the list of North American distance records, which includes tropo results. • On October 18, 1975, Rijn Muntjewerff, the Netherlands, received UHF channel E34 Pajala, Sweden, at a distance of . • On June 13, 1989, Shel Remington, Keaau, Hawaii, received several 88–108 MHz FM signals from Tijuana, Mexico, at a distance of . • 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 and low-power (LPTV) station W38BB from Raleigh, North Carolina, at a distance of • 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 (at the time, the record for UHF DTV). • On December 9, 2004, Polish DXer Maciej Ługowski received the British TV stations Channel 5 (on UHF ch.37 from the Croydon transmitter) and BBC2 (UHF ch.46 from Bluebell Hill transmitter) near Warsaw, Poland at and , respectively. • On October 15, 2006, a German DXer known on YouTube as EifelDX received the Norge Mux on channel E58, transmitter Oslo, with a distance of . • 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 , and Barstow logged WHDR 93.1 Miami, Florida, at a distance of . • On December 17, 2007, Polish DXer Maciej Ługowski received BBC Radio Scotland on 93,7 MHz from Keelylang Hill (Orkney Islands) transmitter near Warsaw, Poland at distance. BBC Scotland reception continued for next two days. • On November 3, 2008, Swedish Radio Amateur Kjell Jarl SM7GVF contacted Russian Radio Amateur RA6HHT at a distance of on 144Mhz. • On April 23, 2009, a San Antonio-area DXer received WFTS-TV 28's digital signal from Tampa, Florida, at a distance of .