--- title: "Yagi parasitic elements and LPDA comparison" subject: "Propagation and Antenna" chapter: 3 key: "semester_6/propagation_and_antenna:yagi-parasitic-elements-vs-lpda" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/yagi-parasitic-elements-vs-lpda site: BCT Engineering --- # Yagi parasitic elements and LPDA comparison Propagation and Antenna · Chapter 3: Antenna Classification - Asked in: 2080 Asoj, 1+2+3 marks; 2076 Baisakh, 1+2+4 marks; 2075 Baisakh, 1+2+4 marks (asked 3 times) - 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-parasitic-elements-vs-lpda - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3 ## Question Name the parasitic elements used in Yagi-Uda array. Explain their significance in the array. Compare Yagi-Uda antenna with Log periodic dipole array. ## Short answer The parasitic elements are one reflector and several directors. The longer inductive reflector cancels backward radiation, while shorter capacitive directors provide end-fire phasing, increasing gain and narrowing the beam. Unlike the narrowband Yagi with one driven element, the LPDA feeds all scaled elements and gives very wide bandwidth. ## Answer #### Parasitic elements of a Yagi-Uda array The Yagi-Uda array has one **driven element** (fed λ/2 or folded dipole) and two kinds of **parasitic elements** (not fed; excited by mutual coupling): 1. **Reflector** – one element, slightly longer than λ/2, placed behind the driven element. 2. **Directors** – one or more elements, slightly shorter than λ/2, placed in front. ```text R DE D1 D2 D3 | | | | | | | | | | ====> beam | -o- | | | | | | | | longest fed shorter -> ``` #### Significance - **Reflector**: being longer than resonant length, it is **inductive**; its induced current lags so that its field cancels radiation in the backward direction and reinforces the forward direction. It sets the **front-to-back ratio**. Usually one reflector is enough; more add little. - **Directors**: shorter, so **capacitive**; their currents lead and give a progressive phase towards the front, guiding the wave like an end-fire array. Each extra director increases gain (about 1 dB per director for the first few) and narrows the beam. - Parasitic coupling lowers the input impedance of the driven element, so a folded dipole (≈ 300 Ω alone) is often used to restore a convenient impedance. #### Yagi-Uda vs log periodic dipole array (LPDA) | Point | Yagi-Uda | LPDA | |---|---|---| | Elements fed | Only one (driven); rest parasitic | All elements fed by a transposed line | | Element sizes | Nearly equal (~0.45–0.5λ) | Grow by ratio τ along the antenna | | Bandwidth | Narrow (≈ 2–5 %) | Very wide (10:1 possible), frequency independent | | Gain | Higher for same boom, 7–15 dBi | Moderate, 6–10 dBi, almost constant | | Beam direction | Towards the directors | Towards the shortest elements | | Input impedance | Varies with frequency | Nearly constant over band | | Design | Simple, empirical | Uses τ, σ, apex angle α | | Use | TV for one channel, fixed links | Multiband TV, EMC testing, HF wideband | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The parasitic elements of a Yagi-Uda are A. a reflector and one or more directors B. two driven dipoles C. only a folded dipole D. a feeder and a balun Answer: A. The driven element is fed; reflector and directors are excited by mutual coupling. (Key point: Parasitic elements) 2. The reflector is slightly longer than lambda/2, so it is A. capacitive, and increases gain per element B. inductive, and sets the front-to-back ratio C. resistive, and sets bandwidth D. fed directly from the transmitter Answer: B. Its current lags and cancels radiation backward, setting the front-to-back ratio. (Key point: Reflector behaviour) 3. Each director is shorter than resonance, so it is A. inductive, with lagging current B. unable to radiate C. capacitive, with leading current that guides the wave forward D. longer than the driven element Answer: C. Directors give progressive phase forward like an end-fire array and add roughly 1 dB each at first. (Key point: Directors) 4. Which comparison is correct? A. Yagi feeds all elements; LPDA feeds one B. Both have 2-5% bandwidth C. LPDA has higher gain than Yagi on the same boom D. LPDA feeds all elements and has very wide bandwidth; Yagi feeds one element and is narrowband Answer: D. LPDA bandwidth can reach 10:1 with moderate gain, while Yagi is narrowband with higher gain. (Key point: Yagi vs LPDA) ## Related in Propagation and Antenna - [Yagi vs log-periodic; LP design](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ayagi-vs-log-periodic-design) — Propagation and Antenna, chapter 3 - [Short note: Parasitic array antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aparasitic-array-short-note) — Propagation and Antenna, chapter 3 - [Log-periodic antenna construction and design](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alog-periodic-antenna) — Propagation and Antenna, chapter 3, asked 3× - [Yagi-Uda construction, working and design](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ayagi-construction-design) — Propagation and Antenna, chapter 3, asked 2× - [UHF band and log-periodic antenna](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Auhf-band-log-periodic) — Propagation and Antenna, chapter 3 - [Yagi-Uda design, advantages and applications](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ayagi-advantages-applications) — Propagation and Antenna, chapter 3 ## Related in other subjects - [Short note: λ/2 dipole antenna](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Ashort-note-dipole-antenna) — Basic Electronics Engineering, chapter 5 - [Accessing array elements using pointer](https://www.bctengineer.com/question.md?key=semester_1%2Fcomputer_programming%3Apointer-access-array-elements) — Computer Programming, chapter 9