- High gain in the direction of the satellite (because the free-space attenuation between
the satellite and the earth station is around 200 dB)
- Very low gain in all directions other than the satellite (so that the antenna does not
cause nor receive too much interference or noise)
- Good polarisation isolation (e.g. good rejection of vertically polarised signals when
receiving horizontally polarised signals)
- High efficiency
Good pointing accuracy because high antenna gain means narrow bandwidth (e.g. an antenna
with a diameter of 13m has a 3 dB beamwidth of around 0.1° at a frequency of 14 GHz)
An earth-station comprises:
- A feed and reflector system.
- A pedestal or mount which supports the reflector and feed and enables the antenna to be
accurately pointed towards the satellite.
The feed and reflector have to be made very accurately in order to maximise the gain on-axis,
minimise the sidelobes and ensure good polarisation isolation.
Large antennas (which have narrow beams) normally have tracking systems which keep the antenna
pointing at the satellite even when the satellite moves. Even for tracking antennas, the
reflector, feed and mount must be very rigid so as to ensure maximum beam stability, even
during high winds.
The most commonly used mount for large earth stations is the elevation-over-azimuth (EL-AZ)
mount. The azimuth axis is vertical and the elevation axis is horizontal and the EL-AZ mount
gives full steerability.
Single Axis Mount.
If the beam of an hour-angle/declination mount is pointed at a distant star in the equatorial
plane then the beam points parallel to the equator.
If the beam is pointed at a satellite in the GSO then the beam will point at an angle (d) to
the celestial equator. This angle varies only slightly with satellite position. The satellite
may be tracked over a limited arc of the GSO by rotating it about the hour-angle axis alone.
By tilting the axis of rotation away from the hour-angle axis it is possible to improve the