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A Yagi-Uda antenna. From left to right, the elements mounted on the boom are called the reflector, driven element, and director. The reflector is easily identified as being a bit (5%) longer than the driven element, and the director a bit (5%) shorter.

A Yagi-Uda Antenna, commonly known simply as a Yagi antenna or Yagi, is a directional antenna system consisting of an array of a dipole and additional closely coupled parasitic elements (usually a reflector and one or more directors). The dipole in the array is driven, and another element, slightly longer, operates as a reflector. Other shorter parasitic elements are typically 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 via 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.

Directional antennas, such as the Yagi-Uda, are also commonly referred to as beam antennas.

Yagi-Uda antenna signal-gathering action compared with other end-fire, back-fire and traveling-wave types.

Yagi-Uda antennas include one or more director elements, which, by virtue of their being arranged at approximately a one-eighth-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.

All of 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.

The antenna gain is a function of the number of dipole elements and can be approximated (for the main lobe) as

G_T = 1.66 * N

Where N is the number of elements (dipoles) in the Yagi-Uda antenna

Developed 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" or LC) 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 affect its response in the desired direction.

Shown below is the coverage that could be provided by a 3- or 4-element Yagi-Uda array on the 41-meter shortwave radio broadcasting band. The transmitter driving the array has a power output of 20 kilowatts. The array's main lobe bearing is indicated by the red line from the proposed transmitter location in British Columbia, Canada. This predicted coverage was calculated by the VOACAP program.

The Yagi-Uda antenna was invented in 1926 by Shintaro Uda of Tohoku Imperial University, Sendai, Japan, with the collaboration of Hidetsugu Yagi, also of Tohoku Imperial University. Yagi published the first English-language reference on the antenna in a 1928 survey article on short wave research in Japan 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. The Japanese military authorities first became aware of this technology after the Battle of Singapore when they captured the notes of a British radar technician that mentioned "yagi antenna". Japanese intelligence officers did not even recognise that Yagi was a Japanese name in this context. When "questioned" the technician said it was an antenna named after a Japanese professor. (This story is analogous to the story of American intelligence officers interrogating German rocket scientists and finding out that Robert Goddard was the real pioneer of rocket technology even though he was not well known in the US at that time.)

Despite its being invented in Japan, many Japanese radar engineers were unaware of the design until very late in the war, due to internal fighting between the Army and Navy. A horizontally polarized array can be seen under the left leading edge of Grumman F4F, F6F, TBF Avenger carrier based Navy aircraft. Vertically polarized arrays can be seen on the cheeks of the P-61 and on the nose cones of many WWII aircraft, notably some versions of the German Junkers Ju 88 R1 fighter-bomber, and the British Bristol Beaufighter night-fighter, and Short Sunderland flying-boat. Indeed, the latter had so many antenna elements arranged on its back it was nicknamed the "Flying Porcupine" by German airmen.

Yagi-Uda antennas are widely used by amateur radio operators worldwide for communication on frequencies from shortwave, through VHF/UHF, and into microwave bands. Hams often homebrew this type of antenna, and have provided many technical papers and software to the engineering community.

Hidetsugu Yagi attempted wireless energy transfer in February of 1926 with this antenna. Yagi and Uda published their first report on the wave projector directional antenna. Yagi managed to demonstrate a proof of concept, but the engineering problems proved to be more onerous than conventional systems.

• Antenna (radio)
• Cardioid
• Radio direction finder
• Radio direction finding

Journals

• H .Yagi, Beam transmission of ultra-shortwaves , Proceedings of the IRE, vol. 16, pp. 715-740, June 1928. The URL is to a 1997 IEEE reprint of the classic article. See also Beam Transmission Of Ultra Short Waves: An Introduction To The Classic Paper By H. Yagi by D.M. Pozar, in Proceedings of the IEEE, Volume 85, Issue 11, Nov. 1997 Page(s):1857 - 1863.
• "Scanning the Past: A History of Electrical Engineering from the Past". Proceedings of the IEEE Vol. 81, No. 6, 1993.
• Shozo Usami and Gentei Sato, "Directive Short Wave Antenna, 1924". IEEE Milestones, IEEE History Center, IEEE, 2005.
• D. Jefferies, "Yagi-Uda antennas". 2004.