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 late July and into early 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. As of April, 2010, it was possible for many Americans to see Canadian and Mexican analog 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.
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). A 1945 FCC engineering study concluded that Sporadic E caused interference issues 1% of the time for a station broadcasting at 42 MHz, but only .01% for one at 84 MHz. 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.
Notable sporadic E DX receptions
- In 1939, there were some news reports of reception of an early Italian television service in England about 900 miles (1,400 km) away.
- The Medford Mail Tribune in Medford, Oregon reported on June 1, 1953, that KGNC-TV, Channel 4 in Amarillo, and KFEL-TV, Channel 2 from Denver had been received on the Trowbridge and Flynn Electric Company's television set at their Court Street warehouse and, with a pre-amplifier, a New York station's test pattern was reportedly picked up.
- The June 4, 1953 issue of the Brimfield News in Brimfield, Illinois reported that area residents "'saw' Salt Lake City Monday (via Television)." It reported that a local farm family witnessed interference to WHBF-TV, Channel 4 of Rock Island, Illinois by KDYL-TV in Salt Lake City, which "blocked out all their favorite programs."
- On August 2, 1957, the world record for high-band (channels 7 – 13) sporadic E television DX was extended to approximately 2,300 miles (3,700 km) with the reception of the YVLV channel 9 relay from Maracaibo, Venezuela, by Bobby Grimes in Little Rock, Arkansas. Two hours later, Bedford Brown of Hot Springs, Arkansas, also received the channel 9 station, along with multi-hop sporadic E reception from Venezuela on channels 2, 4 and 5. Brazilian television on channel 2 and Argentina on channel 3 were also received via transequatorial propagation (TEP).
- On June 30, 1975, Glenn Hauser of Enid, Oklahoma, logged WJCT-TV 7, WFLA-TV 8, WJHG-TV 7, WFTV-TV 9, & WTVT-TV 13 during intense Sporadic E conditions. The distances were all around 1,020–1,090 miles (1,640–1,750 km). Bob Seybold in Dunkirk, NY also noted band III Sporadic E, from KOAM-TV 7 on June 16.
- In mid-July 1979, Several residents of Laurel, MD, reported reception of Channel 3 stations: KDAL-TV (now KDLH), Duluth, MN; KMTV, Omaha, NE; and KTBS-TV, Shreveport, LA during a sporadic E propagation event that was reinforced by near-peak sunspot activity that summer. At the end of the KTBS-TV 5PM newscast, one of their anchors was heard to announce "We'd like to welcome some of our new viewers in Laurel, MD, tonight...".
- In June 1981, Rijn Muntjewerff (the Netherlands) received 55.25 MHz TV-2 Guaiba, Porto Alegre, Brazil, via a combination of sporadic E and afternoon TEP at a distance of 6,320 miles (10,170 km).
- On May 30, 2003, Girard Westerberg made the first known reception of digital television by sporadic E when he decoded the PSIP ID of KOTA-DT (broadcasting on channel 2 in Rapid City, South Dakota) in Lexington, Kentucky, 1,062 miles (1,709 km) away.
- On June 11, 2003, Laszlo Tringer (Enying, Hungary) received Saudi Radio's Holy Quran programme from Al Baha transmitter site 2,274 miles or 3,660 kilometres away. It's interfered with local station "Blue Danube".
- On June 26, 2003, Paul Logan (Lisnaskea, Northern Ireland) was the first DXer to receive transatlantic Sporadic E at frequencies above 88 MHz. Stations received included 88.5 MHz WHCF Bangor, Maine (2,732 miles or 4,397 kilometres), and 97.5 MHz WFRY Watertown, New York (3,040 miles or 4,890 kilometres). David Hamilton from Cumnock in Ayrshire, Scotland received CBTB from Baie Verte, Newfoundland and Labrador, Canada on 97.1 MHz on this day also. 
- On July 20, 2003, Jozsef Nemeth from (Győr, Hungary) received TR3 Radio Miras on OIRT FM 70.61 MHz from Türkmenistan, Uly Balkan transmitter 1,895 miles or 3,050 kilometres away.
- On July 10, 2004, Matt Sittel achieved what was then the longest DTV reception, receiving KVBC-DT (channel 2, Las Vegas, Nevada) at a distance of 1,088 miles (1,751 km) (note the NBC logo in the upper right corner of the picture).
- On July 7, 2004, several UK TV DXers received channels A2, A3, A4 and A5 from Puerto Rico via multi-hop Sporadic E at distances of some 4,000+ miles (6,400+ km).
- On June 15, 2005, Danny Oglethorpe in Shreveport, Louisiana received a test signal from KBEJ-TV (channel 2, Fredericksburg, Texas) by Sporadic E at a very short distance for this propagation mode: 327 miles (526 km).
- On August 3, 2007, Albu Daniel from Bucharest, Romania received via multi-hop Sporadic E CCTV1 channel C1 Baoji, Shaanxi, China at distance 6,784 kilometres (4,215 mi).
- In the summer of 2008, the recent introduction of the Coupon-eligible converter box, which was highly affordable, and tolerated multipath interference better than older ATSC receivers, allowed many DXers in North America to receive and identify ATSC digital TV signals by Sporadic-E, something that had been very difficult to do in previous years.
- On July 7, 2008, Daniel Albu from Bucharest, Romania received via multi-hop Sporadic E SNRT Cahin (90,4 MHz) El Jadida Morocco at a distance of 3,216 kilometres (1,998 mi).
- On May 25, 2009 Daniel Albu from Bucharest, Romania received 2 radio stations from the United Arab Emirates, Radio Aziziah 88,7 MHz, and Holy Qu'ran Radio 88,2 MHz from Dubai at a distance of 3,369 kilometres (2,093 mi).
- On June 26, 2009, Paul Logan (Lisnaskea, Northern Ireland) had transatlantic Sporadic-E receptions on the FM band from eight US States and one Canadian Province. The most distant signal received was that of 90.7 WVAS Radio in Montgomery, Alabama at 6456 km / 4012 miles. This reception was recorded and later confirmed by WVAS Newsreader Marcus Hyles.
- A new world distance record for FM reception via Sporadic-E of 4,302 miles (6,923 km) was achieved by Mike Fallon in Sussex, England on May 31, 2010 when the religious station La Voz de la Luz in Salvaléon de Higüey, Dominican Republic was received and recorded on 88.7 MHz from 12:48 UTC for approximately 20 minutes. The recording was verified by the station to be their output. The station transmitted at 1 kW.
- On May 31, 2010, Paul Logan (Lisnaskea, Northern Ireland) received signals from 89.7 WRTU Radio, San Juan, Puerto Rico at a distance of 3,946 miles (6,350 km). The reception was recorded and later confirmed by the station's Program Director Carlos R. Camunas.
- 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.
- On May 26, 2021, Mathew Amador (Palmdale, California) received a multi-hop Sporadic-E reception of WBVM, located in Tampa, Florida, at a distance of 2,153 miles (3,465 kilometers). Confirmation of the station was through the RDS (Radio Data System) with a valid Program Identification (PI) code of 597A from the originating station. 
- amfmdx.net. Sporadic E Reference Page. "Archived copy". Archived from the original on 2007-06-24. Retrieved 2007-08-20.CS1 maint: archived copy as title (link) Accessed 3 July 2008.
- "FCC Expected To Decide FM's Place in Spectrum About May 1", Broadcasting, April 23, 1945, page 20.
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- "Early Television in Italy". 16 March 2007. Archived from the original on 16 March 2007.
- Ronald Kramer. "History of Television in Southern Oregon". Western States Museum of Broadcasting. Archived from the original on 2016-01-12. Retrieved 2009-08-16.
- "The Brimfield News". Retrieved 2020-08-06.
- "Rijn Muntjewerff's 1961–2005 TV DX". Todd Emslie's TV DX Page. Retrieved August 29, 2005.
- Welcome to DX FM Archived 2009-04-12 at the Wayback Machine
- AM/FM DX Hungary (2 May 2010). "RetroDX: Saudi Arabia with double-hop e-skip in 2003!" – via YouTube.
- "Trans-Atlantic FM 26 June 2003". www.dxradio.co.uk.
- "mp3 recording from 70.61 MHz". ucoz.hu.
- "KBEJ-2 via Es". www.tvdxtips.com.
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- "shinese.png – Click to see more photos". ImageShack.
- "YouTube". www.youtube.com.
- Sound Haaly Q'uran 88,2 MHz[permanent dead link].
- Sporadic E causing strange phenomena for Aussie radio stations - Radioinfo.com.au. Retrieved 26 November 2016
- SkyWaveBirdDX FM Logs - skywavebirddx.neocities.org Retrieved 31 May 2021
- Davies, Kenneth (1990). Ionospheric Radio. IEE Electromagnetic Waves Series #31. London, UK: Peter Peregrinus Ltd/The Institution of Electrical Engineers. pp. 184–186. ISBN 978-0-86341-186-1.
- Radio Science (1972): "Special Issue on sporadic E", 7, (3)
- Radio Science (1975): "Special Issue on recent advances in the physics and chemistry of the E region", 10, (3)
- Mid-Latitude Sporadic-E: A Review
- Smith, Ernest K. Worldwide Occurrence of Sporadic E. U.S. Department of Commerce, National Bureau of Standards, 1957.
- 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