
A high-gain antenna (HGA) is an antenna with a focused, narrow radiowave beam width. This narrow beam width allows more precise targeting of the radio signal - also known as a directional antenna. Most commonly referred to during space missions, these antennas are also in use all over Earth, most successfully in flat, open areas where no mountains...
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http://en.wikipedia.org/wiki/High-gain_antenna

A dish-shaped spacecraft antenna principally used for high rate communication with Earth This type of antenna is highly directionally and must be pointed to within a fraction of a degree of Earth.
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http://www.braeunig.us/space/glossary.htm
(HGA) A type of antenna used on board spacecraft to provide high amplification of either transmitted or received radio signals. HGAs typically consist of parabolic dishes and must be accurately pointed to be useful.
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http://www.daviddarling.info/encyclopedia/H/high-gain_antenna.html
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