A basic Yagi consists of two or three straight elements, each measuring approximately1/2 electrical wavelengths. The antenna can be balanced or unbalanced. The Yagi is inherently a balanced antenna, but it can be fed with coaxial cable and a device called a balunat the point where the feed line joins the driven element.
The driven element of a Yagi is the equivalent of a center-fed, half-wave dipole antenna. Parallel to the driven element, and approximately 0.2 to 0.5 wavelength on either side of it, are straight rods or wires called reflectors and directors. A reflector is placed behind the driven element and is slightly longer than 1/2 wavelength; a director is placed in front of the driven element and is slightly shorter than 1/2 wavelength. A typical Yagi has one reflector and one or more directors. The antenna propagates electromagnetic field energy in the direction running from the driven element toward the director(s), and is most sensitive to incoming electromagnetic field energy in this same direction.
|Radiation pattern of Yagi-Uda antenna|
|Animation of radiation pattern of Yagi-Uda antenna|