A Yagi-Uda Antenna, commonly known simply as a Yagi antenna, is a antenna consisting of an array of a dipole and additional parasitic elements. The dipole in the array is driven, and another element, slightly longer, operates as a reflector. Other shorter parasitic elements can be added in front of the dipole as directors. This arrangement gives the antenna directionality that a single dipole lacks. Yagis are directional along the axis perpendicular to the dipole in the plane of the elements, from the reflector through the driven element and out the director(s); if you hold out your arms to form a dipole and have the reflector behind you, you would receive signals with maximum gain from in front of you.
Yagi antennas which include one or more director elements, which, by virtue of their being arranged at approximately a quarter-wavelength mutual spacing and being progressively slightly shorter than a half wavelength, direct signals of increasingly higher frequencies onto the active dipole. Thus the complete antenna achieves a distinct response bandwidth determined by the length, diameter, and spacing of all the individual elements; but its overall gain is proportional to its length, rather than simply the number of elements.
All the elements usually lie in the same plane, typically supported on a single boom or crossbar. 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. This however reduces the efficiency of the array on any given frequency, and radically affects its response in the desired direction.
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. Arrays can be seen on the nose cones of many WWII aircraft, notably some versions of the German Junkers Ju 88 fighter-bomber and the British Avro Lancaster bomber.
Related concepts: Antenna theory
