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What is a Yagi-Uda antenna?

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A Yagi-Uda antenna is an endfire array of one driven element (usually a dipole or folded dipole), a slightly longer reflector behind it and one or more slightly shorter directors in front, all parasitically excited by mutual coupling. It was developed by Shintaro Uda and Hidetsugu Yagi at Tohoku Imperial University and described in English by Yagi in 1928.[1] Gain rises with boom length, from roughly 7 dBi for three elements to about 15 dBi or more for long, many-director designs.[2][3]

How does a Yagi work?

Each parasitic element carries current induced by its neighbors. An element longer than resonance is inductive, so its current lags and it acts as a reflector; shorter elements are capacitive and act as directors, phasing the currents to form a traveling wave toward the director end.[3] Typical reflector spacing is 0.15–0.25λ; directors are spaced about 0.2–0.35λ.

What gain does a Yagi have for a given length?

Gain depends mainly on boom length, not element count. NBS Technical Note 688 tabulated optimized designs measured at 400 MHz: for example a boom of 0.4λ with three elements gave about 7.1 dBd, a boom of 1.2λ about 10.2 dBd and a boom of 4.2λ about 14.2 dBd.[2] Treat these as figures for the specific element diameters and frequency studied.

How is a Yagi matched?

Parasitic elements lower the driven element’s impedance well below 73 Ω, often to 20–30 Ω. Common fixes are a folded dipole driven element, a gamma match or a hairpin (beta) match, followed by a balun.[4]

What are typical trade-offs?

Maximum gain, maximum front-to-back ratio and widest bandwidth do not coincide; optimization with NEC trades them off. Yagis are narrowband (a few percent), which suits TV channels, amateur bands and point-to-point links.[3]

Frequently asked questions

Why is the reflector longer?

So it is inductive; its current lags and reinforces radiation forward.

How many directors are useful?

Gain grows slowly with length: each doubling of boom length adds roughly 2–3 dB in long Yagis.

Is a Yagi horizontally polarized?

It is polarized parallel to its elements; mount the elements vertically for vertical polarization.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

  1. “Beam Transmission of Ultra Short Waves”. H. Yagi, Proceedings of the IRE, vol. 16, no. 6, pp. 715-741. June 1928.
  2. “Yagi Antenna Design, NBS Technical Note 688”. P. P. Viezbicke, U.S. National Bureau of Standards. December 1976.
  3. “Antenna Theory: Analysis and Design, 4th ed.”. C. A. Balanis, Wiley. 2016.
  4. “The ARRL Antenna Book for Radio Communications”. ARRL. multiple editions.
Categories: Antenna types