Satellite antenna
- Overview, of the typical RF antenna design types used with satellites,
both on the ground and on the satellite. This includes satellite television (tv)
reception.
A variety of forms of antenna can be used for transmitting to
and receiving from satellites. The most common type of satellite antenna is the
parabolic reflector, however this is not the only type of antenna that can be
used. The actual type of antenna will depend upon what the overall application
and the requirements.
Antenna gain
The distances over which signals travel to some satellites is
very large. Geostationary ones are a particular case. This means that path
losses are high and accordingly signal levels are low. In addition to this the
power levels that can be transmitted by satellites are limited by the fact that
all the power has be generated from solar panels. As a result the antennas that
are used are often high gain directional varieties. The parabolic reflector is
one of the most popular.
Antennas on satellites
Although there is fundamentally no difference between the
antennas on satellites and those on the ground there are a number of different
requirements that need to be taken into account. In the first instance the
environmental conditions are very different. As conditions in space are
particularly harsh the antennas need to be built to withstand this. Temperatures
vary considerably between light and dark and this will cause expansion and
contraction. The materials that are sued in the conduction need to be carefully
chosen.
The gain and directivity of the antenna need to be chosen to
meet the needs of the satellite. For most geostationary satellites the use of
directional antennas with gain is mandatory in view of the path losses incurred.
These satellites are more likely to cover a give area of the Earth, and as they
remain in the same position this is normally not a problem. However the attitude
of the satellite and its antenna must be carefully maintained to ensure the
antenna is aligned in the correct direction. The antennas on board the satellite
are typically limited in size to around 2 - 3 metres by the space that is
available on the satellite structure.
For satellites in low earth orbits, considerably less
directive antennas are normally used. Signals are likely to be received and
transmitted over a much wider angle, and these will change as the satellites
move. Accordingly these satellites seldom use parabolic reflector antennas.
Ground antennas
Ground antennas used for receing satellite signals and
transmitting to the satellites vary considerably according to their application.
Again parabolic reflectors are the most widely used, but Yagi antennas may be
used on occasions.
The size of the antennas may vary considerably. The parabolic
reflectors used for satellite television reception are very small. However those
used for professional applications are much larger and may range up to several
tens of metres in size.
The satellite antennas are carefully chosen by the system
designer to match the particular requirements. It is possible to calculate the
exact specification for the antenna, knowing the path loss, signal to noise
ratio, transmitter power levels, receiver sensitivities, etc. A small 70
centimetre antenna may be sufficient for direct reception of satellite TV
programmes but would not be suitable for transmitting programmes up to the
satellite where a much higher signal level is required to ensure the best
possible picture is radiated back to Earth.
Satellite television antennas
It has already been mentioned that satellite television
antennas use parabolic reflector or "dish" antennas. They are also incorporate
what is termed an LNB. This is a Low Noise Block converter. The satellite
transmits signals at frequencies between 12.2 and 12.7 GHz. Signals at these
frequencies would be very quickly attenuated by any coaxial feeder that was
used. As feeder lengths may run into several metres or more in many
installations, this would mean that the signals that reached the television
would be very weak. To overcome this problem the LNB is installed at the feed
point of the antenna. Its job is two fold. It amplifies the signal, but more
importantly it converts it down to a frequency (usually 950 to 1450MHz) where
the loss introduced by the coaxial feeder is considerably less. The
amplification provided by the LNB also enables the loss introduced by the cable
to be less critical. By performing these two functions it means that domestic
coaxial cable can be used satisfactorily, while maintaining sufficiently high
signal levels at the receiver.
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