Tropospheric propagation

Lore & Background

Tropospheric propagation is directly affected by weather conditions extending over hundreds of miles. During very settled, warm anticyclonic weather (high pressure), usually weak signals from distant transmitters improve in strength. Such conditions can cause co-channel interference, such as horizontal lines or extra floating pictures with analog broadcasts and break-up with digital broadcasts. A settled high-pressure system favors signals traveling along the prevailing isobar pattern. These conditions can occur at any time, but summer and autumn are the best periods. Observable characteristics include clear, cloudless days with little wind; at sunset and sunrise, temperature gradients form an inversion layer that allows VHF and UHF signals to propagate well beyond the normal radio horizon. Fog also produces good tropospheric results due to inversion effects, often arising towards nightfall and clearing with sunrise over 4–5 hours.

Reader's Guide

Tropospheric ducting is a type of radio propagation that tends to happen during stable, anticyclonic weather. When a signal encounters a temperature inversion, the higher refractive index bends the signal. Ducting affects all frequencies, with signals enhanced up to 800 miles (1,300 km) typically, though some have received tropo beyond 1,000 miles (1,600 km). Stable signals from 500+ miles (800+ km) are not uncommon when the refractive index is high. Ducting is relatively common during summer and autumn, resulting from changes in refractive index at boundaries between air masses of different temperatures and humidities. Temperature inversions occur most frequently along coastal areas bordering large bodies of water. Higher frequencies above 90 MHz are more favorably propagated. Ideal paths are relatively flat land or sea paths; mountainous terrain can form an effective barrier. In certain parts of the world, such as the Mediterranean Sea and the Persian Gulf, ducting conditions can become established for many months, allowing regular quality reception over 1,000 miles (1,600 km). Tropospheric signals exhibit a slow cycle of fading and can produce signals strong enough for noise-free stereo, RDS data, solid HD Radio locks, noise-free color TV, stable DTV, and stable DAB reception. Virtually all long-distance reception of digital television occurs by tropospheric ducting.

Did You Know?

• Tropospheric ducting over water has produced VHF/UHF reception ranging from 1,000 to 3,000 miles (1,600–4,800 km), including between California and Hawaii, Brazil and Africa, and Australia and New Zealand.
• On June 13, 1989, Shel Remington in Keaau, Hawaii received FM signals from Tijuana, Mexico at a distance of 2,536 miles (4,081 km).
• A US listening post was built in Ethiopia to exploit a common ducting of signals from southern Russia.
• Tropospheric ducting has been occasionally observed down to 40 MHz, but signal levels are usually very weak; frequencies above 90 MHz are more favorably propagated.