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TV-FM DX

From Academic Kids

(Redirected from Tropo duct)
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The world's largest antenna system designed for receiving distant 88-108 MHz broadcast FM signals is located in southern Sweden.

TV DX and FM DX are two terms, customarily grouped together, that refer to long-distance reception of TV and FM radio stations, respectively. The term DX is an old telegraph term meaning "long distance." These terms refer to the active search for distant radio or television stations received during unusual tropospheric lower atmospheric weather-related, or E-layer and F2-layer upper atmospheric ionospheric conditions.

VHF/UHF television and radio signals are normally limited to a maximum "deep fringe" reception service area of approximately 40-100 miles. However, providing favourable atmospheric conditions are present, television and radio signals can sometimes be received at hundreds or even thousands of miles outside their intended coverage area. These signals are received using a large outdoor antenna system connected to a sensitive TV or FM tuner and/or receiver.

While only a limited number of local stations can be normally 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 can arise after more distant signals are either intentionally or accidentally discovered, leading to a serious interest in improving the aerial and receiving installation for the purpose of actively seeking long-range television and radio reception. The TV-FM DX hobby is somewhat similar in scope to other radio/electronic related hobbies such as amateur radio or short-wave radio, and organizations 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.[1] (http://anarc.org/wtfda)

Contents

History

After the introduction of the Alexandra Palace, London 405-line BBC channel B1 TV service in 1936, it soon became apparent that television reception was also possible well outside the original intended service area. For example, in February 1938, engineers at the RCA Research Station, Riverhead, Long Island, New York, accidentally received 3,000-mile trans-Atlantic F2 reception of the London 45.0 MHz 405-line channel B1 TV service. The flickering black-and-white footage, which is characteristic of F2 propagation, includes 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 programs. This reception was coincidentally recorded on 16mm movie film, and is now considered to be the only surviving example of pre-war live high-definition British television. [2] (http://www.apts.org.uk/recording.htm)

The BBC temporarily ceased transmissions on 1 September 1939, as World War II began. After the BBC channel B1 television service re-commenced in 1946, distant reception reports were received from various parts of the world, including Italy, South Africa, India, Middle East, North America and the Caribbean.

In May 1940, the Federal Communications Commission formally allocated the 42-49 MHz band for FM radio broadcasting. It was soon apparent that distant FM signals from up to 1,400 miles distance would often interfere with local stations during the summer months. Because the 42-49 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.

On February 1942, the first known published long-distance reception report of a FM broadcast station on 41.5 MHz was reported by FM magazine: "Zenith Radio Corporation, operating W51C, has received a letter from a listener in Monterey, 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 airline miles, from which consistent reception of the 50 kw. transmitter has been reported." [3] (http://members.aol.com/jeff560/chronofm.html)

In June 1945, the FCC decided that FM would have to move from the established 42-49 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. [4] (http://ieee.cincinnati.fuse.net/reiman/09_1994.html)

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 10,800 miles with the reception of England's BBC channel 1 in various parts of Australia. Most notably, George Palmer in Melbourne, Victoria, received viewable pictures from the BBC London channel B1 station. This BBC F2 reception was recorded on to movie film. [5] (http://www.geocities.com/toddemslie/George_Palmer_TVDX.html)

In the early 1960s, the U.K. magazine Practical Television first published a regular TV DX column edited by Charles Rafarel. By 1970, Charles' DX TV column had attracted considerable interest from TV DXers worldwide. After Charles' death in 1971, U.K. TV DXer Roger Bunney continued the monthly long distance television column, which is still in current publication.

Roger Bunney also published several TV DX books, including Long Distance Television Reception (TV-DX) for the Enthusiast 1981 ISBN 0900162716, and A TV DXer's Handbook 1986 ISBN 0-85934-150-X.

Tropospheric propagation

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Meteorologist Bill Hepburn's forecast maps often provide early indications of potential tropospheric DX openings.

The service area from a TV or FM radio transmitter extends to just beyond the optical horizon, at which point signals start to rapidly reduce in strength. Viewers living in such a "deep fringe" reception area will notice that during certain conditions weak signals normally masked by noise, increase in signal strength to allow quality reception. Such conditions are related to the current state of the troposphere.

Tropospheric propagated signals travel in the part of the atmosphere adjacent to the surface and extending to some 25,000 feet. Such signals are thus directly affected by weather conditions extending over some hundreds of miles. During very settled, warm, anti-cyclonic weather (i.e., high pressure), usually weak snowy TV signals from distant transmitters improve in signal strength. Another symptom during such conditions may be interference to the local transmitter, resulting in co-channel interference, which may be in the form of horizontal lines or an extra floating picture. A settled high-pressure system gives the classic conditions for enhanced tropospheric propagation, in particular favouring signals which travel along the prevailing isobar pattern rather than across it. Such weather conditions can occur at any time, but generally the summer and autumn months are the best periods. In certain favourable locations, enhanced tropospheric propagation may enable reception of UHF TV signals up to 1,000 miles or more.

The observable characteristics of such high-pressure systems are usually clear, cloudless days with little or no wind. At sunset the upper air cools, as does the surface temperature, but at different rates. This produces a boundary or temperature gradient, which allows an inversion level to form - a similar effect occurs at sunrise. The inversion is capable of allowing VHF and UHF signal propagation well beyond the normal radio horizon distance.

The inversion effectively reduces sky wave radiation from a transmitter - normally VHF and UHF signals travel on into space when they reach the horizon, the refractive index of the ionosphere preventing signal return. With temperature inversion, however, the signal is to a large extent refracted over the horizon rather than continuing along a direct path into outer space.

Fog also produces good tropospheric results, again due to inversion effects. Fog occurs during high pressure weather, and if such conditions result in a large belt of fog with clear sky above, there will be heating of the upper fog level and thus an inversion. This situation often arises towards night fall, continues overnight and clears with the sunrise over a period of around 4-5 hours.

Tropospheric ducting

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This example of 1,340-mile tropospheric ducting reception shows Auckland, New Zealand ch4 TV received by Robert Copeman, Sydney, Australia.

Tropospheric ducting is a type of radio propagation that tends to happen during periods of stable, anti-cyclonic weather. In this propagation method, when the signal encounters a rise in temperature in the atmosphere instead of the normal decrease (known as a temperature inversion), the higher refractive index of the atmosphere there will cause the signal to be bent. Tropospheric ducting affects all frequencies, and signals enhanced this way tend to travel up to 800 miles (though some people have received "tropo", as it is usually abbreviated, from beyond 1000 miles), while with tropospheric bending, stable signals with good signal strength from 500+ miles away are not uncommon when the refractive index of the atmosphere is fairly high.

Tropospheric ducting of UHF television signals is relatively common during the summer and autumn months, and is the result of change in the refractive index of the atmosphere at the boundary between air masses of different temperatures and humidities. Using an analogy, it can be said that the denser air at ground level slows the wave front a little more than does the rare upper air, imparting a downward curve to the wave travel.

Ducting can occur on a very large scale when a large mass of cold air is overrun by warm air. This is termed a temperature inversion, and the boundary between the two air masses may extend for 1,000 miles (1,800 km) or more along a stationary weather front.

Temperature inversions occur most frequently along coastal areas bordering large bodies of water. This is the result of natural onshore movement of cool, humid air shortly after sunset when the ground air cools more quickly than the upper air layers. The same action may take place in the morning when the rising sun warms the upper layers.

Even though tropospheric ducting has been occasionally observed down to 40 MHz, the signal levels are usually very weak. Higher frequencies above 90 MHz are generally more favourably propagated.

High mountainous areas and undulating terrain between the transmitter and receiver can form an effective barrier to tropospheric signals. Ideally, a relatively flat land path between the transmitter and receiver is ideal for tropospheric ducting. Sea paths also tend to produce superior results.

In certain parts of the world, notably the Mediterranean Sea and the Persian Gulf, tropospheric ducting conditions can become established for many months of the year to the extent that viewers receive regular quality reception of television signals over distances up to around 1,000 miles. Such conditions are normally optimum during very hot settled summer weather.

Tropospheric ducting over water, particularly between California and Hawaii, Brazil and Africa, Australia and New Zealand, Australia and Indonesia, and Bahrain and Pakistan, has produced VHF/UHF reception ranging from 1,000 to 3,000 miles (4,500 km).

Tropospheric signals exhibit a slow cycle of fading and will occasionally produce signals sufficiently strong for noise-free stereo reception on FM or noise-free TV pictures, sometimes in full color.

Virtually all long distance reception of digital television occurs by tropospheric ducting (due to most, but not all, DTV stations broadcasting in the UHF band).

Notable tropospheric DX receptions

Sporadic E propagation (E-skip)

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This example of 1,400-mile Sporadic E reception shows Te Aroha, New Zealand ch1 TV received by Todd Emslie, Sydney, Australia.

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 and scatter 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 many miles beyond their intended area of reception. E-skip is unrelated to tropospheric ducting.

By means of short-wave radio it is possible to transmit signals to distant countries around the world. Such communication is dependant upon a number of reflecting layers in the ionosphere, high above the Earth's surface known as the E, F1, and F2 layers. The E layer E region lies at an approximate distance of 65 miles (105 km) above the Earth's surface. Under normal conditions the E layer reflects Short-Wave signals (at night, when the D layer dissolves, Mediumwave signals are reflected as well). Normally, VHF and UHF signals pass through the E and F layers into outer space. At certain times however, intense patches of ionisation form in the E layer, a phenomenon known as Sporadic E. Incident VHF signals that strike these patches are reflected back to Earth. During such conditions television and radio transmissions in band 1 (45-88 MHz), band 2 (88-108 MHz), and very occasionally Band 3 (175-220 MHz), are capable of being reflected, allowing distant reception at distances over considerable distances.

Although Sporadic E can occur at any time of the year, the most active period is during the summer months, from early May to August (Northern Hemisphere), and early November to February (Southern Hemisphere). A small peak of activity is also usually noted in mid-winter.

The length of a single-hop E-skip path varies between approximately 450-1,500 miles. At times, double-hop Sporadic E can propagate signals over a 1,900-2,900 mile path. During periods of extremely widespread Es ionization, multi-hop signals up to 60 MHz have been received out to 5,000 miles.

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 polarization shift can occur, single-hop Sporadic E signals tend to remain in the original transmitted polarization. Long-skip (900-1,500 miles) Sporadic E television signals tend to be more stable and relatively free of multi-path images. Shorter-skip (400-800 miles) signals tend to be reflected from more than one part of the Sporadic E layer, resulting in multiple images and ghosting, with at times phase reversal.

Sporadic E usually affects the lower VHF band I (TV channels 2-6) and band II (88-108 MHz FM broadcast band). The typical expected distances are about 600 to 1,400 miles. However, under exceptional circumstances, a highly ionized Es cloud can propagate band I VHF signals down to approximately 350 miles. When short skip Es reception occurs, i.e., under 500 miles in band I, there is a greater possibility that the ionised Es cloud will be capable of reflecting a signal at a much higher frequency - i.e., a VHF channel 7 TV signal - since a sharp reflection angle (short skip) favours low frequencies, a shallower reflection angle from the same ionised cloud will favour a higher frequency.

At high, i.e., polar latitudes, Sporadic E can accompany auroras and associated disturbed magnetic conditions.

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

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.

Notable Sporadic E DX receptions

  • TV DX is not a new phenomenon: in 1939, there were some news reports of reception of an early Italian television service in England (about 900 miles away). [8] (http://www.etedeschi.ndirect.co.uk/database/it.tv.htm)
  • On August 2nd, 1957, the world record for high-band (channels 7-13) sporadic E television DX was extended to approximately 2,300 miles 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 30th, 1975, Glen Hauser of Enid, Oklahoma, logged WJCT-TV7, WFLA-TV8, WJHG-TV7, WFTV-TV9, & WTVT-TV13 during intense Sporadic E conditions. The distances were all around 1,020-1,090 miles. Bob Seybold also noted band III Sporadic E, Dunkirk, NY KOAM-TV7 on June 16th.
  • In June 1982, Ryn Muntjewerff (Holland) received 55.25 MHz TV-2 Guaiba, Porto Alegre, Brazil, via multi-hop Sporadic E at a distance of 6,320 miles.
  • On May 30th, 2003, Girard Westerberg [9] (http://www.dxfm.com/) made the first known reception of digital television by sporadic E when he decoded the PSIP (Program and System Information Protocol) ID of KOTA-DT (broadcasting on channel 2 in Rapid City, South Dakota) in Lexington, Kentucky, 1,062 miles away.
  • On June 26th, 2003, Paul Logan (Lisnaskea, Northern Ireland) was the first DXer to receive trans-Atlantic Sporadic E at frequencies above 88 MHz. Stations received included 88.5 MHz WHCF Bangor, Maine (2,732 miles), and 97.5 MHz WFRY Watertown, New York (3,040 miles). The WFRY-FM reception is also the current world record for 88-108 MHz Sporadic E. [10] (http://www.dxradio.co.uk/transatlanticfm.html)
  • On June 26th, 2003, several Irish and British DXers received signals from across the Atlantic via double-hop Sporadic E [11] (http://www.skywaves.info/fmta.html) (it was also achieved on June 7th [12] (http://www.skywaves.info/tadx07-06-03.html) and July 20th [13] (http://www.geocities.com/yogi540/tafm3.html) that same year).
  • On May 12th, 2004, Matthew C. Sittel [14] (http://members.cox.net/mcsdx/) of Bellevue, Nebraska received WKYC-DT [15] (http://members.cox.net/mcsdx/wkycdt02.jpg) (channel 2 in Cleveland, Ohio) via sporadic E at a distance of 740 miles. A partially decoded video frame was also obtained.
  • On May 26th, 2004, Girard Westerberg decoded several perfect frames of digital TV video, again from KOTA-DT, along with a few other DXers in his area.
  • On July 6th, 2004, a spectacular intense high MUF Sporadic E opening allowed David Pierce [16] (http://airchecksdx.com/) to receive KAKE-TV [17] (http://airchecksdx.com/tv212.htm) (channel 10, Wichita, Kansas) in Woodbridge, Virginia, 1,098 miles away, and allowed Mike Bugaj [18] (http://pages.cthome.net/fmdx) to receive KATV [19] (http://fmdx.usclargo.com/hibandes.html) (channel 7, Little Rock, Arkansas) in Enfield, Connecticut, 1,176 miles away.
  • On July 10th, 2004, Matt Sittel achieved the longest DTV reception to date [20] (http://members.cox.net/mcsdx/dxphotos02.htm), receiving KVBC-DT (channel 2, Las Vegas, Nevada) at a distance of 1,088 miles (note the NBC logo in the upper right corner of the picture).
  • On July 7th 2004, several UK TV DXers received channels A2, A3, A4 and A5 from Puerto Rico via multi-hop Sporadic-E at distances of some 4000+ miles.

F2 propagation (F2-skip)

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This example of 8,490-mile F2 reception shows Grünten Germany chE2 TV received by Tony Mann, Perth, Western Australia.

Solar activity has a cycle of approximately 11 years. During this period, sunspot activity rises to a peak and gradually falls again to a low level. When sunspot activity increases, the reflecting capabilities of the F layer surrounding the Earth enable short wave communications. The highest-reflecting layer, the F2 layer, which is approximately 200 miles above the Earth, receives ultraviolet radiation from the sun, causing ionisation of the gasses within this layer. During the winter daytime when sunspot activity is at a maximum, the F2 layer can become intensely ionised due to radiation from the sun. When solar activity is sufficiently high, the MUF (Maximum Usable Frequency) rises, hence the ionisation density is sufficient to reflect signals well into the lower 30-50 MHz VHF spectrum. Since the height of the F2 layer is some 200 miles, it follows that single-hop F2 signals will be received at thousands rather than hundreds of miles. A single-hop F2 signal will usually be around 2,000 miles minimum. A maximum F2 single-hop can reach up to approximately 3,000 miles. Multi-hop F2 propagation has enabled low-band VHF reception to over 11,000 miles.

Since F2 reception is directly related to radiation from the Sun on both a daily basis and in relation to the sunspot cycle, it follows that for optimum reception the centre of the signal path will be roughly at mid-day.

The F2 layer tends to predominately propagate signals below 40 MHz, which includes the 27 MHz CB band, and 28 MHz 10-metre Amateur band. Less frequently, television signals in the 45-55 MHz VHF band are also propagated over considerable distances. Television pictures propagated via F2 tend to suffer from characteristic multiple images and smearing.

Notable F2 DX receptions

In 1958, the FM broadcast radio DX record was set by DXer Gordon Simkin in southern California when he logged a 45 MHz commercial FM station from Korea via trans-Pacific F2 propagation at a distance of 5,000 miles.

In October 1979, Anthony Mann (Perth, Western Australia) received 48.25 MHz audio and 51.75 MHz video from the Holme Moss BBC channel B2 television transmitter. This F2 reception is a world record for reception from a BBC 405-line channel B2 transmitter. [21] (http://internal.physics.uwa.edu.au/~agm/tvdx.html)

During October to December 1979, United Kingdom DXers Roger Bunney (Hampshire, UK), Hugh Cocks (Sussex, UK), Mike Allmark (Leeds, UK), and Ray Davies (Norwich, UK) all received viewable television pictures from Australian channel TVQ 0 Brisbane (46.26 MHz) via multi-hop F2 propagation.

On January 31st, 1981, Todd Emslie, Sydney, Australia, received 41.5 MHz channel B1 television audio transmitted from Crystal Palace, London by the BBC's television service, 10,560 miles away. This BBC B1 reception was also recorded on to audio tape. [22] (http://www.geocities.com/toddemslie/index.html)

Transequatorial propagation (TEP)

Discovered in 1947, transequatorial spread-F (TE) propagation makes it possible for reception of television and radio stations between 3,000 and 5,000 miles 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 1957-1958 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 (both South Africa and Rhodesia/Zimbabwe), and between Central and South America by TEP.

There are two distinctly different types of TEP: afternoon TEP and evening TEP.

Afternoon TEP

Afternoon TEP peaks during the mid afternoon and early evening hours and is generally limited to distances of 4,000 to 5,000 miles. Signals propagated by this mode are limited to approximately 60 MHz. Afternoon TEP signals tend to have high signal strength and suffer moderate distortion due to multipath reflections.

Evening TEP

The second type of TEP peaks in the evening around 1900 to 2300 hours local time. Signals are possible up to 220 MHz, and even very rarely on 432 MHz.

Evening TEP is quenched by moderate to severe geomagnetic disturbances. The occurrence of evening TEP is more heavily dependent on high solar activity than is the afternoon type.

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, Australia. [23] (http://www.geocities.com/toddemslie/Darwin_VHFdxpedition.html)

Earth-Moon-Earth (EME) propagation (Moonbounce)

The Arecibo Radio Telescope is the world's largest antenna ever used for EME DX.
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The Arecibo Radio Telescope is the world's largest antenna ever used for EME DX.

Since 1953, radio amateurs have been experimenting with lunar communications by reflecting VHF and UHF signals off the moon. Moonbounce allows communication on Earth between any two points that can observe the moon at a common time.[24] (http://www.arrl.org/news/features/2002/01/21/1/)

Since the Moon's mean distance from the Earth is 239,000 miles, path losses are very high. It follows that a typical 240 dB total path loss places great demand on high gain receiving antennas, high power transmissions, and sensitive receiving systems. Even when all these factors are observed, the resulting signal level is often just above the noise.

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. This means that the only aspect of the TV signal that could be detected is the field scan modulation (AM vision carrier). FM broadcast signals also feature wide frequency modulation, hence EME reception is generally not possible. Indeed, there are no published records of VHF/UHF EME amateur radio contacts using frequency modulation.

While not yet confirmed, the 305-metre Arecibo radio telescope may be capable of receiving weak viewable EME television pictures. Based on mathematical calculations assuming 60 dB antenna gain, FM broadcast EME reception may also be possible using the Arecibo dish antenna.

Notable Earth-Moon-Earth (EME) DX receptions

During the mid 1970s, John Yurek, K3PGP [25] (http://www.k3pgp.org/lasereme.htm), using a home-constructed 24 foot 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. At the time of the experiment there were only two known transmitters operating in the United States on UHF television channel 68, the main reason why this channel was selected for EME experiments.

For three nights in December 1978, 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. [26] (http://history.nasa.gov/CP-2156/ch5.4.htm)

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 analyzer software, could be used to successfully detect extremely weak UHF television carriers via EME. [27] (http://internal.physics.uwa.edu.au/~agm/eme.html)

Auroral propagation

An Aurora is most likely to occur during periods of high solar activity when there is a high probability of a large solar flare. When such an eruption occurs, charging particles from the flare spiral towards the Earth arriving about a day later. These particles are affected by the various radiation belts surrounding the Earth and activity is concentrated around the Earth's magnetic poles, resulting in an aurora which disrupts short-wave communications due to ionospheric/magnetic storms in the D, E, and F layers. Various visual effects are also seen in the sky towards the north - aptly called the Northern Lights. The same effect occurs in the Southern Hemisphere, but the visual effects are towards the south.

The aurora produces a reflecting sheet which tends to lie in a vertical plane. The result of this vertical ionospheric "curtain" is reflection of signals well into the upper VHF band. Since the reflecting sheet lies towards the poles, it follows that reflected signals will arrive from that general direction. If the solar flare is extremely large, it may persist for some 27 days resulting in a second aurora when the Sun has rotated. There is a tendency for Auroras to occur around the March/April, September/October equinox periods. Signals propagated by Aurora have a characteristic hum effect, which makes vision and audio reception difficult. Vision carriers, as heard on a communications receiver, no longer can be heard as a pure tone. Often two phases occur in an aurora, one in the afternoon, and a later one during the evening period. Frequencies up to 200 MHz can be affected by auroral propagation.

Meteor scatter propagation

Meteor scatter occurs when a signal bounces off a meteor's ionized trail.

When a meteor strikes the Earth's atmosphere, a cylindrical region of free electrons is formed at the height of the E layer. This slender, ionized column is relatively long, and when first formed is sufficiently dense to reflect and scatter television and radio signals, generally 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. A signal reflected by such meteor ionisation can vary in duration from fractions of a second up to several minutes for very intense ionised trails. Frequencies in the range of 50 to 80 MHz have been found to be optimum for meteor scatter propagation. The 88-108 MHz FM broadcast band is also highly suited for meteor scatter experiments. During the major meteor showers, with extremely intense trails, band III (175-220 MHz) signal reception can occur.

Ionized trails generally reflect lower frequencies for longer periods compared to higher frequencies. For example, a 8 second burst on 45.25 MHz may only cause a 4 second burst at 90.5 MHz.

The effect of a single meteor of medium size (1 cm) 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 the Earth's atmosphere, while perhaps too weak to provide long-term ionization, is thought to contribute to the existence of the nighttime E layer.

The optimum time for receiving RF reflections off random meteors is the early morning period, when the velocity of the Earth relative to the velocity of the particles is greatest. Random meteor bursts can also be heard throughout a twenty-four hour period.

The major meteor showers are detailed below:

Satellite TVRO UHF TV DX

Although not by strict definition terrestrial TV DX, satellite UHF TVRO reception is related in certain aspects. For example, reception of satellite signals requires sensitive receiving systems and large outdoor antenna systems. However, unlike terrestrial TV DX, satellite UHF TV reception is far easier to predict. The geosynchronous satellite at 36,000 km height is a line of sight reception source. If the satellite is above the horizon, it can be generally received, if it is below the horizon, reception is not possible.

Notable Satellite TVRO UHF TV DX receptions

In December 1975, Stephen Birkhill, Sheffield, England, was the first DXer to receive viewable pictures from the 860 MHz Indian ATS-6 satellite, which was in synchronous orbit over Central Africa, for the purpose of providing educational television programs to the Indian subcontinent. [28] (http://www.rwt.co.uk/rwthist.htm)

In 1978, Ian Roberts, in South Africa, received 714 MHz UHF television pictures from the Soviet Strat-T satellite. [29] (http://www.qsl.net/zs6bte/)

See also

References

External links

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