The yagi-Uda antenna is one of the constructions that significantly influenced the practical antenna technology of the 20th century. Its principle originated in Japan in the mid-20s of the last century and even then it used an idea that is well known to radio amateurs: one powered element and a system of passive, interconnected elements. In 2026, we are therefore commemorating approximately a century since the creation and first publications of this antenna.
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Uda, Yagi and 1926
Shintaro Uda and Hidetsugu Yagi from Tohoku Imperial University in Sendai, Japan are behind the construction. Work on a directional antenna for short waves had already been going on since 1924. Uda published the results of his research in 1925, followed in 1926 by a joint publication by Yagi and Uda describing a new directional antenna with parasitic elements.
Yagi played a significant role in the international spread of the principle. In 1928, he published an English article in the Proceedings of the Institute of Radio Engineers, thanks to which the construction reached a wider professional public. It was this publication that made a significant contribution to the fact that the name Yagi antenna became established in practice, although Uda played a fundamental role in the actual experimental development of the principle.
The more accurate designation Yagi-Uda therefore recalls both authors. However, the name "Yagi" became significantly more widespread in technical literature and later also among radio amateurs.
How the Yagi-Uda antenna works
A basic Yagi-Uda has one powered element, usually a half-wave dipole, and at least one parasitic element. Behind the radiator there is a reflector, in front of it one or more directors. Parasitic elements are not galvanically connected to the power supply. Energy is transferred to them by electromagnetic coupling and induced currents are created in the elements.
The length and mutual distance of the elements determine the phase and amplitude of the induced currents. The reflector is usually longer than the emitter and the directors are shorter. The electromagnetic fields created in this way add up in the direction of the directors and cancel to a greater extent in the opposite direction. The result is a distinctly directional, so-called end-fire radiation pattern.
So it is not a simple "reflector" that would mechanically reflect the radio wave. Correct functioning is the result of the mutual connection of the elements and the phase ratios of their currents. That is why the lengths of the elements and their spacing are of fundamental importance in the design of the Yagi.
Gain, F/B, Lobe Width and Bandwidth
Yagi gain depends on the number of elements, their lengths, spacing, conductor diameter and overall geometry. By adding directors, directionality and profit can generally be increased, while the contribution of the next element gradually decreases. An important parameter is also the front-back F/B ratio, i.e. the ratio of signal intensity in the main direction to the signal from the rear direction.
As the directionality increases, the width of the main lobe also changes. Longer and optimized arrays with more elements can create a narrower radiation pattern. However, bandwidth cannot be ignored at the cost of high directivity. The Yagi is naturally resonant and both its gain and F/B deteriorate outside the optimized frequency range.
| Parameter | Meaning |
|---|---|
| Profit | The ability to focus radiated or received energy in the desired direction. |
| F/B ratio | The ratio of the signal strength in the main direction to the signal strength from the rear direction. |
| The width of the main lobe | The angle that characterizes the directivity of the antenna in the main direction (usually for a drop of -3dB) |
| Bandwidth | The frequency range in which the antenna maintains the required parameters. |
Video: the principle of the Yagi-Uda antenna
The technical interpretation of the Yagi-Uda antenna principle was prepared by the Universitat Politècnica de València. The video clearly shows the role of the emitter, reflector and directors and their effect on the radiation pattern.
From a simple Yagi to special constructions
The Yagi-Uda principle makes it possible to create many practical variants. A classic single-band Yagi may have only a reflector, emitter and one director, but also dozens of elements. In radio amateur practice, there are multi-element VHF and UHF Yagis with high directivity, but also multi-band constructions in which elements or passive circuits are designed for several frequency ranges.
Related directional antennas include, for example Moxon constructions and various trapped or shortened antennas. When designing, it is necessary to distinguish which solution is still a classic Yagi-Uda system and which already uses a different principle. A separate group is represented by modifications combined with waveguide or other structures.
Yagi in WWII radar technology
Významným historickým použitím bola radarová technika. Yagi-Uda antény sa počas druhej svetovej vojny používali v radarových systémoch viacerých štátov vrátane Spojeného kráľovstva, USA, Nemecka a Japonska. Ich smerovosť, jednoduchá konštrukcia a vhodnosť pre vyššie frekvencie z nich urobili praktický prvok radarových systémov, vrátane palubných radarov.
The historical paradox is that the name "Yagi" itself, according to historical sources, was less known in Japan during the war than would be expected given the Japanese origin of the design. The international expansion was significantly influenced by the English publication of Yagi from 1928.
Video: Yagi and radar
The historical context of the use of Yagi antennas in the radar technology of the Second World War is also brought closer by the following video dedicated to the development of radars and their antennas.
Television boom after the war
After the Second World War, the Yagi gained another large area of application in receiving television technology. The development of VHF and later UHF television broadcasting created the need for directional antennas with higher gain. Therefore, Yagi appeared en masse on the roofs of family houses and became one of the most famous outdoor receiving antennas.
Televízne Yagi zároveň ukázali jeden zo základných kompromisov konštrukcie: vysoká smerovosť a zisk sú spojené s obmedzenou šírkou pásma. S rastúcim počtom televíznych kanálov a požiadavkou na širší frekvenčný rozsah sa preto začali presadzovať aj širokopásmové konštrukcie, napríklad log-periodické antény.
Yagi in amateur radio
Pre rádioamatérov sa Yagi-Uda stala jednou zo základných smerových antén najmä na VHF a vyšších frekvenciách. Na 144 MHz, 432 MHz a 1296 MHz umožňuje relatívne jednoduchou mechanickou konštrukciou dosiahnuť smerovosť potrebnú pre DX, contesting, meteor scatter, EME a satelitnú prevádzku.
An advantage is also the possibility of creating antenna systems from several identical Yagis. Stacking can increase the gain and adjust the vertical or horizontal radiation pattern. However, with such systems, the distance between the antennas, phasing and power supply must be solved precisely. Practical designs for 432 MHz and 1296 MHz today use NEC simulations and subsequent measurements to verify the result.
From spreadsheets to computer optimization
The first designs were created by a combination of theory, experiment and repeated measurements. Today, the Yagi geometry can be modeled by electromagnetic simulators based on, for example, NEC. The program allows you to change the lengths of the elements, their diameters and spacing, and monitor the gain, impedance, F/B and radiation diagram. Thus, modern designs can optimize several parameters at once instead of tuning one element step by step.
However, simulation is not the last step in practical construction. Element diameter, mounting method, boom, radiator insulation, power supply and mechanical tolerances change the electrical properties. Therefore, the subsequent measurement of the impedance and radiation properties of the real antenna is also important for computer design.
Why is Yagi-Uda still relevant?
Po sto rokoch zostáva princíp rovnaký: jeden napájaný prvok, parazitné prvky, vzájomná väzba a presne definované fázy prúdov. Zmenili sa materiály, možnosti simulácie a meracia technika, nie základná elektromagnetická podstata. Práve kombinácia mechanickej jednoduchosti, smerovosti a možnosti optimalizácie robí z Yagi-Uda antény stále praktickú voľbu pre rádioamatérske VHF, UHF, EME aj satelitné aplikácie.