--- title: "Yagi-Uda construction, working and design" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:yagi-construction-design" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/yagi-construction-design site: BCT Engineering --- # Yagi-Uda construction, working and design Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2074 Bhadra, 8 marks; 2069 Bhadra, 10 marks (asked 2 times) - 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-construction-design - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question Describe the construction, working principle, and design of Yagi-Uda antenna with necessary diagrams. ## Short answer A Yagi-Uda antenna has a driven dipole, one reflector about 5% longer, and several directors about 5% shorter, mounted on a boom. Mutual coupling cancels backward radiation and adds forward, giving an end-fire beam. Gain 7-15 dBi, narrow bandwidth, used for TV/FM reception. ## Answer A **Yagi-Uda antenna** is an end-fire array of one **driven** half-wave (usually folded) dipole and several **parasitic** elements — one **reflector** behind and one or more **directors** in front — mounted parallel to each other on a common boom. It gives a moderate-gain unidirectional beam with a single feed point. #### Construction ```text R DE D1 D2 D3 | | | | | | | | | | --+-----(o)------+------+------+--> boom & beam | feed | | | | | | | | ≈0.5λ 0.46- 0.45λ shorter -> 0.48λ ``` - **Driven element**: resonant λ/2 dipole or folded dipole connected to the feeder. - **Reflector**: one element ≈ 5 % longer than the driven element (≈ 0.5λ), spaced 0.15–0.25λ behind. - **Directors**: elements ≈ 5 % shorter than the preceding one (≈ 0.45λ downwards), spaced 0.1–0.35λ. - Elements are metal tubes/rods fixed at their centres (voltage zero) to a metal boom, so no insulation is needed. #### Working principle 1. RF current in the driven element radiates; this field induces currents in the parasitic elements by **mutual coupling**, and they re-radiate. 2. The **reflector**, longer than resonant length, has an **inductive** impedance; its current lags. With spacing ≈ 0.15–0.25λ, its re-radiated field cancels the driven element's field in the backward direction and adds in the forward direction. 3. Each **director**, shorter than resonant length, is **capacitive**; its current leads. The directors form a chain with progressive phase, so the wave is guided forward like an **end-fire array**. 4. Result: a unidirectional beam along the boom towards the directors. More directors → higher gain and narrower beam (with diminishing returns). 5. Parasitic coupling lowers the driven element's impedance (to ≈ 20–30 Ω for a simple dipole), so a **folded dipole** (≈ 4 × 73 ≈ 292 Ω alone) is used to give a convenient ≈ 50–75 Ω or 300 Ω input. ```text Radiation pattern (top view) .--. back .' '. ==o===(R DE D D)======> main lobe '. .' '--' small back lobe; front-to-back ≈ 10-20 dB ``` #### Design procedure Design rules (standard empirical rules): ```text 1. λc = c / fc (fc = centre frequency of band) 2. Driven element L_DE = 0.46λ to 0.48λ 3. Reflector L_R = 1.05 L_DE (≈ 0.5λ) 4. Directors L_D1 = 0.95 L_DE, L_Dk+1 = 0.95 L_Dk 5. Spacing R-DE S_R = 0.25λ (0.15-0.25λ) Spacing DE-D, D-D S_D = 0.15λ (0.1-0.35λ) 6. Element diameter d ≈ 0.0025λ to 0.01λ ``` **Example**: 5-element Yagi at fc = 200 MHz, L_DE = 0.48λ. ```text λc = 3×10⁸ / 200×10⁶ = 1.5 m L_DE = 0.48 × 1.5 = 0.720 m L_R = 1.05 × 0.720 = 0.756 m L_D1 = 0.95 × 0.720 = 0.684 m L_D2 = 0.95 × 0.684 = 0.650 m L_D3 = 0.95 × 0.650 = 0.617 m S_R = 0.25 × 1.5 = 0.375 m S_D = 0.15 × 1.5 = 0.225 m Boom = 0.375 + 3(0.225) = 1.05 m ``` #### Characteristics | Property | Typical value | |---|---| | Gain | 7 dBi (3 elements) to 15 dBi (10+) | | Beamwidth | ≈ 30°–60° | | Front-to-back ratio | 10–20 dB | | Bandwidth | Narrow, ≈ 2–5 % | | Polarization | Linear, plane of elements | **Advantages**: light, cheap, simple single feed, good gain and directivity. **Disadvantages**: narrow bandwidth, gain limited (~ 15–17 dBi), sensitive to element lengths. **Applications**: TV and FM reception, VHF/UHF point-to-point links, amateur radio, radar. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The reflector is about A. 5% shorter than the driven element B. twice as long C. 5% longer than the driven element D. equal to the driven element Answer: C. L_R = 1.05 L_DE, so it is inductive. (Key point: Reflector length) 2. A folded dipole is used because A. it increases the gain by 10 dB B. it makes elements need insulation C. it eliminates directors D. parasitic coupling lowers the driven element impedance Answer: D. A folded dipole (about 292 ohm alone) restores a convenient input impedance. (Key point: Reason for folded dipole) 3. Elements can be fixed to a metal boom without insulation because A. they are mounted at their centres, which are voltage nulls B. they are at current maxima C. the boom is non-conducting D. they are very short Answer: A. At the centre of a half-wave element voltage is zero. (Key point: Mounting) 4. For L_DE = 0.48 lambda at 200 MHz, L_DE is A. 1.44 m B. 0.720 m C. 0.360 m D. 0.756 m Answer: B. lambda = 1.5 m, 0.48 x 1.5 = 0.72 m. 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