Yagi–Uda antenna
A Yagi–Uda antenna, or simply Yagi antenna, is a
Reflector elements (usually only one is used) are slightly longer than the driven dipole and placed behind the driven element, opposite the direction of intended transmission. Directors, on the other hand, are a little shorter and placed in front of the driven element in the intended direction.
Also called a
Origins
![](http://upload.wikimedia.org/wikipedia/commons/thumb/8/80/FuG_220_and_FuG_202_radar_of_Me_110_1945.jpg/170px-FuG_220_and_FuG_202_radar_of_Me_110_1945.jpg)
The antenna was invented by Shintaro Uda of Tohoku Imperial University, Japan,[5] in 1926, with a lesser role played by Hidetsugu Yagi.[6][7]
However, the name Yagi has become more familiar, while the name of Uda, who applied the idea in practice or established the conception through experiment, is often omitted. This appears to have been due to the fact that Yagi based his work on Uda's pre-announcement[5] and developed the principle of the absorption phenomenon Yagi had announced earlier.[8] Yagi filed a patent application in Japan on the new idea, without Uda's name in it, and later transferred the patent to the Marconi Company in the UK.[9] Incidentally, in the US, the patent was transferred to RCA Corporation.[10]
Yagi antennas were first widely used during World War II in radar systems by Japan, Germany, the United Kingdom, and the United States.[7] After the war, they saw extensive development as home television antennas.
Description
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The Yagi–Uda antenna typically consists of a number of parallel thin rod elements, each approximately a half wave in length. Rarely, the elements are discs rather than rods. Often they are supported on a perpendicular crossbar or "boom" along their centers.
Conveniently, the dipole parasitic elements have a node (point of zero RF voltage) 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.[4] They are usually bolted or welded to the antenna's central support boom.[4] The most common form of the driven element is one fed at its centre so its two halves must be insulated where the boom supports them.
The gain increases with the number of parasitic elements used.[4] Only one reflector is normally used since the improvement of gain with additional reflectors is small, but more reflectors may be employed for other reasons such as wider bandwidth. Yagis have been built with 40 directors[3] and more.[11]
The bandwidth of an antenna is, by one definition, the width of the band of frequencies having a gain within 3 dB (one-half the power) of its maximum gain. The Yagi–Uda array in its basic form has a narrow bandwidth, 2–3 percent of the centre frequency.[4] There is a tradeoff between gain and bandwidth, with the bandwidth narrowing as more elements are used.[4] For applications that require wider bandwidths, such as terrestrial television, Yagi–Uda antennas commonly feature trigonal reflectors, and larger diameter conductors, in order to cover the relevant portions of the VHF and UHF bands.[12] Wider bandwidth can also be achieved by the use of "traps", as described below.
Yagi–Uda antennas used for amateur radio are sometimes designed to operate on multiple bands. These elaborate designs create electrical breaks along each element (both sides) at which point a parallel LC (inductor and capacitor) 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 rotator system above a tower, the combination of antennas for three amateur bands in one unit is a 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
![](http://upload.wikimedia.org/wikipedia/commons/thumb/4/42/Two_meter_yagi.jpg/260px-Two_meter_yagi.jpg)
Consider a Yagi–Uda consisting of a reflector, driven element and a single director as shown here. The driven element is typically a
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. The principal part of the current in a loaded receiving antenna is distributed as in a center-driven antenna. It is proportional to the effective length of the antenna and is in
Parasitic elements involved in Yagi–Uda antennas are not exactly resonant but are somewhat shorter (or longer) than 1⁄2λ so that the phase of the element's current is modified with respect to its excitation from the driven element. The so-called reflector element, being longer than 1⁄2λ, has an inductive reactance, which means the phase of its current lags the phase of the open-circuit voltage that would be induced by the received field. The phase delay is thus larger than 90 degrees and, if the reflector element is made sufficiently long, the phase delay may be imagined to approach 180 degrees, so that the incident wave and the wave reemitted by the reflector interfere destructively in the forward direction (i.e. looking from the driven element towards the passive element). The director element, on the other hand, being shorter than 1⁄2λ, has a capacitive reactance with the voltage phase lagging that of the current.[14] The phase delay is thus smaller than 90 degrees and, if the director element is made sufficiently short, the phase delay may be imagined to approach zero and the incident wave and the wave reemitted by the reflector interfere constructively in the forward direction.
Interference also occurs in the backward direction. This interference is influenced by the distance between the driven and the passive element, because the propagation delays of the incident wave (from the driven element to the passive element) and of the reradiated wave (from the passive element back to the driven element) have to be taken into account. To illustrate the effect, we assume zero and 180 degrees phase delay for the reemission of director and reflector, respectively, and assume a distance of a quarter wavelength between the driven and the passive element. Under these conditions the wave reemitted by the director interferes destructively with the wave emitted by the driven element in the backward direction (away from the passive element), and the wave reemitted by the reflector interferes constructively.
In reality, the phase delay of passive dipole elements does not reach the extreme values of zero and 180 degrees. Thus, 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
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 assumption of an additional 90 degrees (leading or lagging) phase shift of the reemitted wave is not valid. Typically, the phase shift in the passive element is much smaller. Moreover, to increase the effect of the passive radiators, they should be placed close to the driven element, so that they can collect and reemit a significant part of the primary radiation.
-
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 together, (destructive interference), thereby reducing the power emitted in that direction.
-
Illustration of forward gain of a two element Yagi–Uda array using only a driven element (left) and a director (right). The wave (green) from the driven element excites a current in the passive director which reradiates a wave (blue) having a particular phase shift (see explanation in text, note that the dimensions are not to scale with the numbers in the text). The addition of these waves (bottom) is increased in the forward direction, but leads to partial cancellation in the reverse direction.
A more realistic model of a Yagi–Uda array using just a driven element and a director is illustrated in the accompanying diagram. The wave generated by the driven element (green) propagates in both the forward and reverse directions (as well as other directions, not shown). The director receives that wave slightly delayed in time (amounting to a phase delay of about 45° which will be important for the reverse direction calculations later). Due to the director's shorter length, the current generated in the director is advanced in phase (by about 20°) with respect to the incident field and emits an electromagnetic field, which lags (under far-field conditions) this current by 90°. The net effect is a wave emitted by the director (blue) which is about 70° (20° - 90°) retarded with respect to that from the driven element (green), in this particular design. These waves combine to produce the net forward wave (bottom, right) with an amplitude somewhat larger than the individual waves.
In the reverse direction, on the other hand, the additional delay of the wave from the director (blue) due to the spacing between the two elements (about 45° of phase delay traversed twice) causes it to be about 160° (70° + 2 × 45°) out of phase with the wave from the driven element (green). The net effect of these two waves, when added (bottom, left), is partial cancellation. The combination of the director's position and shorter length has thus obtained a unidirectional rather than the bidirectional response of the driven (half-wave dipole) element alone.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/0/0d/Zij-en.png/220px-Zij-en.png)
When a passive radiator is placed close (less than a quarter wavelength distance) to the driven dipole, it interacts with the near field, in which the phase-to-distance relation is not governed by propagation delay, as would be the case in the far field. Thus, the amplitude and phase relation between the driven and the passive element cannot be understood with a model of successive collection and reemission of a wave that has become completely disconnected from the primary radiating element. Instead, the two antenna elements form a coupled system, in which, for example, the self-impedance (or radiation resistance) of the driven element is strongly influenced by the passive element. A full analysis of such a system requires computing the mutual impedances between the dipole elements[15] which implicitly takes into account the propagation delay due to the finite spacing between elements and near-field coupling effects. We model element number j as having a feedpoint at the centre with a voltage Vj and a current Ij flowing into it. Just considering two such elements we can write the voltage at each feedpoint in terms of the currents using the mutual impedances Zij:
Z11 and Z22 are simply the ordinary driving point impedances of a dipole, thus 73 + j43 ohms for a half-wave element (or purely resistive for one slightly shorter, as is usually desired for the driven element). Due to the differences in the elements' lengths Z11 and Z22 have a substantially different reactive component. Due to reciprocity we know that Z21 = Z12. Now the difficult computation is in determining that mutual impedance Z21 which requires a numerical solution. This has been computed for two exact half-wave dipole elements at various spacings in the accompanying graph.
The solution of the system then is as follows. Let the driven element be designated 1 so that V1 and I1 are the voltage and current supplied by the transmitter. The parasitic element is designated 2, and since it is shorted at its "feedpoint" we can write that V2 = 0. Using the above relationships, then, we can solve for I2 in terms of I1:
and so
- .
This is the current induced in the parasitic element due to the current I1 in the driven element. We can also solve for the voltage V1 at the feedpoint of the driven element using the earlier equation:
where we have substituted Z12 = Z21. The ratio of voltage to current at this point is the driving point impedance Zdp of the 2-element Yagi:
With only the driven element present the driving point impedance would have simply been Z11, but has now been modified by the presence of the parasitic element. And now knowing the phase (and amplitude) of I2 in relation to I1 as computed above allows us to determine the radiation pattern (gain as a function of direction) due to the currents flowing in these two elements. Solution of such an antenna with more than two elements proceeds along the same lines, setting each Vj = 0 for all but the driven element, and solving for the currents in each element (and the voltage V1 at the feedpoint).[16] Generally the mutual coupling tends to lower the impedance of the primary radiator and thus, folded dipole antennas are frequently used because of their large radiation resistance, which is reduced to the typical 50 to 75 Ohm range by coupling with the passive elements.
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a7/Yagi_uda_antenna.jpg/170px-Yagi_uda_antenna.jpg)
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 2n − 1 parameters to adjust (the element lengths and relative spacings), this iterative analysis method is not 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 trial and error, 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 United States 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.[17] 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.
History
The Yagi–Uda antenna was invented in 1926 by Shintaro Uda of Tohoku Imperial University,[5] Sendai, Japan, with the guidance of Hidetsugu Yagi, also of Tohoku Imperial University.[6] Yagi and Uda published their first report on the wave projector directional antenna. Yagi demonstrated a proof of concept, but the engineering problems proved to be more onerous than conventional systems.[18]
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 who provided the conception which was originally vague expression to Uda, always acknowledged Uda's principal contribution towards the design which will currently be recognized as the reduction to practice, and if the novelty is not considered, the proper name for the antenna is, as above, the Yagi–Uda antenna (or array).
![](http://upload.wikimedia.org/wikipedia/commons/thumb/1/1e/Nakajima_J1N1-S_Gekko_land_based_night_fighter_at_the_Steven_F._Udvar-Hazy_Center.jpg/220px-Nakajima_J1N1-S_Gekko_land_based_night_fighter_at_the_Steven_F._Udvar-Hazy_Center.jpg)
The Yagi was first widely used during
![](http://upload.wikimedia.org/wikipedia/commons/thumb/f/fa/ASV_Mk_II_radar_transmitter_antenna_on_Bristol_Beaufort.jpg/220px-ASV_Mk_II_radar_transmitter_antenna_on_Bristol_Beaufort.jpg)
A
After World War 2, the advent of
The Yagi–Uda antenna was named an
See also
- Antenna (radio)
- Antenna array
- Numerical Electromagnetics Code
- Radio direction finder
- Radio direction finding
Notes
- Robert Goddardwas the real pioneer of rocket technology even though he was not well known in the US at that time.
References
- Citations
- ISBN 0080511988.
- ^ a b c d e f g "What Is a Yagi Antenna?". wiseGEEK website. Conjecture Corp. 2014. Retrieved 18 September 2014.
- ^ ISBN 978-1118209752.
- ^ a b c d e f g h i j k l m Wolff, Christian (2010). "Yagi Antenna". Radar Basics. Radartutorial.eu. Retrieved 18 September 2014.
- ^ a b c d e Uda, S. (December 1925). "On the Wireless Beam of Short Electric Waves". The Journal of the Institute of Electrical Engineers of Japan. Institute of Electrical Engineers of Japan: 1128. (This was the preface and notice in advance for a series of eleven papers, of the same title, by Uda, between 1926 and 1929, on the antenna. However, it seems that Uda's pre-announcement caused his invention to lose its novelty and become unpatentable. He would not have been informed by Professor Yagi about those.)
- ^ . Retrieved 11 September 2014.
- ^ ISBN 0471783013.
- . Retrieved 3 December 2022.
- '^ "Y. Mushiake, '"Notes on the History of Yagi-Uda Antenna." IEEE Antennas and Propagation Magazine, Vol. 56, No. 1, February 2014. pp. 255-257". Sm.rim.or.jp. Retrieved 4 July 2014.
- ^ a b "Milestones:Directive Short Wave Antenna, 1924". Engineering and Technology History Wiki. IEEE. December 2022. Retrieved 1 December 2022.
- ^ [1], VE1FA transatlantic Yagi
- ^ Common TV Antenna Types
- ISBN 9780674182172.
- ISBN 9780070353961.
- ISBN 0-471-90167-9
- ^ S. Uda; Y. Mushiake (1954). Yagi-Uda Antenna. Sendai, Japan: The Research Institute of Electrical Communication, Tohoku University.
- ^ Yagi Antenna Design, Peter P. Viezbicke, National Bureau of Standard Technical Note 688, December 1976
- ^ a b Brown, 1999, p. 138
- ^ Graf, Rudolf F. (June 1959). "Make Your Own UHF Yagi Antenna". Popular Mechanics, pp. 144–145, 214.
- ^ 2001 IEEE Antennas and Propagation Society International Symposium By IEEE Antennas and Propagation Society. International Symposium.
- S2CID 8215383. Retrieved 14 October 2022.
- ^ The Sunderland flying-boat queen, Volume 1 By John Evans, Page 5
- ^ "HyperScale 48D001 Ju 88 G-6 and Mistel S-3C Collection decals". Hyperscale.com. Retrieved 15 April 2012.
- Bibliography
- Brown, Louis (1999). A radar history of World War II: technical and military imperatives. CRC Press. ISBN 0-7503-0659-9
- S. Uda, "High angle radiation of short electric waves". Proceedings of the IRE, vol. 15, pp. 377–385, May 1927.
- S. Uda, "Radiotelegraphy and radiotelephony on half-meter waves". Proceedings of the IRE, vol. 18, pp. 1047–1063, June 1930.
- J. E. Brittain, Scanning the Past, Shintaro Uda and the Wave Projector, Proc. IEEE, May 1997, pp. 800–801.
- 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.
- Toma KAWANISHI (7 April 2020). "Conceptualizing engineering as a science: Hidetsugu Yagi as a promoter of engineering research". Philosophy and History of Science Studies. 14 (14). Philosophy and History of Science, Kyoto University: 1–24. doi:10.14989/250442.
- Griffiths, Hugh (1 January 2022). "The Yagi Antenna". IEEE Aerospace and Electronic Systems Magazine. 37 (1). IEEE: 4–5. S2CID 245708628. Retrieved 12 December 2022.
External links
![](http://upload.wikimedia.org/wikipedia/en/thumb/4/4a/Commons-logo.svg/30px-Commons-logo.svg.png)
- Yagi-Uda antenna History". History of antenna invention and its patents.
- D. Jefferies, "Yagi-Uda antennas Archived 2005-12-25 at the Wayback Machine". 2004.
- 'Yagi–Uda emitter used for AESA(active electronically scanned array)' low-frequency radars patents.google.com
- Yagi-Uda Antenna. Simple information on basic design, project and measure of Yagi–Uda antenna. 2008
- Yagi-Uda Antennas www.antenna-theory.com
- Yagi Antenna calculator and computer designs 2020"