TV and FM DX

TV DX and FM DX is the active search for distant radio or television stations received during unusual atmospheric conditions. The term DX is an old telegraphic term meaning "long distance."
VHF/UHF television and radio signals are normally limited to a maximum "deep fringe" reception service area of approximately in areas where the broadcast spectrum is congested, and about 50 percent farther in the absence of interference. 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. These signals are often received using a large outdoor antenna system connected to a sensitive TV or FM receiver, although this may not always be the case. Many times smaller antennas and receivers, such as those in vehicles, will receive stations farther than normal depending on how favourable conditions are.
While only a limited number of local stations can normally be received at satisfactory signal strengths in any given area, tuning into other channels may reveal weaker signals from adjacent areas. More consistently strong signals, especially those accentuated by unusual atmospheric conditions, can be achieved by improving the antenna system. The development of interest in TV-FM DX as a hobby has grown over time, with enthusiasts installing and upgrading HF/UHF antennae for the purpose of gaining a higher range. The TV-FM DX hobby is similar to other radio/electronic related hobbies such as amateur radio, Medium Wave DX, or short-wave radio; and organisations such as the Worldwide TV-FM DX Association have developed to coordinate and foster the further study and enjoyment of VHF/UHF television and FM broadcast DX.

History

After the Alexandra Palace, London 405-line BBC channel B1 TV service was introduced in 1936, it soon became apparent that television could be received well outside the original intended service area.
For example, in November 1938, engineers at the RCA Research Station, Riverhead, Long Island, accidentally received a 3,000-mile transatlantic F2 broadcast of the London 45.0 MHz, 405-line BBC Television service.
The flickering black-and-white footage 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.
This reception was recorded on 16 mm movie film, and is now considered to be the only surviving example of pre-war, live British television.
The BBC temporarily ceased transmissions on September 1, 1939 as World War II began. 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, 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 distance would often interfere with local stations during the summer months.
Because the 42 – 50 MHz FM signals were originally intended to only cover a relatively confined service area, the sporadic long-distance signal propagation was seen as a nuisance, especially by station management.
In February 1942, the first known published long-distance FM broadcast station reception report was reported by FM magazine. The report provided details of 45.1 MHz W51C Chicago, Illinois, received in Monterrey, Mexico: "Zenith Radio Corporation, operating W51C, has received a letter from a listener in Monterrey, Mexico, telling of daily reception of this station between 3:00 P.M. and 6:00 P.M. This is the greatest distance, 1,100 miles, from which consistent reception of the 50 transmitter has been reported."
In June 1945, the FCC decided that FM would have to move from the established 42 – 50 MHz pre-war band to a new band at 88 – 108 MHz. According to 1945 and 1946 FCC documents, the three major factors which the commission considered in its decision to place FM in the 88 – 108 MHz band were sporadic E co-channel interference, F2 layer interference, and extent of coverage.
During the 1950s to early 1960s, long-distance television reports started to circulate via popular U.S. electronics hobbyist periodicals such as DXing Horizons, Popular Electronics, Television Horizons, Radio Horizons, and Radio-Electronics. In January 1960, the TV DX interest was further promoted via Robert B. Cooper's regular DXing Horizons column.
In 1957, the world record for TV DX was extended to with the reception of Britain's channel BBC TV in various parts of Australia. Most notably, George Palmer in Melbourne, Victoria, received viewable pictures and audio of a news program from the BBC TV London station. This BBC F2 reception was recorded on movie film.
During the early 1960s, the U.K. magazine Practical Television first published a regular TV DX column edited by Charles Rafarel. By 1970, Rafarel's column had attracted considerable interest from TV DXers worldwide. After Rafarel's death in 1971, UK TV DXer Roger Bunney continued the monthly column, which continued to be published by Television Magazine. With the demise of Television Magazine in June 2008, Bunney's column finished after 36 years of publication. In addition to the monthly TV DX column, Bunney has also published several TV DX books, including Long Distance Television Reception for the Enthusiast 1981, and A TV DXer's Handbook 1986.

Tropospheric propagation

refers to the way radio signals travel through the lowest layer of the Earth's atmosphere, the troposphere, at altitudes up to about to 17 km. Weather conditions in the lower atmosphere can produce radio propagation over greater ranges than normal. If a temperature inversion occurs, with upper air warmer than lower air, VHF and UHF radio waves can be refracted over the Earth's surface instead of following a straight-line path into space or into the ground. Such "tropospheric ducting" can carry signals for 800 km or more, far beyond usual range.

F2 propagation

The F2 layer is found about 200 miles above the Earth's surface and can reflect radio waves back toward the Earth. When the layer is particularly strong during periods of high sunspot activity, FM and TV reception can take place over 2000 miles or more, as the signal effectively "bounces" off the high atmospheric layer.

Sporadic E propagation (E-skip)

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. When frequencies reflect off multiple patches, it is referred to as multi-hop skip. E-skip allows radio waves to travel a thousand miles or even more beyond their intended area of reception. E-skip is unrelated to tropospheric ducting.
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 Sporadic E television signals tend to be more stable and relatively free of multipath images. Shorter-skip 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 and band II. The typical expected distances are about. However, under exceptional circumstances, a highly ionized Es cloud can propagate band I VHF signals down to approximately. When short-skip Es reception occurs, i.e., under 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 favours low frequencies, a shallower reflection angle from the same ionized cloud will favour a higher frequency.
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.

Transequatorial propagation (TEP)

Discovered in 1947, transequatorial spread-F propagation makes it possible for reception of television and radio stations between 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, and between Central and South America by TEP.
"Afternoon" and "evening" are two distinctly different types of trans-equatorial propagation.