Sporadic E propagation
Encyclopedia
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 27–110 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 widely used in Western Europe), CB radio (27 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 is an abnormal event, not the usual condition, 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. 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.
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.
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...
using characteristics of the Earth's ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
. 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
Maximum usable frequency
Maximum usable frequency describes, in radio transmission, using reflection from the regular ionized layers of the ionosphere, the upper frequency limit that can be used for transmission between two points at a specified time, independent of transmitter power...
also varies widely, but most commonly falls in the 27–110 MHz range, which includes the FM broadcast band
FM broadcast band
The FM broadcast band, used for FM broadcast radio by radio stations, differs between different parts of the world. In Europe and Africa , it spans from 87.5 to 108.0 megahertz , while in America it goes only from 88.0 to 108.0 MHz. The FM broadcast band in Japan uses 76.0 to 90 MHz...
(87.5–108 MHz), Band I VHF television
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
(American channels 2-6, Russian channels 1-3, and European channels 2-4, the latter no longer widely used in Western Europe), CB radio (27 MHz) and the amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
10- and 6-meter
6-meter band
The 6-meter band is a portion of the VHF radio spectrum allocated to amateur radio use. Although located in the lower portion of the VHF band, it nonetheless occasionally displays propagation mechanisms characteristic of the HF bands. This normally occurs close to sunspot maximum, when solar...
bands. Strong events have allowed propagation at frequencies as high as 250 MHz.
As its name suggests, sporadic E is an abnormal event, not the usual condition, 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. 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
Christmas
Christmas or Christmas Day is an annual holiday generally celebrated on December 25 by billions of people around the world. It is a Christian feast that commemorates the birth of Jesus Christ, liturgically closing the Advent season and initiating the season of Christmastide, which lasts twelve days...
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.
Further reading
- Mid-Latitude Sporadic-E: A Review
- Smith, Ernest K. Worldwide Occurrence of Sporadic E. U.S. Department of Commerce, National Bureau of Standards, 1957.
- DX-Sherlock's real-time Sporadic-E MUF maps
- DX-Sherlock's real-time VHF&up propagation ticker
- One account of the DTV transition/Es event on June 12, 2009
- Sporadic E overview