A directional antenna is a radio-frequency (RF) wireless antenna designed to function more effectively in some directions than in others. The purpose of that directionality is improving transmission and reception of communications and reducing interference.
In consumer applications, the most common directional antenna is the dish used with satellite Internet and satellite television installations. Other types of directional antennas, such as the Yagi antenna, quad antenna, billboard antenna, and helical antenna, are used in a variety of applications. For example, a Yagi antenna that is mounted outdoors can work well for a wireless Internet connection in a remote area, because the Yagi drastically improves the range compared with a conventional indoor antenna.
For most terrestrial wireless communications purposes, antenna directionality matters only in the azimuth, or horizontal, plane. For satellite and space-communications applications, both the azimuth and elevation (angle above the horizon) are important. A straight, vertically-oriented antenna such as a dipole measuring 1/2 wavelength from end-to-end is omnidirectional in the azimuth plane, meaning that it radiates and receives equally well in all horizontal directions. In any elevation plane, however, a vertical dipole exhibits the most gain parallel to the earth's surface and the least gain directly upward. A horizontally oriented dipole antenna produces more gain off the sides than off the ends in the azimuth plane, so it is bidirectional for terrestrial communications purposes. Horizontal dipole antennas find favor primarily among amateur radio operators.
Directional antennas usually exhibit unidirectional properties. In other words, their maximum gain (increase in efficiency) occurs in a single direction. So-called bidirectional antennas have two high-gain directions, usually oriented opposite to each other in space. An omnidirectional antenna radiates or intercepts radio-frequency (RF) electromagnetic fields equally well in all horizontal directions in a flat, two-dimensional (2D) geometric plane.
Continue reading about directional antennas:
Cisco Systems discusses the characteristics of directional antennas versus omnidirectional (nondirectional) antennas.
Engadget describes the construction of a homemade dish antenna for wireless Internet use.
YagiAntenna.net explains how a Yagi works.