TABLE OF CONTENTS Even though the satellite appears to be stationary for a geostationary orbit, large antennas with very narrow beam widths generally require an automatic tracking device to provide the best performance. Smaller antennas generally do not require a tracking device. Digital satellite systems for television and movies use a small antenna and do not require any tracking device. Once the antenna is installed at the site it does not have to be adjusted for several years.
The gain of a ground station antenna is largely dependent on the diameter of the antenna and is also frequency related. The following equation for the gain of the parabolic antenna is often used for satellite communications systems:
where
n=efficiency factor <1,
D=diameter of the parabolic dish,
?=wavelength.
A low-cost system operating in the Ku and Ka bands using small antennas approximately 1 to 2 meters in diameter is the very small aperture terminal (VSAT). VSATs provide two-way communications to a central location, the hub. They are used mainly for businesses, schools, and remote areas and connect remote computers and data equipment to the hub via satellites.
Satellite communications uses three types of antenna systems.
Primary focus antenna system. The feed is positioned in front of the primary reflector, and the signal is reflected once from the feed to the intended direction of radiation. The single reflector antenna system is generally less expensive and provides the simplest...
