--- title: "Yagi-Uda design, advantages and applications" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:yagi-advantages-applications" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/yagi-advantages-applications site: BCT Engineering --- # Yagi-Uda design, advantages and applications Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2075 Bhadra, 7 marks - Video: [Yagi Uda Antenna (Basics, Types, Structure, Radiation, Designing, Properties, Pros & Cons) Explained](https://www.youtube.com/watch?v=8_GbiRk_gLI) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/yagi-advantages-applications - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question Explain the working principle, construction, design, advantages and application of Yagi-Uda antenna. ## Short answer Yagi-Uda antenna is an end-fire array with reflector and directors. It provides high gain (7-15 dBi), light weight, and unidirectional beam. Applications include VHF/UHF TV reception, point-to-point links, amateur radio, and radar. ## Answer 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 ```text 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) ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How do reflector and directors differ in working? A. Reflector shorter, capacitive B. Longer reflector is inductive and cancels backward radiation; shorter directors are capacitive and guide the wave forward C. All are driven D. Directors are longer Answer: B. The phase of induced currents makes the array end-fire. (Key point: Reflector action) 2. Which rule set is used? A. Equal lengths B. L_R = 0.5 L_DE C. L_DE = 0.48 lambda, L_R = 1.05 L_DE, each director 0.95 of previous D. Directors 1.05 times longer Answer: C. Rules of thumb. (Key point: Design rules) 3. For 500 MHz, lambda = 0.6 m: what is L_DE? A. 15 cm B. 30.24 cm C. 27.36 cm D. 28.8 cm Answer: D. 0.48 x 60 = 28.8 cm. (Key point: Example check) 4. Which is a disadvantage of the Yagi? A. Narrow bandwidth (2-5 %) B. Wide bandwidth C. No gain D. Bidirectional Answer: A. Sensitive to dimensions and narrowband. 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