Sporadic E propagation

Lore & Background

Sporadic E is an unpredictable event that can happen at almost any time, though it displays strong seasonal and diurnal patterns. Activity peaks predictably near the solstices in both hemispheres. In the mid-latitude Northern Hemisphere, activity usually begins in mid-May, peaks in early June, trails off after mid-July, and becomes much less reliable by early August. A smaller peak occurs during the winter solstice. For the Southern Hemisphere, the timeframes are inversed. Communication distances of 800–2,200 km can occur using a single E cloud, depending on cloud height and density. The maximum usable frequency (MUF) varies widely but most commonly falls in the 25–150 MHz range, including the FM broadcast band, VHF television bands, CB radio, and amateur radio bands. On very rare occasions, a MUF of 225 MHz can be attained.

No conclusive theory has yet been formulated as to the origin of sporadic E. One theory involves wind shear forming mid-latitude E from vertical shears in the horizontal neutral wind. Scholars also attribute other factors such as diurnal and semi-diurnal tides. Attempts to connect sporadic E with the eleven-year sunspot cycle have provided tentative correlations: a positive correlation between sunspot maximum and E activity in Europe, and a negative correlation in Australasia. Some research implies a correlation with iron/magnesium micrometeoroid ablation in the ablation zone, 100 to 140 km above the Earth's surface. Equatorial sporadic E is a regular daytime occurrence over equatorial regions, peaking around midday local time. At polar latitudes, sporadic E can accompany auroras and associated disturbed magnetic conditions and is called auroral E.

Reader's Guide

Sporadic E propagation has significant implications for long-distance VHF communication and broadcasting. Television and FM signals received via sporadic E can be extremely strong, ranging from just detectable to overloading. Single-hop E signals tend to remain in the original transmitted polarization, and long single-hop (900–1,500 mi) sporadic E television signals tend to be more stable and relatively free of multipath images. Shorter-skip (400–800 mi) signals tend to be reflected from more than one part of the sporadic E layer, resulting in multiple images and ghosting. A 1945 FCC engineering study concluded that E caused interference issues 1% of the time for a station broadcasting at 42 MHz, but only 0.01% for one at 84 MHz. Under exceptional circumstances, a highly ionized E cloud can propagate band I VHF signals down to approximately 350 mi. When short-skip E reception occurs under 500 mi in band I, there is a greater possibility that the cloud will be capable of reflecting a signal at a much higher frequency, such as VHF band 3, potentially enabling DVB-T reception. Notable events include the reception of analog TV stations from great distances during the DTV transition in the United States, and transatlantic sporadic E receptions above 88 MHz. The legacy of sporadic E continues as it allows unexpected contacts and receptions, even during periods of low solar activity, and may become more relevant with new digital television standards.

Did You Know?

• Sporadic E can support communication distances of 800–2,200 km using a single E cloud.
• The maximum usable frequency for sporadic E most commonly falls in the 25–150 MHz range, but can rarely reach 225 MHz.
• Equatorial sporadic E is a regular daytime occurrence peaking around midday local time for stations within ±10° of the geomagnetic equator.
• On June 26, 2003, a DXer in Ireland received transatlantic sporadic E signals at frequencies above 88 MHz from stations in Maine and New York.