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TV and FM DX

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TV DX and FM DX are two terms, customarily grouped together, that refer to "long distance reception" of TV and FM radio stations, respectively. These terms refer to the search for distant radio or television stations that can be received during unusual tropospheric lower atmospheric weather-related conditions, or E-layer (E-skip) and F2-layer (F2 skip) upper atmospheric ionospheric conditions. An outdoor antenna system connected to a FM tuner or TV receiver are used to receive distant stations.

Tropospheric Propagation

Main article: Tropospheric ducting

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 (CCI), 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 accross 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 far beyond the normal radio horizon distance.

The inversion effectively reduces skywave 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

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 more favourably propagated. UHF TV frequencies are especially propagated by tropospheric modes.

High elevations and mountainous areas form an effective barrier to tropospheric signals. Thus, if your receiving location is at a low elevation, tropospheric reception may be relatively poor.

In certain parts of the world, notably the Mediterranean and the Arabian Gulf, tropospheric ducting conditions can become established for many months of the year to the extent that viewers enjoy 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).

Meteorologist Bill Hepburn's tropospheric forecast maps [1] provide a very good indication of potential tropospheric openings.

Virtually all DX reception of digital television occurs by tropospheric ducting (due to most, but not all, DTV stations broadcasting in the UHF band). Signals have a slow cycle of fading and will produce signals strong enough for noise-free stereo reception on FM or clear TV pictures, sometimes in full color.

E-skip and Sporadic E

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 high above the Earth's surface known as the E, F1, and F2 layers. The E layer lies at an approximate distance of 70 miles, and under normal conditions reflects Short Wave signals. VHF and UHF signals normally pass through the E and F2 layers into outer space. At certain times however, patches of the E layer become intensely ionised and reflect VHF signals 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 400 miles.

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 the Northern Hemisphere. Conversely, there seems to be an inverse correlation between maximum sunspot activity and Es activity in the Southern Hemisphere.

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 1,900-3,000 mile path. During periods of extremely widespread Es ionisation, 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 polarisation shift can occur, single-hop Sporadic E signals tend to remain in the original transmitted polarisation. 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.

E-skip, also called Sporadic E, 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 hundreds or even thousands of miles beyond their intended area of reception. E-skip is unrelated to tropospheric ducting.

E-skip usually affects the lower VHF channels (channels 2-6 and the FM band), and usually enhances stations from beyond 600 miles; however, under exceptional circumstances, a highly ionized cloud can propagate E-skip receptions over a distance as little as 450 miles, and can even go up to channel 10.

Another form of E-skip, called skywave, occurs every night in the mediumwave and lower shortwave bands, allowing broadcast stations and amateur radio operators on those frequencies to be heard from across the continent.

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.

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

E-skip can sometimes support reflections for distances of up to 2,400 km at frequencies up to 150 MHz or, on rare occasions, even higher.

Source: Federal Standard 1037C

Notable E-skip DX receptions

F2 propagation

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 relecting capabilities of the F2 layer surrounding the Earth, enable Short Wave communications. The Highest reflecting layer, the F2 layer, which is approximately 200 miles above the Earth, receives Ultra Violet radiation from the Sun, causing ionisation of the gasses within this layer. If Solar activity is sufficiently high the ionisation density is sufficient to reflect signals well into the lower VHF spectrum. During periods of high Solar activity, the MUF (Maximum Useable Frequency) rises and the F2 layer is able to reflect signals over considerable distances. A maximum F2 single-hop can reach up to approximately 2,800 miles.

F2 propagation tends to predominately propagate signals below 40 MHz, although F2 reception has been know to occur as high as 60 MHz. Signals have been known to travel very far through this method of propagation. Pictures propagated via F2 tend to suffer from from characteristic multiple images and smearing.

Notable F2 DX reception

On January 31, 1981, Todd Emslie [14] received 41.5 MHz AM channel B1 TV audio transmitted from Crystal Palace, London by the BBC's television service in Sydney, Australia, 10,560 miles away. (At the time, the BBC were still using VHF frequencies to broadcast television.)

Meteor scatter

Meteor scatter occurs when a signal reflects off a meteor. The signals come out as sporadic bursts.

See also

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