--- title: "Yagi construction and 800 MHz design" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:yagi-design-800mhz" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/yagi-design-800mhz site: BCT Engineering --- # Yagi construction and 800 MHz design Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2073 Magh, 5+6 marks - Video: [Yagi-Uda Antenna (Basics, Structure, Radiation, Designing, Applications, Pros & Cons) Explained](https://www.youtube.com/watch?v=QJRSTRTt1ZQ) — 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-design-800mhz - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question Explain the construction, working principle and design of a Yagi-antenna. Design a 5-element Yagi antenna with operating frequency of 800 MHz and dipole as driven element. Take effective dipole length of 0.48λ spacing. ## Short answer At 800 MHz, λ = 37.5 cm. Element lengths: reflector 18.90 cm, driven 18.00 cm, directors 17.10, 16.25, 15.43 cm. Spacings: R-DE 9.375 cm, director spacings 5.625 cm; boom length 26.25 cm. ## Answer #### Yagi antenna: construction and working A **Yagi-Uda antenna** is an end-fire array of one **driven** λ/2 (folded) dipole, one **reflector** behind it and several **directors** in front, mounted parallel on a boom. Only the driven element is fed. ```text R DE D1 D2 D3 | | | | | --+-----(o)------+------+------+---> beam | feed | | | ≈0.5λ 0.48λ shorter towards the front ``` **Working**: the driven element's field induces currents in the parasitic elements by mutual coupling. The reflector (longer, **inductive**, current lags) cancels radiation backwards and reinforces it forwards. The directors (shorter, **capacitive**, current leads) produce a progressive phase that guides energy forward, as in an end-fire array. The beam points towards the directors; gain rises with the number of directors (≈ 7 dBi for 3 elements, ≈ 9–10 dBi for 5). **Design procedure**: choose λ from the frequency; take the driven element ≈ 0.46–0.48λ; reflector ≈ 5 % longer; each director ≈ 5 % shorter than the previous; reflector spacing ≈ 0.15–0.25λ and director spacing ≈ 0.1–0.35λ. #### Design: 5 elements at 800 MHz Design rules used (standard empirical Yagi rules): ```text λc = c / fc L_DE = 0.48 λc (driven, given) L_R = 1.05 L_DE (≈ 0.5λc) (reflector 5 % longer) L_D1 = 0.95 L_DE, L_Dk+1 = 0.95 L_Dk (5 % shorter each) S_R = 0.25 λc (reflector to driven) S_D = 0.15 λc (driven to D1 and between directors) ``` Interpretation: "effective dipole length of 0.48λ" is taken as the driven element length. **Step 1: Wavelength** ```text λc = 3×10⁸ / 800×10⁶ = 0.375 m = 37.5 cm ``` **Step 2: Element lengths** ```text L_DE = 0.48 × 37.5 = 18.00 cm L_R = 1.05 × 18.00 = 18.90 cm L_D1 = 0.95 × 18.00 = 17.10 cm L_D2 = 0.95 × 17.10 = 16.25 cm L_D3 = 0.95 × 16.245 = 15.43 cm ``` **Step 3: Spacings and boom** ```text S_R = 0.25 × 37.5 = 9.375 cm S_D = 0.15 × 37.5 = 5.625 cm Boom = 9.375 + 3 × 5.625 = 26.25 cm ``` | Element | Length (cm) | Distance from R (cm) | |---|---|---| | Reflector | 18.90 | 0 | | Driven | 18.00 | 9.375 | | Director 1 | 17.10 | 15.00 | | Director 2 | 16.25 | 20.625 | | Director 3 | 15.43 | 26.25 | ```text R DE D1 D2 D3 | | | | | 18.90 18.00 17.10 16.25 15.43 cm | (o) | | | ===> beam |<9.375>|<5.625>|<5.625>|<5.625>| <-------- boom 26.25 cm --------> ``` **Answer: λ = 37.5 cm; R = 18.90 cm, DE = 18.00 cm, D1 = 17.10 cm, D2 = 16.25 cm, D3 = 15.43 cm; spacings 9.375 cm (R–DE) and 5.625 cm (others); boom 26.25 cm.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a Yagi-Uda antenna, how do the reflector and directors differ in length and reactance? A. Reflector is longer and inductive; directors are shorter and capacitive B. Reflector is shorter and capacitive; directors are longer and inductive C. Reflector and directors are both longer than the driven element and inductive D. Reflector and directors are all exactly equal to the driven element Answer: A. The reflector is about 5% longer than the driven element (inductive, current lags), while directors are shorter (capacitive, current leads), which makes the beam point toward the directors. (Key point: Role of reflector and directors) 2. Which element(s) of the Yagi antenna are connected to the feed? A. All elements B. Only the reflector C. Only the driven element; the others get current by mutual coupling D. Reflector and first director Answer: C. Only the driven element is fed; parasitic elements receive induced currents by mutual coupling. (Key point: Which element is fed) 3. For 800 MHz, what are the wavelength and the driven element length (0.48λ)? A. λ = 37.5 cm, L_DE = 18.0 cm B. λ = 3.75 cm, L_DE = 1.8 cm C. λ = 37.5 cm, L_DE = 37.5 cm D. λ = 26.7 cm, L_DE = 12.8 cm Answer: A. λ = 3x10^8 / 800x10^6 = 0.375 m = 37.5 cm, and 0.48 x 37.5 = 18.0 cm. (Key point: Wavelength and driven element) 4. With each director 5% shorter than the previous, and S_R = 0.25λ, S_D = 0.15λ, which result is correct for the 5-element design? A. D1 = 18.9 cm and boom = 26.25 cm B. D1 = 17.10 cm, D2 = 16.25 cm, boom = 26.25 cm C. D1 = 17.10 cm, D2 = 17.10 cm, boom = 18.75 cm D. D1 = 16.25 cm, D2 = 17.10 cm, boom = 26.25 cm Answer: B. D1 = 0.95 x 18 = 17.10 cm, D2 = 0.95 x 17.10 = 16.25 cm; boom = 9.375 + 3 x 5.625 = 26.25 cm. 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