Yagi-Uda design, advantages and applications

A Yagi-Uda antenna is an end-fire array of a driven λ/2 dipole (often folded) with one reflector behind and one or more directors in front, all parallel and mounted on a common boom. Only the driven element is fed; the others are parasitic.

Construction

   R      DE      D1     D2     D3
   |      |       |      |      |
 --+-----(o)------+------+------+--> beam
   |     feed     |      |      |
 longest        shorter ->
  <0.15-0.25λ><-- 0.1-0.35λ -->

• Reflector ≈ 0.5λ (5 % longer than driven element).
• Driven element ≈ 0.46–0.48λ, folded dipole for impedance.
• Directors ≈ 0.45λ and shorter, each ≈ 5 % less than the previous.
• Elements fixed at centres to a metal boom.

Working principle

The driven element's field induces currents in the parasitic elements by mutual coupling. The longer reflector is inductive (current lags) and cancels backward radiation; the shorter directors are capacitive (current leads) and guide the wave forward with progressive phase, like an end-fire array. The result is a unidirectional beam towards the directors.

Design (rules of thumb)

λ = c/f
L_DE = 0.48λ,  L_R = 1.05 L_DE,  L_Dk+1 = 0.95 L_Dk
S_R  = 0.25λ,  S_D = 0.15λ

Example, 3 elements at 500 MHz (λ = 0.6 m): L_DE = 28.8 cm, L_R = 30.24 cm, L_D = 27.36 cm, S_R = 15 cm, S_D = 9 cm.

Advantages

• High gain (7–15 dBi) and good front-to-back ratio with a single feed point.
• Light, compact, cheap and easy to build and mount.
• Unidirectional beam reduces interference and ghost images.
• Folded-dipole driven element gives convenient 300 Ω/75 Ω matching.

Disadvantages

• Narrow bandwidth (≈ 2–5 %); gain limited to ~ 17 dBi; sensitive to element dimensions.

Applications

• VHF/UHF TV and FM reception (rooftop antennas).
• Point-to-point VHF/UHF links, amateur radio, radio direction finding, radar and RFID readers.