VHF/UHF television and radio signals are normally limited to a maximum "deep fringe" reception service area of approximately 40 – 100 miles (60 – 160 kilometers) 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 can sometimes 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 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 listener's antenna and receiving installation for the purpose of actively seeking long-range television and radio reception. The TV-FM DX hobby is somewhat 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.
For example, in February 1938, engineers at the RCA Research Station, Riverhead, Long Island, accidentally received a 3,000-mile (4,800 km) transatlantic F2 reception of the London 45.0 MHz, 405-line channel B1 TV 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.
The BBC temporarily ceased transmissions on September 1, 1939 as World War II began. After the BBC channel B1 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) 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 (2,250 km) 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 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 miles, from which consistent reception of the 50 [kW] 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 10,800 miles (17,400 km) with the reception of England's BBC channel 1 in various parts of Australia. Most notably, George Palmer in Melbourne, Victoria, received viewable pictures and audio of a news program from the BBC London channel B1 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 December 2006, 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 (TV-DX) for the Enthusiast 1981 ISBN 0-900162-71-6, and A TV DXer's Handbook 1986 ISBN 0-85934-150-X.
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 (7,620 m). Such signals are thus directly affected by weather conditions extending over some hundreds of miles. During very settled, warm anticyclonic weather (i.e., high pressure), usually weak signals from distant transmitters improve in strength. Another symptom during such conditions may be interference to the local transmitter resulting in co-channel interference, usually horizontal lines or an extra floating picture. A settled high-pressure system gives the characteristic 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 (1,600 km) 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 is a type of radio propagation that tends to happen during periods of stable, anticyclonic 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 (1,300 km) (though some people have received "tropo" beyond 1,000 miles / 1,600 km), while with tropospheric-bending, stable signals with good signal strength from 500+ miles (800+ km) 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 (1,600 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 regularly receive quality reception of signals over distances of 1,000 miles (1,600 km). 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 (1,600 – 4,800 km).
Tropospheric signals exhibit a slow cycle of fading and will occasionally produce signals sufficiently strong for noise-free stereo, reception of RDS data, and solid locks of HD Radio streams on FM or noise-free, color TV pictures.
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.
By means of short-wave radio it is possible to transmit signals to distant countries around the world. Such communication is dependent upon a number of reflecting layers in the ionosphere, high above earth's surface known as the E, F1 and F2 layers. The E layer region lies at an approximate distance of 65 miles (105 km) above 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 reception at 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 and 1,500 miles (720 – 2,400 km). At times, double-hop Sporadic E can propagate signals over a 1,900 to 2,900-mile (3,100 – 4,700 km) path. During periods of extremely widespread Es ionisation, multi-hop signals up to 60 MHz have been received out to 5,000 miles (8,000 km).
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 / 1,450 – 2,400 km) Sporadic E television signals tend to be more stable and relatively free of multipath images. Shorter-skip (400 – 800 miles / 650 – 1,300 km) 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) and band II (88 – 108 MHz FM broadcast band). The typical expected distances are about 600 to 1,400 miles (970 – 2,250 km). However, under exceptional circumstances, a highly ionized Es cloud can propagate band I VHF signals down to approximately 350 miles (550 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.
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.
On July 20 2003, Paul Logan, Lisnaskea, Northern Ireland achieved a second reception of CBAF Moncton, New Brunswick on 88.5 MHz at 02:15 local time.
The E layer of the ionosphere is not the only layer that can reflect VHF television signals. Less frequently, the higher F2 layer can also propagate VHF signals several thousand miles beyond their intended area of reception.
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 F1 layer surrounding earth enable high frequency short-wave communications. The highest-reflecting layer, the F2 layer, which is approximately 200 miles (320 km) above earth, receives ultraviolet radiation from the sun, causing ionisation of the gases within this layer. During the daytime when sunspot activity is at a maximum, the F2 layer can become intensely ionized 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 30 – 50 MHz VHF spectrum. Since the MUF progressively increases, F2 reception on lower frequencies can indicate potential low band 45-55 MHz VHF TV paths. A rising MUF will initially affect the 27 MHz CB band, and the amateur 28 MHz 10 meters band before reaching 45-55 MHz TV. The F2 MUF generally increases at a slower rate compared to the Es MUF.
Since the height of the F2 layer is some 200 miles (320 km), 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 (3,200 km) minimum. A maximum F2 single-hop can reach up to approximately 3,000 miles (4,800 km). Multi-hop F2 propagation has enabled low-band VHF reception to over 11,000 miles (17,700 km).
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 midday.
The F2 layer tends to predominantly propagate signals below 40 MHz, which includes the 27 MHz CB band, and 28 MHz 10-metre Amateur radio band. Less frequently, television signals in the 45 – 55 MHz VHF band are also propagated over considerable distances. In North America, F2 is most likely to only affect VHF channel 2.
Television pictures propagated via F2 tend to suffer from characteristic ghosting and smearing. Picture degradation and signal strength attenuation increases with each subsequent F2 hop.
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. 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 (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.
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.
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.
Since the moon's mean distance from earth is 239,000 miles (384,000 km), 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. There are no published records of VHF/UHF EME amateur radio contacts using FM.
While not yet confirmed, the 1000 ft (305 m) Arecibo radio telescope may be capable of receiving weak EME television signals. Based on mathematical calculations assuming 60 dB antenna gain, FM broadcast EME reception may also be possible using the Arecibo dish antenna.
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.
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, charged particles from the flare may spiral towards earth arriving about a day later. This may or may not cause Aurora: if the interstellar magnetic field has same polarity, the particles do not get coupled to geomagentic field efficiently. Besides sunspot-related active solar surface areas, there are other solar pheomena that produce particles causing Auroras, such as re-occurring coronal holes spraying out intense solar wind. These charged particles are affected and captured by geomagentic field and the various radiation belts surrounding earth. 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. 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 Auroral even starts by onset of geomagnetic storm, followed by number of sub-storms over the next day or so.
The aurora produces a reflecting sheet (or metric sized columns) 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. The reflection is very aspect sensitive. Since the reflecting sheet lies towards the poles, it follows that reflected signals will arrive from that general direction. An active region or coronal hole 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, when the geomagnetic field is at right angle to Sun for efficient charged particle coupling. Signals propagated by aurora have a characteristic hum effect, which makes video and audio reception difficult. Video carriers, as heard on a communications receiver, no longer can be heard as a pure tone.
A typical radio aurora occurs in the afternoon, which produces strong and distorted signals for few hours. The local midnight sub-storming usually produces weaker signals, but with less distortion by Doppler from gyrating electrons.
Frequencies up to 200 MHz can be affected by auroral propagation.
Meteor scatter occurs when a signal bounces off a meteor's ionized trail.
When a meteor strikes 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 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. A signal reflected by such meteor ionisation can vary in duration from fractions of a second up to several minutes for intensely ionized trails. The events are classified as overdense and underdense, depending on the electron line-density (related to used frequency) of the trail plasma. The signal from overdense trail has a longer signal decay associated with fading and is a physically a reflection from the ionized cylinder surface, while an underdense trail gives a signals of short duration, which rises fast and decays exponentially and is scatter from individual electrons inside the trail.
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 (and produce stronger signals) compared to higher frequencies. For example, an 8-second burst on 45.25 MHz may only cause a 4-second burst at 90.5 MHz.
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.
The optimum time for receiving RF reflections off sporadic meteors is the early morning period, when the velocity of earth relative to the velocity of the particles is greatest which also increases the number of meteors occurring on the morning-side of the earth, but some sporadic meteor reflections can received at any time of the day, least in the early evening.
The annual major meteor showers are detailed below:
For observing meteor shower-related radio signals, the shower's radiant must be above the (propagation mid path) horizon. Otherwise no meteor of the shower can hit the atmoshere along the propagation path and no reflections from shower's meteor trails can be observed.