Coverage area

The coverage area of the emission of a satellite corresponds to enlightened terrestrial surface by a satellite. This zone on the ground is materialized by curves of intensity of equal power, or Iso-p.i.r.e, is expressed in dBW. This formula of fast and clear presentation easily makes it possible to dimension the performance of a satellite antenna, or factor of quality (G/T).

More the WORST is the weak more parabola must have a profit (thus a diameter) important, the factor of noise of the universal Tête intervening, but little.

For the determination of the size (diameter) the parabola one is based on the following universal basic relation, knowing that the frequency is worth 12.5 GHz and that the factor of standard noise is of 0.6 dB: P.I.R.E 50 dBW = 60 cm ( or 36 dB ) is ~14.5 dBK

then, as one moves away from the center of the beam to 50 dBw, by losing 1 dBw, one adds 12.25% with the preceding diameter, for (Re) gaining 1 dB, so on… It is still noticed that each time the level of signal (WORSE) loses 6 dB, the diameter of the parabola must be doubled.

This formula of basic presentation (validated by Eutelsat and ITS, Astra) takes account of degradations hydrometeors standardized, but does not retain, because more discrete, the influence of the rise in the antenna on the factor of quality of the antenna.

Thus the diameter (round-off) of parabola standardized evolves/moves, for P.i.r.e nominal and to preserve an appreciably constant level, as follows:

49 dBw: 0.65 m
48 dBw: 0.75 m
47 dBw: 0.85 m
46 dBw: 0.95 m
45 dBw: 1.05 m
44 dBw: 1.20 m
43 dBw: 1.35 m
42 dBw: 1.50 m
41 dBw: 1.70 m
40 dBw: 1.90 m
39 dBw: 2.15 m
38 dBw: 2.40 m
37 dBw: 2.70 m
36 dBw: 3.00 m

Up to 1.80 m in diameter the parabolas with shifted hearth can bring an output of about 70%, beyond this diameter, enlightened since a centered hearth, 60 to 65%.

The satellites undergo the laws of the celestial mechanics which results in a certain instability standardized into orbit. This particularly affects their narrow beams which move thus in liking of the day, or dépointage of the sending antennas. These phenomena involve, especially on the edge of the most external, materialized coverage area or not, of the variations of intensity of signal which can reach 10 dB. One can say that more the Iso curves are brought closer more the variations are sensitive.

The profit of the antenna will have to thus be adapted according to the “WORSE practice”, therefore punctually most unfavourable, without forgetting the safety margin before dysfunction. The use of a more powerful head (0.4 dB) makes it possible to compensate for only the very limited falls of signal.

The use of parabolas of a diameter lower than 60 cm, even in the zones more strongly enlightened > 51 dBw, is disadvised, since their aperture can more not discriminate the signals (at identical frequency and polarization) produced by adjacent satellites, with 3° variation. By convention and to simplify one says that a satellite coverage area covers or lights, 100 % of a national territory, but there remains in general of very rare natural remote regions which represent in any event a marginal percentage compared to the extent of a country like the France, 555.000 km ². In Denmark, in the Netherlands, in Belgium etc the remote regions are almost non-existent, whereas in Switzerland they are more present. These natural not-enlightened zones are especially possible in the deep throats and valleys, on the sides abrupt, ~ Northern side, of the young mountains (the Alps, the Pyrenees) and as soon as the angle of the talus is higher than the local angle of elevation, towards the satellite, around 34°, but for example, via Atlantic Bird 3, the slopes can reach 41° in the Pyrenees before screening.

The satellite remote regions correspond in general to little populated zones even mainly uninhabited. The potential of 100% of hearths (or people) is thus almost reached. (99.xx%) knowing that the number of households with television sets (declared!) in France east of approximately 24 million.

The artificial remote regions due to an important construction vertical, rather high and close to a reception point, are known, but no document " national" evoke the subject, because it is about a phenomenon more located even, ex the Tour Montparnasse.

The trees, particularly in summer, a house, a wall, etc can also obstruct a reception, but one cannot enter these cases, even frequent, in the determination of the importance of a satellite coverage area.

The satellite reception, by the 3 most used satellites, Atlantic Bird 3, Hot-Bird, Astra would seem to interest 10% (approximately) of the value of the above mentioned potential.

The coverage area into terrestrial ex, TNT, is surface " vue" by the sending antennas at the top of a tower of remote transmission (see transmitting television) according to the geomorphology, by taking account of the particularism of the point reception, height of the antennas above the ground. The local obstacles create hertzian remote regions. The echoes (several shifted images), jammings, the interferences and the signals of insufficient quality, which had with an exploitation in a too weak field, obstacle or diagram of unfavourable sending antenna, in spite of the use of a powerful antenna, are entered in the remote regions VHF - UHF.

The minimum level advised at exit dipole evolves/moves around 30 dBµV or ~ 20 dB C/N (case of the own spectrum…) in TNT against > 40 dBµV in analogy.

In TNT one seldom expresses oneself in term of rate related in an coverage area, but in rate of users or hearths being able to be served by technology TNT i.e. the numerical TV by terrestrial terrestrial network, not including the TV by technology via the phone line (flow or streaming DSL) and cables it. For these 2 vectors one speaks about subscribers, therefore of subscription-based services. In France they are a few million to have chosen this type of reception which is characterized mainly by the absence of personal antennas.

Thus, the TNT will have, in the long term, a rate of initialized 85 % (for a population of 62 million people) but a deposit rate of zone about 75 %, since the transmitters of Télévision are located firstly (even only) the urbanized zones with strong population density.

The best rate of initialized is that obtained since network VHF I and III allocated with C +, since it would be close to 95% of the téléspecateurs, therefore approaching the satellite output. In UHF the output drops appreciably, average 90%, since the signals approach an optical ray more and more, i.e. the point of reception (antenna) often owes " voir" the transmitter.

See also :

The terrestrial Curve in ground network.

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