The High Gain Antenna reference article from the English Wikipedia on 24-Apr-2004
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High Gain Antenna

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The High Gain Antenna (HGA) is an antenna with a focused, narrow radiowave beam width. This narrow beam width allows for more precise targeting of where you want the radio signal to go - 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 lie to disrupt radiowaves.

See also: Low Gain Antenna