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
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.
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
The Clarke Belt used by
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.
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
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
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
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 GHz FSS+MPS
11.7 - 12.5 DBS
12.5 - 12.75 FSS (telecommunications)
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.
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
The footprint diagram shows
the area of coverage and the required antenna sizes in the
central and outlying areas.
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
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)
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
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
The 14/18V electrically
controlled polarizer is integrated within the LNC, and requires
no additional connection other than to the LNC over a coax
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
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:
- Programs are financed by
viewer subscription rather than advertising revenues.
- Programs are meant for a
- Programs to be broadcast have
been acquired with copyright clearance for specific
geographical areas only.
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.
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
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
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
- Not very selective, with
possibility of interference if the number of satellites is
- Not very sensitive, but
sufficiently sensitive to receive MPS satellites in central
- 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
- 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.
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
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
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
- 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.
As more and more information is
being handled in digital format, the future for satellite is also
digital. In the near future, transmissions will take place in
digital format and this offers some advantages. The prime reason
for digital broadcasting is that with analog broadcasting only one
channel per transponder can be transmitted, whereas with digital
broadcasting this can be 10 channels per transponder. This means a
substantial cost reduction per channel. Due to compression
techniques, more information can be put on the same channel
bandwidth currently being used, which allows more flexibility. For
instance, the sender can opt for higher resolution, or for a lower
resolution but more channels. In general, digital broadcasting
will bring an increase of choices to consumers. Besides a likely
increase of the number of programs, the same programs will also be
broadcast several times per hour or day, to give the consumer more
flexibility in when to watch a program. Also, channels will become
increasingly focused on specific subjects, such as documentaries,
movies, sports, and perhaps even more specific than that (for
example only football or nature documentaries).