Earth Station Antenna
An Earth station antenna is the antenna used at a ground, maritime, aeronautical, transportable, or mobile terminal to transmit signals to a satellite, receive signals from a satellite, or perform both functions. It is one of the most important parts of the Earth station because it determines how effectively radio-frequency energy is directed toward the satellite on transmit and collected from the satellite on receive. Its performance has a direct effect on link margin, interference, antenna pointing requirements, terminal size, and overall satellite communications service quality.
Most fixed Earth station antennas used in traditional satellite communications are parabolic reflector antennas. A parabolic reflector concentrates energy from a feed into a narrow beam on transmission and focuses incoming energy onto the feed on reception. This provides high antenna gain, which is essential because satellite signals must travel over very long distances. Large gateway and teleport antennas may be several meters to tens of meters in diameter, while small VSAT antennas may be less than 1 m to a few meters in diameter. Receive-only television or data antennas may be smaller still, depending on satellite power, frequency band, and required performance.
The antenna’s gain is a central design parameter. Higher gain increases the effective isotropic radiated power (EIRP) on the uplink and improves the received signal level on the downlink. On receive, antenna gain is considered together with system noise temperature in the figure of merit known as G/T. A larger, more efficient, accurately pointed antenna usually provides better G/T and can support weaker satellite signals, higher data rates, or greater fade margin. However, higher gain also means narrower beamwidth, so the antenna must be pointed more accurately.
Earth station antennas must normally be aimed at the intended satellite in azimuth and elevation. For geostationary satellites, the apparent satellite position is fixed in the sky, so a fixed antenna can usually be aligned during installation and then left in place, apart from occasional adjustment. For low Earth orbit and medium Earth orbit systems, the satellite moves relative to the Earth station, so the antenna must track the satellite or use electronic beam steering. Tracking may be mechanical, electronic, or a combination of both.
Antenna pointing accuracy is especially important for transmitting Earth stations. If the antenna is mispointed, the wanted satellite receives less signal, and more energy may be radiated toward adjacent satellites or other systems. This can create interference. For this reason, Earth station antennas are often required to meet off-axis gain limits and sidelobe performance requirements. Larger antennas generally provide better discrimination between satellites in the geostationary arc, but they also require more precise mechanical alignment.
Polarization is another important feature of Earth station antennas. Satellite systems commonly use linear polarization, such as horizontal and vertical, or circular polarization, such as right-hand and left-hand circular polarization. Orthogonal polarizations allow the same frequency band to be reused, increasing capacity. The Earth station antenna must therefore be correctly aligned or configured for the required polarization. Poor polarization alignment can cause cross-polarization interference and reduce link performance.
Earth station antennas vary according to application. A large gateway antenna may use a high-performance reflector with precise tracking, redundant radio-frequency equipment, and environmental control. A VSAT antenna may be designed for low cost and easy installation. A maritime antenna may be stabilized and enclosed in a radome so that it can remain pointed at the satellite as the vessel rolls, pitches, and yaws. An aeronautical antenna may be low-profile and aerodynamically faired. A modern LEO broadband terminal may use an electronically steered flat-panel array rather than a dish.
The antenna system includes more than the radiating aperture. It may include the feed, orthomode transducer, polarizer, diplexer, low-noise block converter, block upconverter, high-power amplifier, waveguide, cables, pedestal, tracking controller, radome, and grounding or lightning protection. Losses between the antenna feed and the receiver or transmitter are important because they reduce EIRP on transmit and degrade G/T on receive. For receive systems, losses ahead of the low-noise amplifier are particularly harmful because they attenuate the wanted signal before low-noise amplification.
Environmental and installation factors also affect Earth station antenna performance. Wind, rain, ice, snow, salt spray, temperature changes, foundation movement, mast flexure, and nearby obstructions can reduce gain, alter pointing, increase noise, or cause signal blockage. At higher frequencies, such as Ku-band and Ka-band, surface accuracy, radome loss, rain attenuation, and pointing accuracy become increasingly important.
In satellite communications, the Earth station antenna is therefore not simply a dish or terminal accessory. It is the radio-frequency aperture that links the terrestrial equipment to the satellite. Its size, gain, beamwidth, polarization, sidelobe pattern, pointing accuracy, and environmental design are central to reliable satellite link operation.