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A three-element Yagi–Uda antenna used for long-distance communication in the bands by an station. The longer reflector element ( left), the ( centre), and the shorter director ( right) each have a so-called trap (parallel ) inserted along their conductors on each side, allowing the antenna to be used on more than one frequency band. A Yagi–Uda antenna, commonly known as a Yagi antenna, is a consisting of multiple parallel elements in a line, usually made of metal rods. Yagi–Uda antennas consist of a single connected to the or with a, and additional ' which are not connected to the transmitter or receiver: a so-called and one or more directors.
It was invented in 1926 by of, and (with a lesser role played by his colleague). The reflector element is slightly longer than the driven dipole, whereas the directors are a little shorter. The parasitic elements absorb and reradiate the radio waves from the driven element with a different, modifying the dipole's. The waves from the multiple elements and to enhance radiation in a single direction, achieving a very substantial increase in the antenna's compared to a simple dipole. Also called a 'beam antenna', or 'parasitic array', the Yagi is very widely used as a high-gain antenna on the, and bands. It has moderate to high which depends on the number of elements used, typically limited to about 20, unidirectional (end-fire) beam pattern with high of up to 20 db.
And is lightweight, inexpensive and simple to construct. The of a Yagi antenna, the range over which it has high gain, is narrow, a few percent of the center frequency, and decreases with increasing gain, so it is often used in fixed-frequency applications. The largest and best-known use is as rooftop terrestrial, but it is also used for point-to-point fixed communication links, in radar antennas, and for long distance communication by shortwave broadcasting stations.
Contents. Origins The antenna was invented in 1926 by of, with a lesser role played by his colleague. However the 'Yagi' name has become more familiar with the name of Uda often omitted. This appears to have been due to Yagi filing a patent on the idea in Japan without Uda's name in it, and later transferring the patent to the in the UK.
Yagi antennas were first widely used during in systems by the British, US, Germans and Japanese. After the war they saw extensive development as home.
Description. Yagi–Uda antenna with a ( left), half-wave driven element ( centre), and ( right). Exact spacings and element lengths vary somewhat according to specific designs. The Yagi–Uda antenna consists of a number of parallel thin rod elements in a line, usually half-wave long, typically supported on a perpendicular crossbar or 'boom' along their centers. There is a single driven in the center (consisting of two rods each connected to one side of the transmission line), and a variable number of, a single reflector on one side and optionally one or more directors on the other side.
The parasitic elements are not electrically connected to the transmitter or receiver, and serve as, reradiating the radio waves to modify the. Typical spacings between elements vary from about 1⁄ 10 to ¼ of a wavelength, depending on the specific design.
The directors are slightly shorter than the driven element, while the reflector(s) are slightly longer. The is unidirectional, with the along the axis perpendicular to the elements in the plane of the elements, off the end with the directors. Conveniently, the dipole parasitic elements have a (point of zero ) at their centre, so they can be attached to a conductive metal support at that point without need of insulation, without disturbing their electrical operation. They are usually bolted or welded to the antenna's central support boom. The driven element is fed at centre so its two halves must be insulated where the boom supports them. The gain increases with the number of parasitic elements used. Only one reflector is used since the improvement of gain with additional reflectors is negligible, but Yagis have been built with up to 30–40 directors.
The of the antenna is the frequency range between the frequencies at which the gain drops 3 dB (one-half the power) below its maximum. The Yagi–Uda array in its basic form has very narrow bandwidth, 2–3 percent of the centre frequency. There is a tradeoff between gain and bandwidth, with the bandwidth narrowing as more elements are used.
For applications that require wider bandwidths, such as, Yagi–Uda antennas commonly feature trigonal reflectors, and larger diameter conductors, in order to cover the relevant portions of the VHF and UHF bands. Wider bandwidth can also be achieved by the use of 'traps', as described below. Yagi–Uda antennas used for are sometimes designed to operate on multiple bands. These elaborate designs create electrical breaks along each element (both sides) at which point a parallel ( and ) circuit is inserted. This so-called trap has the effect of truncating the element at the higher frequency band, making it approximately a half wavelength in length.
At the lower frequency, the entire element (including the remaining inductance due to the trap) is close to half-wave resonance, implementing a different Yagi–Uda antenna. Using a second set of traps, a 'triband' antenna can be resonant at three different bands. Given the associated costs of erecting an antenna and rotor system above a tower, the combination of antennas for three amateur bands in one unit is a very practical solution. The use of traps is not without disadvantages, however, as they reduce the bandwidth of the antenna on the individual bands and reduce the antenna's electrical efficiency and subject the antenna to additional mechanical considerations (wind loading, water and insect ingress). Theory of operation. A portable Yagi–Uda antenna for use at 144 MHz (2 m), with segments of yellow tape-measure ribbon for the arms of the driven and parasitic elements.
Consider a Yagi–Uda consisting of a reflector, driven element and a single director as shown here. The driven element is typically a or and is the only member of the structure that is directly excited (electrically connected to the ). All the other elements are considered parasitic. That is, they reradiate power which they receive from the driven element (they also interact with each other). One way of thinking about the operation of such an antenna is to consider a parasitic element to be a normal dipole element of finite diameter fed at its centre, with a short circuit across its feed point.
As is well known in theory, a short circuit reflects all of the incident power 180 degrees out of phase. So one could as well model the operation of the parasitic element as the superposition of a dipole element receiving power and sending it down a transmission line to a matched load, and a transmitter sending the same amount of power up the transmission line back toward the antenna element. If the transmitted voltage wave were 180 degrees out of phase with the received wave at that point, the superposition of the two voltage waves would give zero voltage, equivalent to shorting out the dipole at the feedpoint (making it a solid element, as it is). Thus a half-wave parasitic element radiates a wave 180° out of phase with the incident wave. The fact that the parasitic element involved is not exactly resonant but is somewhat shorter (or longer) than ½ λ modifies the phase of the element's current with respect to its excitation from the driven element. The so-called reflector element, being longer than ½ λ, has an inductive which means the phase of its current lags the phase of the open-circuit voltage that would be induced by the received field. The director element, on the other hand, being shorter than ½ λ, has a capacitive reactance with the voltage phase lagging that of the current.
The elements are given the correct lengths and spacings so that the radio waves radiated by the driven element and those re-radiated by the parasitic elements all arrive at the front of the antenna in-phase, so they superpose and add, increasing signal strength in the forward direction. In other words, the crest of the forward wave from the reflector element reaches the driven element just as the crest of the wave is emitted from that element.
These waves reach the first director element just as the crest of the wave is emitted from that element, and so on. The waves in the reverse direction, cancelling out, so the signal strength radiated in the reverse direction is small. Thus the antenna radiates a unidirectional beam of radio waves from the front (director end) of the antenna. Analysis While the above qualitative explanation is useful for understanding how parasitic elements can enhance the driven elements' radiation in one direction at the expense of the other, the assumptions used are quite inaccurate. Since the so-called reflector, the longer parasitic element, has a current whose phase lags that of the driven element, one would expect the directivity to be in the direction of the reflector, opposite of the actual directional pattern of the Yagi–Uda antenna. In fact, that would be the case were we to construct a phased array with rather closely spaced elements all driven by voltages in phase, as we posited. However these elements are not driven as such but receive their energy from the field created by the driven element, so we will find almost the opposite to be true.
For now, consider that the parasitic element is also of length λ/2. Again looking at the parasitic element as a dipole which has been shorted at the feedpoint, we can see that if the parasitic element were to respond to the driven element with an open-circuit feedpoint voltage in phase with that applied to the driven element (which we'll assume for now) then the reflected wave from the short circuit would induce a current 180° out of phase with the current in the driven element. This would tend to cancel the radiation of the driven element. However, due to the reactance caused by the length difference, the phase lag of the current in the reflector, added to this 180° lag, results in a phase advance, and vice versa for the director. Thus the directivity of the array indeed is in the direction towards the director. How the antenna works. The radio waves from each element are emitted with a phase delay, so that the individual waves emitted in the forward direction (up) are in phase, while the waves in the reverse direction are out of phase.
Therefore, the forward waves add togetherenhancing the power in that direction, while the backward waves partially cancel each other , thereby reducing the power emitted in that direction. At other angles, the power emitted is intermediate between the two extremes.
Two Yagi–Uda antennas on a single mast. The top one includes a corner reflector and three stacked Yagis fed in phase in order to increase gain in the horizontal direction (by cancelling power radiated toward the ground or sky).
The lower antenna is oriented for vertical polarization, with a much lower resonant frequency. Design There are no simple formulas for designing Yagi–Uda antennas due to the complex relationships between physical parameters such as. element length and spacing. element diameter. performance characteristics: gain and input impedance However using the above kinds of iterative analysis one can calculate the performance of a given a set of parameters and adjust them to optimize the gain (perhaps subject to some constraints). Since with an n element Yagi–Uda antenna, there are 2 n − 1 parameters to adjust (the element lengths and relative spacings). This iterative analysis method is not a straightforward.
The mutual impedances plotted above only apply to λ/2 length elements, so these might need to be recomputed to get good accuracy. The current distribution along a real antenna element is only approximately given by the usual assumption of a classical standing wave, requiring a solution of Hallen's integral equation taking into account the other conductors. Such a complete exact analysis considering all of the interactions mentioned is rather overwhelming, and approximations are inevitable on the path to finding a usable antenna. Consequently, these antennas are often empirical designs using an element of, often starting with an existing design modified according to one's hunch. The result might be checked by direct measurement or by computer simulation. A well-known reference employed in the latter approach is a report published by the National Bureau of Standards (NBS) (now the (NIST)) that provides six basic designs derived from measurements conducted at 400 MHz and procedures for adapting these designs to other frequencies. These designs, and those derived from them, are sometimes referred to as 'NBS yagis.'
By adjusting the distance between the adjacent directors it is possible to reduce the back lobe of the radiation pattern. A 1-S night fighter with quadruple Yagi radar transceiver antennas The Yagi–Uda antenna was invented in 1926 by of, with the collaboration of, also of Tohoku Imperial University. Yagi and Uda published their first report on the wave projector directional antenna. Yagi demonstrated a, but the engineering problems proved to be more onerous than conventional systems.
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). Yagi arrays of the German FuG 220 radar on the nose of a late-World War II fighter aircraft. The Yagi was first widely used during for airborne sets, because of its simplicity and directionality. Despite being invented in Japan, many Japanese radar engineers were unaware of the design until very late in the war, partly due to rivalry between the Army and Navy.
The Japanese military authorities first became aware of this technology after the 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.
A array can be seen under the leading edge of carrier-based aircraft and the long range patrol seaplane. Vertically polarized arrays can be seen on the cheeks of the and on the of many WWII aircraft, notably the -equipped examples of the German R-1, and the British night-fighter and flying-boat. Indeed, the latter had so many antenna elements arranged on its back – in addition to its formidable turreted defensive armament in the nose and tail, and atop the hull – it was nicknamed the fliegendes Stachelschwein, or 'Flying Porcupine' by German airmen. The experimental Morgenstern German AI VHF-band radar antenna of 1943–44 used a 'double-Yagi' structure from its 90° angled pairs of Yagi antennas formed from six discrete dipole elements, making it possible to fair the array within a conical, rubber-covered plywood radome on an aircraft's nose, with the extreme tips of the Morgenstern's antenna elements protruding from the radome's surface, with an G-6 of the wing's staff flight using it late in the war for its Lichtenstein SN-2 AI radar.
After World War 2, the advent of motivated extensive development of the Yagi–Uda antenna as a rooftop television reception antenna in the and bands, and to a lesser extent an antenna. Until the development of the in the 1960s, it was the only type of antenna that could give adequate in areas far from the television transmitter. A major drawback was the Yagi's inherently narrow bandwidth. Very complicated Yagi designs were developed to give adequate gain over the broad television bands. TV antennas are still a major application of the Yagi antenna. The Yagi–Uda antenna was named an in 1995. See also.
Notes. Graf, Rudolf F. WiseGEEK website.
Conjecture Corp. Retrieved 18 September 2014. ^ Balanis, Constantine A.
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John Wiley and Sons. ^ Wolff, Christian (2010). Radar Basics. Retrieved 18 September 2014. (December 1925). Of IEE of Japan. Institute of Electrical Engineers of Japan: 1128.
(This was the preface and notice in advance for a series of 11 papers with the same title by Uda between 1926–1929 on the antenna.). ^ Yagi, Hidetsu; Uda, Shintaro (February 1926). Of the Imperial Academy of Japan. Imperial Academy. 2 (2): 49–52.
Retrieved 11 September 2014. ^ Sarkar, T. K.; Mailloux, Robert; Oliner, Arthur A.; et al. John Wiley and Sons.
Retrieved 4 July 2014. Principles of Antenna Theory, Kai Fong Lee, 1984, John Wiley and Sons Ltd.,. S. Mushiake (1954). Sendai, Japan: The Research Institute of Electrical Communication, Tohoku University.
^ Brown, 1999, p. 138. Graf, Rudolf F.
Popular Mechanics, pp. 144–145, 214. Retrieved 15 April 2012. IEEE Global History Network. Retrieved 29 July 2011. Bibliography. Brown, Louis (1999).
Uda, 'High angle radiation of short electric waves'., vol. 15, pp. 377–385, May 1927. Uda, 'Radiotelegraphy and radiotelephony on half-meter waves'. Proceedings of the IRE, vol. 18, pp. 1047–1063, June 1930. Brittain, Scanning the Past, Shintaro Uda and the Wave Projector, Proc.
IEEE, May 1997, pp. 800–801. H.Yagi, 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 by D.M. Pozar, in, Volume 85, Issue 11, Nov.
1997 Page(s):1857–1863. Proceedings of the IEEE Vol. Shozo Usami and Gentei Sato, '.
IEEE Milestones, IEEE History Center, IEEE, 2005. Pozar, David M. Microwave and RF Design of Wireless Systems. John Wiley & Sons Inc. External links Wikimedia Commons has media related to. History of antenna invention and its patents.
Jefferies, '. Simple information on basic design, project and measure of Yagi–Uda antenna. 2008.
www.antenna-theory.com.