A Yagi antenna, also known as a Yagi-Uda array or simply a Yagi, is a unidirectional antenna commonly used in communications when a frequency is above 10 MHz. This type of antenna is popular among Amateur Radio operators. It is even used at some surface installations in satellite communications systems.

A basic Yagi consists of two or three (or more) straight elements, each measuring approximately 1/2 electrical wavelengths. The feedpoint of the antenna can be either balanced or unbalanced. The Yagi is inherently a balanced antenna, but it can be fed with coaxial cable and a device called a balun at the point where the feed line joins the driven element. Simple antenna matching networks can also be used to easily convert a yagi antenna directly to an unbalanced feed. This is the most common type of feed up to at least 6 meters.

The driven element of a Yagi is the equivalent of a center-fed (balanced), 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.

The Yagi antenna not only has a unidirectional radiation and response pattern, but it concentrates the radiation and response. The more directors a Yagi has, the greater the so-called forward gain. When forward gain increases, the width of the frontal pattern sharpens considerably. As more directors are added to a Yagi, it becomes longer. Some Yagi antennas have as many as 10 or even 20 directors in addition to the driven element and one reflector. Long Yagis are rarely used below 50 MHz, because at these frequencies the structure becomes physically unwieldy.