Introduction
In a relatively short time, satellites have become an essential part of global communication. In 1960, the first TV satellite, named Echo, was launched. It was basically not much more than a reflector, which reflected the TV signals it received from earth. Two years later Telstar followed, which was the first so-called active TV satellite. Instead of only reflecting the incoming signals, it also converted the signals in order to avoid interference between the incoming and outgoing signals.Telstar had a rotational speed which was different from the rotational velocity of the earth, so it had to be followed very accurately by both transmission and reception stations. In 1964, this problem was solved, when the first earth-synchronous satellite, Syncom, was launched. Many others have followed since. The most well known is probably Intelsat I, which was launched in 1965. By 1969 the satellite net had expanded to a worldwide communication and TV network.
In December 1982, the Astra I satellite was launched, which generated new interest in satellites from the general public in Europe. With its coming it has become possible for people in Europe to receive TV and radio transmissions with a small dish antenna.
Communication Satellites
All current communication satellites are earth-synchronous or geo-stationary. This means they circle the earth in a specified orbit, at the same speed as the earth itself. As a result, they appear to stand still. All geo-stationary satellites revolve around the earth at a height of 36,000 km, precisely over the equator. Here, the centrifugal and gravitational forces of the earth are in equilibrium, ensuring that the satellites stay in their position and do not fall back to earth. Their speed is approximately 11,000 km per hour and the distance to Central Europe is approximately 41,000 km. As neither the distance nor the position over the equator changes, transmission and receiving stations can remain fixed, maintaining their aim at the satellite. The geo-stationary orbit where the satellites are in is also called the Clarke Belt, named after Arthur C. Clarke. He was a British writer and scientist who first proposed the idea of the geo-stationary orbit used by today's satellites.
Non-geo-stationary satellites are used for applications such as weather observations, military surveillance and experiments. Most of them orbit the earth at a lower altitude than the geo-stationary satellites. Their orbital speed must therefore be faster, or else the earth's gravity would pull them down.The Clarke Belt used by geo-stationary satellites.
Fixed Service Satellites
Fixed Service Satellites (FSS) are satellites designed to transport telephone calls, data transmission and TV signals for broadcasting and cable organizations. Because these satellites have a relatively low power output of 10-20 watts per transmitted channel, it means that a large dish antenna is required for good reception. (Less power means a weaker signal which is harder to pick up, therefore requiring a larger antenna.) However, the advantage of low power satellites is that more programs can be broadcast.
Consumer Satellites - DBS and MPS
A DBS, or Direct Broadcasting Satellite, is a satellite with high transmission powers, especially designed to transmit radio and TV programs. Because of its high power (up to ten times the power of a FSS satellite), its signals can be received with smaller dish antennae of 25-40 cm in central receiving areas.
Another kind of satellite is the Medium Powered Satellite (MPS), which is a satellite with a transmission power of 50 watts. The advantage of this type of satellite is that it has more power than a FSS and its signals can therefore be received much easier. Although it has less power than a DBS, its advantage over a DBS is that it allows the satellite to broadcast more programs. The ASTRA satellite is an example of a MPS. MPS and DBS satellites are also referred to as consumer satellites.
Satellite Positions
All the consumer satellites are located in the same geo-stationary orbit 36,000 km above the equator. Their positions vary from east to west in accordance with international agreements. These agreements about orbital positions allow several satellites to be placed in the same location, so that TV viewers can receive a greater choice of programs with a fixed dish antenna. Also, when a satellite needs to be replaced (the average lifetime of a satellite is about 15 years) the replacement satellite can be put in the same position, so that when the first one 'dies' and falls back to earth, the next one is already in place and continues to broadcast the same stations.
Consumer satellites are located above the equator, in different positions from east to west.The positions of the satellites are controlled by international agreements drawn up by the IRFB (International Radio Frequencies Board). The IRFB also coordinates the frequencies used for satellite broadcasting, to prevent interference which would be caused by two or more satellites using the same frequency. The transmission frequencies used by consumer satellites are in the KU-band, which roughly stretches from 10 to 17 GHz. The range within the KU-band that is actually used by consumer satellites is between 10.7 GHz and 12.75 GHz.
10.7 - 11.7 GHzFSS+MPS 11.7 - 12.5 DBS 12.5 - 12.75 FSS (telecommunications)
Transmission Process
Signals are sent up to the satellite from the earth's surface. The transmission station is called an uplink station. The transmission takes place via frequency modulation (FM). The advantage of FM is that there are no problems regarding the frequency and dynamic range that needs to be transmitted, plus, FM is less sensitive to interference than AM. For practical reasons, conventional TV stations broadcast in AM (called earth or terrestrial TV).The outgoing transmission takes place at a very high frequency of 14,000 MHz (= 14 Gigahertz). To avoid any interference, the incoming signal (downlink) is transmitted at a frequency between 10 and 12 GHz. This is the so-called KU band, which covers the area from 10.7-12.75 GHz. The downlink signal is sent to earth in a focused beam, via a parabolic antenna, that looks quite similar to a receiving dish antenna. From there, it can be picked up by private antenna, shared antenna installations and cable companies.
Footprint
Consumer satellites use a concentrated beam to give a stronger signal over a smaller land area. The area over which the signals can be received is called the footprint of a satellite. Footprint diagrams show the area of coverage, including the antenna size which is needed for good reception in the central and outlying areas. Under normal conditions, good reception within the footprint area is possible for as much as 99.9% of the time. However, exceptional weather conditions can have an adverse effect on reception quality for short periods.
The footprint diagram shows the area of coverage and the required antenna sizes in the central and outlying areas.
LNC
The signals received by the dish antenna are transferred to a frequency converter called the LNC (Low Noise Converter), which is placed in the focal point of the dish antenna. The LNC is also called the LNB (Low Noise Block converter). The LNC converts the incoming signal to a lower frequency in the area between 950 and 2150 MHz, and then amplifies the signal before it is sent to the satellite tuner. Due to the very weak signal levels, it is of vital importance that the amplification takes place free of noise. During the amplification of the frequencies, all frequencies will be amplified, including noise. An important performance parameter of the LNC is therefore its noise factor. The lower the noise factor, the better the picture quality. For good reception and image results, the quality of the LNC and the satellite tuner are of vital importance.
A Low Noise Converter (Low Noise Block Converter) placed in the focal point of the dish antenna.
Polarization
Polarization is a way to give transmission signals a specific direction. It makes the beam more concentrated. Signals transmitted by satellite can be polarized in one of four different ways: linear (horizontal or vertical) or circular (left-hand or right-hand). FSS satellites use horizontal and vertical polarization, whereas DBS satellites use left- and right-hand circular polarization. To use the channels that are available for satellite broadcast as efficiently as possible, both horizontal and vertical polarization (and left- and right-hand circular polarization) can be applied simultaneously per channel or frequency. In such cases the frequency of one of the two is slightly altered, to prevent possible interference. Horizontal and vertical transmissions will therefore not interfere with each another because they are differently polarized. This means twice as many programs can be transmitted per satellite. Consequently, via one and (almost) the same frequency the satellite can broadcast both a horizontal and a vertical polarized signal (H and V), or a left- and right-hand circular polarized signal (LH and RH).
TV signals transmitted by satellites can be polarized in four different ways: (1) vertical, (2) horizontal, (3) left-hand circular and (4) right-hand circular.
Types of Polarizers
In order to select either a horizontal, vertical, right- or left-hand circular signal, the LNC must be provided with a polarizer. There are three types of polarizers: mechanical, ferrite/magnetic and electrically controlled polarizers.
The mechanical polarizer is a small pulse-controlled motor which rotates a metal probe between the horizontal and vertical polarization directions. This system offers high switching precision, with low signal loss. It gives wide-band reception, covering all the different frequency bands. By adding a small circular depolarizer, the polarizer can be modified to also receive circular polarized signals.
The ferrite-magnetic polarizer has no moving parts and gives effectively instantaneous switching, combined with low signal losses. Channels need to be pre-programmed. By adding the small circular depolarizer, this type of polarizer can also be modified to receive circular polarized signals.
The 14/18V electrically controlled polarizer is integrated within the LNC, and requires no additional connection other than to the LNC over a coax cable.
The Satellite Tuner
Signals come in to the satellite tuner via the LNC. A satellite tuner next to the TV tuner is required for satellite reception. Normal TV tuners can only handle signals between 47 to 870 MHz, whereas satellite transmission takes place between 950 and 2150 MHz. TV sets cannot generate specific LNC control signals, nor handle polarization switching. Furthermore, TV tuners cannot process the audio signals from the satellite. Some TV sets and VCRs have satellite tuners built in. In addition to a satellite tuner, one may also need an additional antenna positioner (in case of a polar mount dish), a descrambler box and a smart card reader in order to receive encoded transmissions, all of which can be built into the satellite tuner.All satellite tuners are equipped with a special connection for the existing antenna or cable, which makes replugging unnecessary if you want to switch from conventional to satellite TV and vice versa.
Scrambling and Conditional Access
Not all signals picked up by a dish antenna are suitable for viewing. For several reasons TV signals can be scrambled or given conditional access and can only be watched with the help of a decoder or descrambler. These reasons might be that:
There is a distinction between scrambling and conditional access, although for the viewer without a decoder the result is the same: unclear video and/or audio signals. Scrambling is the jumbling up of a picture and/or a sound channel to make it impossible to watch or listen to a program without a decoder. Conditional access is a form of encoding to protect information with a scrambled signal that tells the decoder how to decode it. Scrambling is therefore applied to the picture, whereas conditional access is applied to the control signal. Scrambled signals require additional decoder boxes or a smart card reader for access.
- Programs are financed by viewer subscription rather than advertising revenues.
- Programs are meant for a selected audience.
- Programs to be broadcast have been acquired with copyright clearance for specific geographical areas only.
The Dish Antenna
Types of Dish Antennae
There are a number of dish antenna types. The first and simplest is the Prime Feed Focus dish, which is a parabolic dish with the LNC mounted centrally at the focus. Because the LNC is mounted centrally, it means that a lot of the incoming signals are blocked by the LNC. Its efficiency of 50% is low compared with the other types. The Prime Feed Focus dishes are mainly used for antennae with diameters over 1.4 meters. Because of its relatively larger surface, the parabolic antenna is less sensitive to small directional deviations and there is a better chance of receiving signals outside the normal footprint. On the other hand, rain and snow can easily collect in the dish and could interfere with the signal.
Prime Feed Focus Dish.The Offset Dish Antenna, has its LNC not mounted centrally, but to the side of the dish. Because the LNC no longer obstructs the signal path, the dish has a better performance than the Prime Feed Focus dish. This allows the dish diameter to be smaller. Another advantage of this type of dish is that it can be positioned almost vertically, whereas the Prime Feed Focus dish needs to be positioned more obliquely. The problem that it could collect rain and snow and give disturbance to the signals is therefore less likely to happen.
Offset Dish Antenna.The Dual Offset Dish Antenna is an improvement on the Offset Dish antenna and has an even better performance. Its efficiency is about 80%. The main feature of this antenna is that it has two dishes: a larger receiving dish and a smaller dish facing the opposite direction which collects the signals from the larger dish and directs it to the LNC.
Dual Offset Dish Antenna.The Flat Antenna is the most compact type and visually the least obtrusive. This type is best suited for receiving signals from DBS satellites in central footprint areas. The LNC is built-in.
Flat Antenna with built-in LNC.
Antenna Sizes
Dish antennae come in various types and sizes, each with their specific characteristics and purposes. The size of the dish required depends upon whether you live in a central footprint area or in an outlying area. There are three sizes: small - 60 to 70 cm diameter, medium - 90 cm and large - 1.20 to 1.50 meters. There are also smaller sized dishes, with a 45 cm diameter, but these are specifically designed for DBS satellites, which because of their high transmission power permit smaller dish antennae.
Small dish - 60 to 70 cm
- Wide opening angle (comparable with wide angle lens) and therefore quite easy to install and tune.
- Not very selective, with possibility of interference if the number of satellites is increased.
- Not very sensitive, but sufficiently sensitive to receive MPS satellites in central receiving areas.
- Thanks to its small size, it can be mounted almost anywhere, such as on a balcony.
- A relatively cheap alternative for satellite reception. For a reasonable price a complete installation including a dish antenna, LNC and satellite tuner can be purchased.
Medium-sized dish - 90 cm
- Acceptable, practical intermediate size between large and small dishes.
- Capable of receiving from many satellites.
- Rotor required.
- Stations which are more difficult to receive do not come through so well.
- The price is between the prices of the small and the large dishes. By purchasing a good quality LNC and a good satellite tuner, the various stations can be received at remarkably good quality.
Large dish - 1,20 to 1,50 m
- Small opening angle (comparable with a telephoto lens) and therefore must be installed and tuned by an expert.
- Very selective, and therefore little chance of interference.
- Only effective with a rotor.
- Much more sensitive than the small dish (hence better quality). Is also suitable to receive satellites which orbit further below the horizon and therefore transmit weaker signals.
- Wind resisting construction required due to the size.
- A large dish with a corresponding high quality LNC and a good satellite tuner will cost considerably more than a small dish.
Mounting Dishes
Before mounting a dish, there are some aspects to be taken into consideration. The dish antenna must have a clear path to the southern skies. There should not be any obstacles between the dish antenna and the satellite, such as buildings and trees. The view should be absolutely clear. The dish must be able to "see" the satellite. Provided these points are taken into consideration, it does not matter whether the antenna is installed on top of a building, on a balcony, or simply on the ground.
An antenna needs to be aligned in two planes, namely horizontally and vertically. It should make an upright angle of 30 degrees. This upright angle is called the elevation of the dish in the vertical plane. The azimuth angle is the position in the horizontal plane and determines how much the dish needs to be turned to the east or the west in order to receive the signals from the desired satellite. For optimum reception quality, the two angles must be adjusted to within a range of _1_. After the first alignment, the dish needs to be fine-tuned by trial and error, until the best signal is received.
When mounting a dish antenna, the elevation and azimuth angles must be carefully adjusted within a 1 degree range.
Polar Mount Principle
With most dish antennae you have to decide at which satellite the dish antenna is to be aimed at before installing it. An alternative to this is to have a polar mount dish. This is a dish that rotates automatically to the position of the satellite selected by the tuner, thus making it easy to tune in to new satellites without having to reinstall the dish antenna.
A polar mount antenna can rotate to the position of the satellite selected by the tuner.
Considerations Regarding Choices
A small dish can only be used to receive satellites in orbit not too far away from the south line. Outside that line the distance to the satellites soon gets much bigger and consequently the reception quality worsens. For this reason small dishes are always fixed and are only tuned once.A large dish antenna has a larger focus which can be aimed much more accurately at a cluster of satellites that orbit closely together, and therefore is more selective than a small dish. Consequently, a large dish will be less troubled than a small antenna by interference problems of the various signals. On the other hand, a mini-dish is comparatively cheap and can always be replaced by a larger dish (and another LNC). Such a small dish can be installed easily, if need be on a windowsill, and is less sensitive to various influences, such as weather conditions.
Cable or Satellite?
Which is better: cable or satellite? This is a question to be considered if you have cable TV, or if you live in an area or place where a cable system will be installed. Should one choose cable, with its subscription costs, or a satellite system, with its purchase costs and the possibly additional costs of a decoder? The following aspects may be relevant:
- A dish antenna might be of interest to people that do not have cable TV. The conventional TV antenna will still be needed to receive earth stations.
- Satellite TV offers a wider program range than cable. The cable network organization has already made a selection out of the available satellite programs, but however extensive the cable offer, its capacity remains limited.
- The quality of satellite reception will often be much better than the quality of the cable signal, provided one uses a good dish antenna plus corresponding reception installation.
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