---
title: "Short note: λ/2 dipole antenna"
subject: "Basic Electronics Engineering"
chapter: 5
key: "semester_2/basic_electronics_engineering:short-note-dipole-antenna"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-dipole-antenna
site: BCT Engineering
---

# Short note: λ/2 dipole antenna

Basic Electronics Engineering · Chapter 5: Communication System

- Asked in: 2067 Mangsir, 6 marks
- Video: [Why dipole antennas are a half wave long](https://www.youtube.com/watch?v=iInJkG8YPkA) — Carl Oliver
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-dipole-antenna
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-5

## Question

Write a short note on λ/2 dipole antenna.

## Short answer

A half-wave (λ/2) dipole consists of a conductor of total length λ/2, center-fed, with each arm λ/4. Its radiation pattern is doughnut-shaped, maximum broadside; input impedance is about 73 Ω; gain is 2.15 dBi. It is the basic reference antenna in many arrays.

## Answer

A **half-wave (λ/2) dipole antenna** is the basic and most widely used antenna. It is a straight conductor of total length equal to half the wavelength (λ/2), split at the centre where the feed line is connected. Each arm is λ/4 long.

```text
     <----- λ/2 ------>
     ===========+=========
      λ/4     feed    λ/4
                |
         transmission line

 Current:   0 -- max -- 0     (current is max at centre,
 Voltage:  max -- 0 -- max     voltage is max at the ends)
```

#### Features

- **Length:** L = λ/2 = c/(2f). In practice the length is about 5 % shorter (end effect), L ≈ 143/f (MHz) metres.
- **Radiation pattern:** doughnut-shaped in 3D; figure-of-eight in the plane containing the wire. Maximum radiation is at right angles to the wire, and zero along its axis.
- **Input (radiation) resistance:** about 73 Ω at the centre, which matches 75 Ω coaxial cable.
- **Gain:** 1.64 (2.15 dBi) over an isotropic antenna; beamwidth about 78°.
- **Polarization:** linear, in the direction of the wire (horizontal or vertical).
- Resonant antenna, so it has narrow bandwidth.

#### Uses

TV and FM receiving antennas, amateur radio, and the driven element in Yagi-Uda arrays; used as a reference for other antennas.

## Check yourself

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

1. A half-wave dipole has total length lambda/2 with:
   A. Two arms of lambda/4 each, fed at the centre
   B. Two arms of lambda/2 each, fed at one end
   C. One arm of lambda/4 fed at the end
   D. Two arms of lambda each, fed at the centre
   Answer: A. The answer states this: Two arms of lambda/4 each, fed at the centre. (Key point: Dipole structure)

2. In a half-wave dipole:
   A. Voltage is maximum at the centre and current at the ends
   B. Current is maximum at the centre and voltage is maximum at the ends
   C. Both are maximum at the centre
   D. Both are maximum at the ends
   Answer: B. The answer states this: Current is maximum at the centre and voltage is maximum at the ends. (Key point: Current and voltage distribution)

3. Where is the radiation of a half-wave dipole maximum?
   A. Along the axis of the wire
   B. Equally in all directions in 3D
   C. At right angles to the wire
   D. Only at the ends of the wire
   Answer: C. The answer states this: At right angles to the wire. (Key point: Radiation pattern)

4. The input resistance of a half-wave dipole is about 73 ohm, which:
   A. Matches 300 ohm twin lead only
   B. Is too low to be fed by any cable
   C. Is equal to the free-space impedance
   D. Matches 75 ohm coaxial cable
   Answer: D. The answer states this: Matches 75 ohm coaxial cable. (Key point: Input resistance)

## Related in other subjects

- [Operation of infinitesimal dipole](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ainfinitesimal-dipole-operation) — Propagation and Antenna, chapter 1, asked 2×
- [Retarded potentials of infinitesimal dipole](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aretarded-potential-infinitesimal-dipole) — Propagation and Antenna, chapter 1
- [Retarded potential importance and infinitesimal dipole](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aretarded-potential-importance-dipole) — Propagation and Antenna, chapter 1
- [Infinitesimal dipole fields and basic parameters](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Ainfinitesimal-dipole-fields-parameters) — Propagation and Antenna, chapter 1