A Yagi-Uda antenna. From left to right, the elements mounted on the boom are called the reflector, driven element, director.A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is an 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 one holds out one's arms to form a dipole and has the reflector behind oneself, one would receive signals with maximum gain from in front of oneself.
Yagi-Uda 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. (See also log-periodic antenna.) 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.
Yagi-Uda antenna signal-gathering action compared to other end-fire, backfire and traveling-wave types.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 a resonant parallel coil and capacitor combination (called a "trap") in the elements. Traps are used in pairs on a multiband antenna. The trap serves to isolate the outer portion of the element from the inner portion for the trap design frequency. In practice, the higher frequency traps are located closest to the boom of the antenna. Typically, a triband beam will have 2 pairs of traps per element. For example, a typical triband Yagi-Uda beam covering the 10, 15 and 20 meter bands would have traps for the 10 and 15 meter bands. The introduction of traps is not without cost-due to their nature, they reduce the overall bandwidth of the antenna and overall 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 Bristol Beaufighter night-fighter and Short Sunderland flying-boat. Indeed, the latter had so many antennae elements arranged on its back it was nicknamed the "Flying Porcupine" by German airmen.