An Earth-station antenna consists of many components such as reflector, feed, Low noise amplifier,BDC,HPA, BUC etc. All the components have an individual role to play and their importance in the system should not be minimized. Among all the antenna plays the main role not only provide the gain necessary to allow proper transmission and reception but also should have radiation characteristics which eliminates the unwanted signals and minimize the interference into other satellites.
Figure 1: Earth station antennas Source: General Dynamics
Antenna specifications can be grouped into several categories namely electrical or RF, control system, structural assembly, pointing and tracking accuracy, environmental requirement, and other specifications like de-icing, primary power distribution, solar radiation and seismic. Considering the total system electrical or RF specifications are the dominant than others. Primary Electrical or RF specifications can be listed as below.
- Antenna Gain
- Antenna noise temperature at different elevation angles
- VSWR ( Voltage Standing Wave Ratio)
- Power handling
- Radiation pattern
- Polarization ( Circular /Linear )
- Axial ratio ( for Circular polarization only)
- Isolation (Tx to Rx & cross pol)
- Antenna Gain to system noise temperature
All the listed parameters except the radiation pattern are determined by the system requirements. Radiation pattern should meet the minimum requirements implemented by International Telecommunications Union (ITU), Intelsat, and U.S Federal Communications Commission (FCC) some of those updated earth-station standards are listed below
- ITU-R S.580-6 Radiation diagrams for use as design objectives for antennas of earth stations operating with geostationary satellites
- FCC 25.209-Federal Communications Commission Antenna performance standards
- IEES-601- Intelsat Earth-station Standards
Above standards specifies reference envelope and permits certain extend for the side-lobs in certain angles. Basically in small antennas, due to the wide beam width, these specs should comply in-order not to interfere other satellites.
Earth-station antennas ca be grouped in to 2 categories, namely single beam and multiple beam antennas. A single beam Earth-station antenna is defined as an antenna which generates a single beam that is pointed towards one satellite. A multiple beam antenna generates multiple beams using a common reflector aperture with multiple feeds illuminating that aperture.
Single Beam Earth-station Antennas can be sub divided in to below types namely
- Axi-symmetric Dual Reflector Antennas
- Cassegrain Antennas
- Gregorian Antennas
- Prime-Focus-Fed Parabolic Antenna
- Offset-Fed Reflector Antennas
- Single Reflector Antenna
- Multiple Reflector antennas
The predominant choice of the designers of earth-station antennas has been the dual reflector antennas. Cassegrain geometry employs a convex secondary reflector while the Gregorian geometry has concave secondary reflector. Both designs has its own pros and cons over each other. The sub reflector is shaped so that the overall efficiency of the antenna has been enhanced. Aperture efficiency can be in 75 %to 80 % range and using geometrical Optics design it can be enhanced up to 90 percent.
Industrial applications General Dynamics uses Casesegrain design for medium and large earth-station antennas other-hand ASC signal (CPI) uses Gregorian design.
Prime-Focus Fed Parabolic antenna is also often employed as an earth station antenna. For moderate to large aperture sizes, this type of antenna has excellent side lobe performance in all angular region except the slip over region around the edge of the reflector.
The 3rd type of Earth station antenna is the offset fed reflector antenna, which can employ single reflector or multiple reflectors. The offset front-fed reflector consisting of a section of a paraboloidal surface minimizes diffraction scattering by eliminating the aperture blocking of the feed and feed support structure.
Side lobe levels are excellent for these type of antennas due to non blockage and aperture efficiency is within 65% to 80% for single reflector, which is much more than axi-symmetric prime focus antenna.
Offset feed dual reflector antenna exhibit high efficiency. In nutshell Offset geometry has major advantage which is completely blockage free and have excellent side lobe performance. The major disadvantage of offset feed antenna is the asymmetrical geometry which increases manufacturing cost.
Multi beam antennas basically has 2 types namely Taurus antenna and Offset-fed Multi beam cassegrain Antenna. Taurus antenna is dual curvature reflector capable of multi beam operation when it is fed with multiple feeds similar to those of the conventional spherical reflector geometry. Some times it is called Simulsat antennas, where simulsat is the manufacturer who is the giant in the Taurus earth-station antenna industry. Due to complexity in design, very few manufacturers in the industry trend to manufacturer Taurus antennas.
Since Earth-station antenna is a vast topic to discuss, from this post hope it will give a basic understanding about various types. In next post I would like to discuss antenna tracking and selecting appropriate antenna controllers for various reflector types.