Yagi antenna
A Yagi-Uda Antenna, commonly known simply as a Yagi antenna, is a antenna consisting of an array of dipoles. One of the dipoles in the array is driven, as in a typical dipole antenna, and another, slightly longer, operates as a reflector. This arrangement gives the antenna directionality that a normal single-element dipole lacks. Yagis are directional along the axis perpendicular to the dipoles; if you hold out your arms to form a dipole, you would receive signals with maximum gain from in front of you.
Yagi antennas often also include one or more "director" dipoles, which, by virtue of their close proximity to each other and the driven dipole, and being slightly shorter than a half wavelength, direct signals of various frequencies onto the active dipole. This technique was commonly used on television antennas in the 1960s and 70s, which included a passive dipole tuned to each channel between 2 and 13.
All the elements usually lie in the same plane, typically supported on a single strut with the dipoles on either side. The parasitic elements do not need to be coplanar, but can be distributed on both sides of the plane of symmetry. Many Yagi-Uda antennas (including the one pictured) are designed to operate on multiple bands; the resulting design is made more complicated by the presence of coils (called "traps") in the elements.
The Yagi-Uda antenna was invented in 1926 by Shintaro Uda of Tohoku University, Sendai, Japan, with the collaboration of Hidetsugu Yagi, also of Tohoku University. Yagi published the first English-language article on the antenna in 1928 and it came to be associated with his name. However, Yagi always acknowledged Uda's principal contribution to the design, and the proper name for the antenna is, as above, the Yagi-Uda antenna (or array).
The Yagi was first widely used during World War II for airborne radar sets, because of its simplicity and directionality. Ironically many Japanese radar engineers were unaware of the design until very late in the war, due to inter-branch fighting between the Army and Navy.
Related concepts: Antenna theory
