Sporadic E propagation
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Sporadic E or Es is an unusual form of radio propagation using characteristics of the Earth's ionosphere. Whereas most forms of skywave propagation use the normal and cyclic ionization properties of the ionosphere's F region to refract (or "bend") radio signals back toward the Earth's surface, sporadic E propagation bounces signals off 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 frequencies not usually well-suited to such communication.
Communication distances of 800–2200 km can occur using 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 25 – 150 MHz range, which includes the FM broadcast band (87.5–108 MHz), Band I VHF television (American channels 2-6, Russian channels 1-3, and European channels 2-4, the latter no longer used in Western Europe), CB radio (27 MHz) and the amateur radio 2-meter, 4-meter, 6-meter, and 10-meter bands. Strong events have allowed propagation at frequencies as high as 250 MHz.
As its name suggests, sporadic E is an abnormal event, but can happen at almost any time; it does, however, 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 through July and into August. A much smaller peak is seen around the winter solstice. Activity usually begins in mid-December in the southern hemisphere, with the days immediately after Christmas being the most active period.
On June 12, 2009, sporadic E allowed some television viewers in the eastern United States to see VHF analog TV stations from other states at great distances, in places and on TV channels where local stations had already done their permanent analog shutdown on the final day of the DTV transition in the United States. This was possible because VHF has been mostly avoided by digital TV stations, leaving the analog stations the last ones on the band. It is still possible (as of April, 2010) for many Americans to see Canadian and Mexican analog stations in this manner when sporadic-E occurs, until those countries do their own analog shutdowns over the following few years. In some cases it is even possible to get DTV Es receptions from well over 1000 miles (1600 km), since some US stations still use Band 1 even for DTV; these signals are characterized for being either extremely clear or extremely blocky. They are also much easier to identify.
On November 24, 2016, many radio listeners from Australia and New Zealand were able to listen to radio stations from other states of Australia, overlapping many radio signals. Many people complained about this, saying that many of their favourite radio stations got replaced by other radio stations from other states. Later, the ACMA confirmed that this was caused by Sporadic E.
Television and FM signals received via Sporadic E can be extremely strong and range in strength over a short period from just detectable to overloading. Although polarisation shift can occur, single-hop Sporadic E signals tend to remain in the original transmitted polarisation. Long single-hop (900–1,500 miles or 1,400–2,400 kilometres) Sporadic E television signals tend to be more stable and relatively free of multipath images. Shorter-skip (400–800 miles or 640–1,290 kilometres) signals tend to be reflected from more than one part of the Sporadic E layer, resulting in multiple images and ghosting, with phase reversal at times. Picture degradation and signal-strength attenuation increases with each subsequent Sporadic E hop.
Sporadic E usually affects the lower VHF band I (TV channels 2–6, E2-E4 and R1-R3) and band II (88–108 MHz FM broadcast band). The typical expected distances are about 600 to 1,400 miles (970 to 2,250 km). However, under exceptional circumstances, a highly ionized Es cloud can propagate band I VHF signals down to approximately 350 miles (560 km). When short-skip Es reception occurs, i.e., under 500 miles (800 km) in band I, there is a greater possibility that the ionized Es cloud will be capable of reflecting a signal at a much higher frequency—i.e., a VHF band 3 channel—since a sharp reflection angle (short skip) favours low frequencies, a shallower reflection angle from the same ionized cloud will favour a higher frequency. In this case even Sporadic E DVB-T reception might be possible if a mux uses VHF band 3, preferably channel E5, especially if QPSK is used, due to its low signal requirements.
At polar latitudes, Sporadic E can accompany auroras and associated disturbed magnetic conditions and is called Auroral-E.
No conclusive theory has yet been formulated as to the origin of Sporadic E. Attempts to connect the incidence of Sporadic E with the eleven-year Sunspot cycle have provided tentative correlations. There seems to be a positive correlation between sunspot maximum and Es activity in Europe. Conversely, there seems to be a negative correlation between maximum sunspot activity and Es activity in Australasia.
Equatorial E-skip is a regular daytime occurrence over the equatorial regions and is common in the temperate latitudes in late spring, early summer and, to a lesser degree, in early winter. For receiving stations located within +/− 10 degrees of the geomagnetic equator, equatorial E-skip can be expected on most days throughout the year, peaking around midday local time.
Unlike equatorial or mid-latitude Es, sporadic E propagation over polar paths is rare and gives unexpected contacts between locations surrounding the Arctic, even during periods of low solar activity.
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- Sporadic E causing strange phenomena for Aussie radio stations - Radioinfo.com.au. Retrieved 26 November 2016
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- VHF PROPAGATION, A GUIDE FOR RADIO AMATEURS (Neubeck, West, CQ Publications)
- One account of the DTV transition/Es event on June 12, 2009
- Sporadic E overview