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.