---
title: "Short note: Parasitic array antenna"
subject: "Propagation and Antenna"
chapter: 3
key: "semester_6/propagation_and_antenna:parasitic-array-short-note"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/parasitic-array-short-note
site: BCT Engineering
---

# Short note: Parasitic array antenna

Propagation and Antenna · Chapter 3: Antenna Classification

- Asked in: 2072 Magh, 3 marks
- Video: [Parasitic Elements in Antennas](https://www.youtube.com/watch?v=0D-US86gtgI) — Stan Gibilisco
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/parasitic-array-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-3

## Question

Write a short note on parasitic array antenna.

## Short answer

A parasitic array has only the driven element fed; parasitic elements are excited by mutual coupling and re-radiate. A longer reflector and shorter directors, spaced 0.1–0.3λ, produce a unidirectional end-fire beam. Yagi–Uda is the classic example; three elements give about 7 dB gain with narrow bandwidth.

## Answer

A **parasitic array** is an antenna array in which only one element (the driven element) is fed from the transmitter; the other elements, called **parasitic elements**, are not connected to the feed line. They get current by **mutual coupling** (induction) from the driven element and re-radiate, so that the total field is strengthened in one direction. The best-known example is the **Yagi–Uda antenna**.

```text
 Reflector  Driven   D1    D2    D3
    |         |      |     |     |   -> max radiation
    |        =|=     |     |     |
    |         |      |     |     |
 <-0.25λ-> <-0.1 to 0.3λ spacing->
```

**Elements**
- **Driven element:** a half-wave dipole or folded dipole (about 0.46–0.48λ), connected to the feeder.
- **Reflector:** about 5% longer than the driven element (≈0.5λ). Its reactance is inductive, so its re-radiated field cancels radiation behind it.
- **Directors:** about 5% shorter (≈0.43–0.45λ). Their reactance is capacitive, so their fields add in the forward direction.
- **Spacing:** 0.1λ to 0.3λ between elements.

**Working:** the current induced in a parasitic element depends on its length (its self-reactance) and its spacing (the phase delay). With the right length and spacing, the fields from all elements add in phase toward the directors and nearly cancel toward the reflector, giving an end-fire, unidirectional beam.

**Features**
- Gain about 7 dB for 3 elements, rising with more directors (but slowly).
- Narrow bandwidth (a few %), since element lengths are resonant.
- Feed impedance of the dipole drops (to about 20–25 Ω), so a folded dipole is used to raise it.
- Simple, light and cheap; used for TV reception, VHF/UHF links and amateur radio.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In a parasitic array, how do the non-driven elements get their current?
   A. From a separate transmitter
   B. Through a transmission line from the feed
   C. By mutual coupling (induction) from the driven element
   D. From the ground
   Answer: C. Only the driven element is fed; parasitic elements are excited by mutual coupling and re-radiate. (Key point: Definition of parasitic array)

2. Which statement about reflector and directors is correct?
   A. Reflector is about 5% longer and inductive; directors about 5% shorter and capacitive
   B. Reflector is about 5% shorter and capacitive; directors about 5% longer and inductive
   C. Both are 5% longer and inductive
   D. Reflector is the same length as the driven element
   Answer: A. The reflector's inductive reactance cancels radiation backward; the directors' capacitive reactance adds radiation forward. (Key point: Reflector vs director)

3. Why is a folded dipole used as the driven element in a parasitic array such as a Yagi?
   A. Dipole impedance drops to about 20-25 Ω, and the folded dipole raises it
   B. To reduce the number of directors
   C. To widen the bandwidth to a decade
   D. To make the beam bidirectional
   Answer: A. Mutual coupling with parasitic elements lowers the feed impedance, and a folded dipole raises it back. (Key point: Features of parasitic array)

## Related in Propagation and Antenna

- [Yagi parasitic elements and LPDA comparison](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ayagi-parasitic-elements-vs-lpda) — Propagation and Antenna, chapter 3, asked 3×
- [Pattern lobes and 180° out-of-phase array](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alobes-importance-opposite-phase-array) — Propagation and Antenna, chapter 2
- [Two-element array: broadside and end fire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Atwo-element-array-broadside-endfire) — Propagation and Antenna, chapter 2
- [Two-element array and end fire pattern](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Atwo-isotropic-endfire-pattern) — Propagation and Antenna, chapter 2
- [Linear array; broadside vs end fire](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Alinear-array-broadside-endfire) — Propagation and Antenna, chapter 2
- [Short note: Broadside vs endfire array](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Abroadside-endfire-short-note) — Propagation and Antenna, chapter 2