TV and FM DX
However, providing favourable atmospheric conditions are present, television and radio signals sometimes can be received hundreds or even thousands of miles outside their intended coverage area.
For example, in November 1938, engineers at the RCA Research Station, Riverhead, Long Island, accidentally received a 3,000-mile (4,800 km) transatlantic F2 broadcast of the London 45.0 MHz, 405-line BBC Television service.
The flickering black-and-white footage (characteristic of F2 propagation) included Jasmine Bligh, one of the original BBC announcers, and a brief shot of Elizabeth Cowell, who also shared announcing duties with Jasmine, an excerpt from an unknown period costume drama and the BBC's station identification logo transmitted at the beginning and end of the day's programmes.
After the BBC Television Service recommenced in 1946, distant reception reports were received from various parts of the world, including Italy, South Africa, India, the Middle East, North America and the Caribbean.
In May 1940, the Federal Communications Commission (FCC), a U.S. government agency, formally allocated the 42 – 50 MHz band for FM radio broadcasting.
It was soon apparent that distant FM signals from up to 1,400 miles (2,300 km) distance would often interfere with local stations during the summer months.
Most notably, George Palmer in Melbourne, Victoria, received viewable pictures and audio of a news program from the BBC TV London station.
[5] During the early 1960s, the U.K. magazine Practical Television first published a regular TV DX column edited by Charles Rafarel.
Sporadic E, also called E-skip, is the phenomenon of irregularly scattered patches of relatively dense ionization that develop seasonally within the E region of the ionosphere and reflect TV and FM frequencies, generally up to about 150 MHz.
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.
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.
However, under exceptional circumstances, a highly ionized Es cloud can propagate band I VHF signals down to approximately 350 miles (560 km).
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.
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).
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, Northern Territory.
Because of the low signal-to-noise ratio, as with amateur-radio practice, EME signals can generally only be detected using narrow-band receiving systems.
During the mid-1970s, John Yurek, K3PGP,[8] using a home-constructed, 24-foot (7.3 m), 0.6-focal-diameter parabolic dish and UHF TV dipole feed-point tuned to channel 68, received KVST-68 Los Angeles (1200 kW ERP) and WBTB-68 Newark, New Jersey via moonbounce.
For three nights in December 1978, astronomer Dr. Woodruff T. Sullivan III used the 305-metre Arecibo radio telescope to observe the Moon at a variety of frequencies.
This experiment demonstrated that the lunar surface is capable of reflecting terrestrial band III (175 – 230 MHz) television signals back to Earth.
In 2002, physicist Dr. Tony Mann demonstrated that a single high-gain UHF yagi antenna, low noise masthead preamplifier, VHF/UHF synthesised communications receiver, and personal computer with FFT spectrum analyser software could be used to successfully detect extremely weak UHF television carriers via EME.
The aurora-producing relativistic electrons eventually precipitate towards Earth's magnetic poles, resulting in an aurora which disrupts short-wave communications (SID) due to ionospheric/magnetic storms in the D, E, and F layers.
There is a tendency for auroras to occur around the March/April, September/October equinox periods, when the geomagnetic field is at right angle to Sun for efficient charged particle coupling.
This slender, ionized column is relatively long, and when first formed is sufficiently dense to reflect and scatter television and radio signals, generally observable from 25 MHz upwards through UHF TV, back to earth.
Consequently, an incident television or radio signal is capable of being reflected up to distances approaching that of conventional Sporadic E propagation, typically about 1500 km (1000 miles).
The effect of a typical visually seen single meteor (of size 0.5 mm) shows up as a sudden "burst" of signal of short duration at a point not normally reached by the transmitter.
The combined effect of several meteors impinging on earth's atmosphere, while perhaps too weak to provide long-term ionisation, is thought to contribute to the existence of the night-time E layer.
