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A Yagi-Uda antenna. From left to beam 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[1] 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, typically 5% longer, effectively operates as a reflector. Other parasitic elements shorter than the dipole may be added in front of the dipole and are referred to as directors. This arrangement gives the antenna increased directionality compared to a single dipole. Directional antennas, such as the Yagi-Uda, are also commonly referred to as beam antennas[2] or high-gain antennas (particularly for transmitting). Many common television antennas are Yagi antennas with added corner reflectors and/or UHF elements.

Contents

Description

Yagi-Uda antenna. Viewed left to right: Reflector element, driven element, director element.

Yagi-Uda antennas 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). Typically, all elements are arranged at approximately a one-quarter-wavelength mutual spacing. (See also log-periodic antenna.) All elements usually lie in the same plane, supported on a single boom or crossbar; however, they do not have to assume this coplanar arrangement. For example, some commercially available Yagi-Uda antennas for television reception have several reflectors arranged to form a corner reflector behind the dipole.

The bandwidth of a Yagi-Uda antenna, which is usually defined as the frequency range for which the antenna provides a good match to the transmission line to which it is attached, is determined by the length, diameter and spacing of the elements. For most designs, bandwidth is low, typically only a few percent of the design frequency.

Yagi-Uda antennas can be designed to operate on multiple bands. Such designs are more complicated, using pairs of resonant parallel coil and capacitor combinations (called a "trap" or LC) in the elements. The trap serves to isolate the outer portion of an element from the inner portion at 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 two pairs of traps per element. For example, a triband design covering the 10, 15 and 20 meter bands would have traps for the 10 and 15 meter bands. The use of traps is not without cost, as they reduce the bandwidth of the antenna on each band and reduce its overall efficiency.

Design

There are no simple formulas for designing Yagi-Uda antennas due to the non-linear relationships between physical parameters such as element length, diameter and position and electrical characteristics such as input impedance and gain. Consequently, designs are found experimentally either through direct measurement or computer simulation, or by modifying existing designs. A well-known reference employed in the latter approach is a report published by the National Bureau of Standards (NBS) (now the National Institute of Standards and Technology (NIST)) that provides six basic designs derived from measurements conducted at 400 Mhz and procedures for adapting these designs to other frequencies.[3] These designs, and those derived from them, are sometimes referred to as "NBS yagis."

History

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. 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. 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.)

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.

Wireless energy transfer

Hidetsugu Yagi attempted wireless energy transfer in February 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.

See also

References

External links

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