YagiUda

The Yagi-Uda antenna, commonly called a Yagi antenna, is a directional radio-frequency antenna that uses a single driven element, typically a dipole, along with one reflector element behind it and one or more director elements in front. The arrangement creates a strong radiation pattern in the forward direction due to the interaction of currents on the parasitic elements, providing increased gain and a favorable front-to-back ratio compared with an isolated dipole. All elements are mounted on a supporting boom and positioned with precise spacing.

The design was developed in 1926 by Hidetsugu Yagi and Shintaro Uda at the Tohoku Imperial University in Japan. The original work demonstrated a highly directional antenna that could achieve significant gain with relatively simple construction, making it suitable for radio reception and transmission experiments of the time. The concept quickly spread worldwide and became a standard approach for many RF applications.

Typical construction and operation: the driven element is about one-half wavelength long at the operating frequency. The reflector is slightly longer than the driven element, and each director is slightly shorter. Element spacing is usually about 0.15 to 0.25 wavelengths. The feed is commonly connected via coax or a balun to match the transmission line to the driven element. The simplicity and high gain of Yagi antennas have led to widespread use in television reception, amateur radio, point-to-point links, and various wireless systems.

Yagi antennas are inherently narrowband but can be made with multiple directors to increase gain; they remain popular for their directional performance and ease of construction.