Definitions

sporadic

Sporadic E propagation

Sporadic E or Es is an unusual form of radio propagation utilizing characteristics of the earth's ionosphere. Whereas most forms of sky-wave propagation use the normal and cyclic ionization properties of the ionosphere's F region to refract (or "bounce") radio signals back toward the earth's surface, sporadic E propagation bounces signals off of smaller "clouds" of unusually ionized atmospheric gas in the lower E region (located at altitudes of approx. 90 to 160 km). This occasionally allows for long-distance communication at VHF and UHF frequencies not usually well-suited to such communication.

Communication distances of 500–1400 miles can occur utilizing a single Es cloud. This variability in distance depends on a number of factors, including cloud height and density. MUF also varies widely, but most commonly falls in the 27–110 MHz range, which includes the FM broadcast band (87.5–108 MHz), and the amateur radio 10 and 6 meter bands. Strong events have allowed propagation at frequencies as high as 250 MHz.

As its name suggests, sporadic E can happen at almost any time, but it does display seasonal patterns. Sporadic E activity peaks predictably in the summertime in both hemispheres. In North America, the peak is most noticeable in mid-to-late June, trailing off though July and into August. Interestingly, a much smaller peak is seen around the winter solstice. Activity usually begins in mid-December in the northern hemisphere, with the days immediately after Christmas being the most active period.

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Real-time Sporadic-E information

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