- 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.
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 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.
By Ian Poole
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