--- title: "Folded dipole and Yagi design 785–895 MHz" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:folded-dipole-yagi-design" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/folded-dipole-yagi-design site: BCT Engineering --- # Folded dipole and Yagi design 785–895 MHz Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2081 Chaitra, 3+6 marks - Video: [EC8701 UNIT 2 FOLDED DIPOLE AND YAGI UDA ANTENNA](https://www.youtube.com/watch?v=tFP56WUJ4G0) — R.M.K. College of Engineering and Technology (ECE) - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/folded-dipole-yagi-design - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question Sketch the current distribution and radiation pattern of folded dipole antenna. Design a 5 element Yagi-Uda antenna for the operating frequency band of 785 MHz to 895 MHz and dipole as a driven element. Take an effective length of the dipole 0.48λc. ## Short answer Folded dipole: in-phase currents on both conductors, half-sine distribution, figure-8 pattern like a dipole, directivity 1.64, impedance 292 Ω. Yagi design: fc=840 MHz, λc=35.71 cm; reflector=18.00 cm, driven=17.14 cm, directors=16.29,15.47,14.70 cm; reflector–driven spacing 8.93 cm, director spacing 5.36 cm, boom 25.0 cm. ## Answer #### Folded dipole: current distribution and pattern A **folded dipole** is two parallel λ/2 conductors joined at both ends, with one of them split at the centre for feeding. The currents in both conductors are in the **same direction and phase**, each following a half-sine distribution, so it radiates like a dipole carrying double current. ```text Current distribution (both arms in phase) _________________________ | --> --> I --> --> | | | |__--> --> ~ --> -->__| max at centre, zero at ends Pattern (plane containing the dipole) 90° .-""-. \ / 0° ----[==]---- 180° nulls along / \ dipole axis '-..-' 270° figure-8 ``` - Input impedance: Z = N² × 73 Ω = 4 × 73 ≈ **292 Ω** for 2 equal conductors. - Pattern: same figure-8 as a λ/2 dipole in the plane containing it; omnidirectional in the plane perpendicular; directivity 1.64. - Wider bandwidth than a simple dipole; ideal driven element of a Yagi. #### Design of 5-element Yagi for 785–895 MHz 5 elements = 1 reflector + 1 driven folded dipole + 3 directors. 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) ``` **Step 1: Centre frequency and wavelength** ```text fc = (f₁ + f₂)/2 = (785 + 895)/2 = 840 MHz λc = 3×10⁸ / 840×10⁶ = 0.3571 m = 35.71 cm ``` **Step 2: Element lengths** ```text L_DE = 0.48 × 35.71 = 17.14 cm L_R = 1.05 × 17.14 = 18.00 cm L_D1 = 0.95 × 17.14 = 16.29 cm L_D2 = 0.95 × 16.29 = 15.47 cm L_D3 = 0.95 × 15.47 = 14.70 cm ``` **Step 3: Spacings and boom** ```text S_R = 0.25 × 35.71 = 8.93 cm S_D = 0.15 × 35.71 = 5.36 cm Boom = 8.93 + 3 × 5.36 = 25.00 cm ``` | Element | Length (cm) | Distance from R (cm) | |---|---|---| | Reflector | 18.00 | 0 | | Driven (folded dipole) | 17.14 | 8.93 | | Director 1 | 16.29 | 14.29 | | Director 2 | 15.47 | 19.64 | | Director 3 | 14.70 | 25.00 | ```text R DE D1 D2 D3 | | | | | 18.0 17.14 16.29 15.47 14.70 cm | (o) | | | ===> beam |<8.93>|<5.36>|<5.36>|<5.36>| <------- boom 25.0 cm ------> ``` The band is 110 MHz wide (≈ 13 % of fc), which is large for a Yagi; the folded dipole driven element helps by its wider bandwidth. Element diameter ≈ 0.005λ ≈ 2 mm. **Answer: fc = 840 MHz, λc = 35.71 cm; R = 18.00 cm, DE = 17.14 cm, D1 = 16.29 cm, D2 = 15.47 cm, D3 = 14.70 cm; spacings 8.93 cm and 5.36 cm; boom 25.0 cm.** ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In a folded dipole, how do the currents in the two parallel conductors behave? A. They are in the same direction and phase, like a dipole with double current B. They are in opposite directions and cancel the radiation C. They are in quadrature, giving circular polarization D. Only the unsplit conductor carries current Answer: A. Both conductors carry in-phase half-sine currents, so the antenna radiates like a dipole carrying double current. (Key point: Folded dipole currents) 2. What is the input impedance of a folded dipole with two equal conductors? A. 73 Ω B. 292 Ω C. 146 Ω D. 584 Ω Answer: B. Z = N^2 x 73 = 4 x 73, about 292 Ω. (Key point: Folded dipole impedance) 3. How is the design wavelength for the 785-895 MHz band found? A. Use fc = 785 MHz, the lower edge B. Use fc = 895 MHz, the upper edge C. Use fc = (785+895)/2 = 840 MHz, so lambda_c = 35.71 cm D. Use fc = 110 MHz, the bandwidth Answer: C. The band centre 840 MHz gives lambda_c = 3x10^8/840x10^6 = 35.71 cm. (Key point: Centre frequency) 4. In the 5-element design, which lengths follow from the rules (L_DE = 17.14 cm)? A. Reflector 17.14 cm, directors 20 cm each B. Reflector 16.29 cm, directors 18.00 cm and longer C. All elements equal to 17.14 cm D. Reflector 18.00 cm, then directors 16.29, 15.47, 14.70 cm Answer: D. Reflector is 5% longer (1.05 x 17.14 = 18.00) and each director is 5% shorter than the previous. 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