Yagi

The Yagi, short for Yagi-Uda antenna, is a directional radio antenna widely used for television reception, amateur radio, and other point-to-point communications. It was developed in 1926 by Hidetsugu Yagi with Shintaro Uda in Japan. The antenna consists of a straight boom with multiple parallel metal elements. The driven element is typically a half-wave dipole that is fed with radio frequency current. In front of the driven element are one or more directors—slightly shorter than the driven element—while behind it is a reflector, slightly longer.

Operation relies on parasitic coupling between elements. The reflector slows currents in the rearward direction, while the directors guide energy toward the front, creating a directional radiation pattern. As more directors are added, the forward gain increases and the beam narrows, at the cost of reduced bandwidth and greater mechanical complexity. The array is inherently narrowband and is commonly matched at the feedpoint with a balun or matching network.

Performance characteristics vary with the number of elements, their lengths, and spacing, but typical three- to five-element Yagis offer gains of roughly 6 to 12 dBi, with higher-element designs achieving greater gain. They also provide a high front-to-back ratio and a relatively simple construction compared to other high-gain antennas.

Applications include television reception, VHF/UHF amateur radio work, and other communications where a directional, relatively broadband antenna is desirable. The design is widely documented and has many practical variants, reflecting its enduring utility in RF systems.