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Sporadic E or Es is an unusual form of radio propagation utilizing characteristics of earth's ionosphere. Whereas most forms of sky-wave propagation use the normal and cyclic ionization properties of the ionosphere to refract (or "bounce") radio signals back toward the earth surface, sporadic E propagation bounces signals off of a "cloud" of unusually ionized atmospheric gas in the ionosphere's E region (located at altitudes from about 90 to 160 km above the surface), occasionally allowing 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.

Sporadic E can happen almost any time -- hence, the name "sporadic" -- but, it does display a seasonal pattern. Sporadic E peaks predictably in the Summer, in either hemisphere. 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 often begins in mid-December in the Northern Hemisphere, with the days immediately after Christmas being the most active period.

Discovered in 1947, transequatorial spread-F (TE) propagation makes it possible for reception of television and radio stations between 3,000 – 5,000 miles (4,800 – 8,000 km) across the equator on frequencies as high as 432 MHz. Reception of lower frequencies in the 30 – 70 MHz range are most common. If sunspot activity is sufficiently high, signals up to 108 MHz are also possible. Reception of TEP signals above 220 MHz is extremely rare. Transmitting and receiving stations should be nearly equidistant from the geomagnetic equator.

The first large-scale VHF TEP communications occurred around 1957 – 58 during the peak of solar cycle 19. Around 1970, the peak of cycle 20, many TEP contacts were made between Australian and Japanese radio amateurs. With the rise of cycle 21 starting around 1977, amateur contacts were made between Greece/Italy and Southern Africa (both South Africa and Rhodesia/Zimbabwe), and between Central and South America by TEP.

There are two distinctly different types of TEP: afternoon TEP and evening TEP.

Afternoon TEP

Afternoon TEP peaks during the mid-afternoon and early evening hours and is generally limited to distances of 4,000 – 5,000 miles (6,400 – 8,000 km). Signals propagated by this mode are limited to approximately 60 MHz. Afternoon TEP signals tend to have high signal strength and suffer moderate distortion due to multipath reflections.

Evening TEP

The second type of TEP peaks in the evening around 1900 to 2300 hours local time. Signals are possible up to 220 MHz, and even very rarely on 432 MHz.

Evening TEP is quenched by moderate to severe geomagnetic disturbances. The occurrence of evening TEP is more heavily dependent on high solar activity than is the afternoon type.

During late September 2001, from 2000 to 2400 local time, VHF television and radio signals from Japan and Korea up to 220 MHz were received via evening transequatorial propagation near Darwin, Australia.