Yagi-Uda antenna

(electromagnetism) An end-fire antenna array having maximum radiation in the direction of the array line; it has one dipole connected to the transmission line and a number of equally spaced unconnected dipoles mounted parallel to the first in the same horizontal plane to serve as directors and reflectors. Also known as Uda antenna; Yagi antenna.

An antenna in which the gain of a single dipole element is enhanced by placing a reflector element behind the dipole (the driver) and one or more director elements in front of it (see illustration). It was invented in 1926 by H. Yagi and S. Uda in Japan. The gain is slightly increased by the reflector and further enhanced by the first director element. Additional director elements further increase the gain and improve the front-to-back ratio, up to a point of diminishing returns. This type of antenna has traditionally been used for local television reception. Its variants have found applications in the more modern communication systems at higher frequencies and smaller sizes, and have even been adapted to printed-circuit techniques in some applications. The same electromagnetic induction principle used in such linear elements can be applied to loop and disk elements as well with similar results. See also Directivity; Electromagnetic induction; Gain; Printed circuit.

Yagi-Uda antenna. Typical radiation patterns are shown. (a) Side view with radiation pattern in the E plane (the plane containing the electric field). (b) Top view with radiation pattern in the H plane (the plane containing the magnetic field).

Since these antennas can be made highly directive with good radiation efficiency, they have found new applications and new manufacturing techniques with miniaturization. They can be printed on microwave circuit substrates with high dielectric constants, which reduces their size even further. The parasitic electromagnetic coupling demonstrated in the Yagi-Uda antenna has been adapted to many new types of miniaturized antennas applicable to mobile communication devices in wide use, and will be used in future wireless Internet devices. See also Antenna (electromagnetism); Internet; Microwave solid-state devices; Mobile radio.