---
title: "Antenna definition and one type"
subject: "Electromagnetism"
chapter: 7
key: "semester_3/electromagnetism:antenna-definition-one-type-properties"
source: https://www.bctengineer.com/q/semester_3/electromagnetism/antenna-definition-one-type-properties
site: BCT Engineering
---

# Antenna definition and one type

Electromagnetism · Chapter 7: Antennas

- Asked in: 2075 Chaitra, 1+1 marks
- Video: [Electro Magnetics - Introduction to Antenna](https://www.youtube.com/watch?v=bIKFmJn7wH0) — TutorialsPoint
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electromagnetism/antenna-definition-one-type-properties
- Whole chapter: https://www.bctengineer.com/md/semester_3/electromagnetism/chapter-7

## Question

Give the definition of an antenna and explain the properties of any one type of antenna that you have studied during your electromagnetism course.

## Short answer

An antenna converts guided electrical energy into radiated waves and vice versa. The half-wave dipole (length ≈ λ/2) has sinusoidal current distribution, omnidirectional doughnut pattern with 78° HPBW, directivity 1.64 (2.15 dBi), radiation resistance ~73 Ω, and linear polarization.

## Answer

#### Definition

An **antenna** is a device that converts electrical energy from a transmission line into electromagnetic waves radiated into space (transmitting) and converts incoming electromagnetic waves into electrical signals (receiving). It is the transition between guided waves and free-space waves.

#### Half-wave dipole antenna: properties

A half-wave dipole consists of two straight conductors, each about λ/4 long (about λ/2 in total), fed at the centre.

```text
 <------ λ/4 ------> <------ λ/4 ------>
 ===================o o===================
                    feed
                 (centre)
```

- **Current distribution:** approximately sinusoidal, maximum at the centre and zero at the ends: I(z) = I₀ cos(kz).
- **Radiation pattern:** E_θ ∝ cos(π/2 cosθ)/sinθ. It is doughnut-shaped: zero along the axis and maximum in the plane perpendicular to the wire (omnidirectional in azimuth).
- **Half-power beamwidth:** 78° in the elevation plane.
- **Directivity:** 1.64 (2.15 dBi); the gain is close to this because the efficiency is high.
- **Radiation resistance:** about 73 Ω (input impedance 73 + j42.5 Ω; it becomes purely resistive when the length is trimmed to about 0.95 × λ/2).
- **Polarisation:** linear, parallel to the dipole axis.
- **Bandwidth:** fairly narrow (about 10 %); thicker conductors increase it.
- **Effective aperture:** A_e = 0.13λ².
- Widely used in TV and FM radio reception, and as a reference antenna for gain measurements.

## Check yourself

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

1. Which best defines an antenna?
   A. A device that stores energy in a field
   B. A transition between guided and free-space waves
   C. A device that only receives signals
   D. A device that amplifies power
   Answer: B. It converts line energy into radiated waves and incoming waves into signals. (Key point: Antenna definition)

2. What is the radiation pattern of a half-wave dipole?
   A. Isotropic
   B. Highly directive pencil beam
   C. Doughnut-shaped, zero along the axis, maximum perpendicular to it
   D. Maximum along the axis
   Answer: C. E_θ ∝ cos(π/2 cosθ)/sinθ is zero along the wire and maximum broadside. (Key point: Dipole pattern)

3. How is current distributed on a half-wave dipole?
   A. Uniform
   B. Sinusoidal, maximum at the centre and zero at the ends
   C. Maximum at the ends
   D. Linearly increasing
   Answer: B. I(z) = I0 cos(kz) is maximum at the feed and zero at the tips. (Key point: Dipole current)

4. Which pair gives the half-wave dipole's directivity and radiation resistance?
   A. 2.15 and 73 Ω
   B. 1.64 and 73 Ω
   C. 1.64 and 377 Ω
   D. 1.5 and 50 Ω
   Answer: B. Directivity is 1.64 (2.15 dBi) and radiation resistance about 73 Ω. (Key point: Dipole figures)

## Related in Electromagnetism

- [Short note: Antenna, types, properties](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Ashort-note-antenna-types-properties) — Electromagnetism, chapter 7, asked 10×

## Related in other subjects

- [Student course department class diagram](https://www.bctengineer.com/question.md?key=semester_6%2Fobject_oriented_analysis_and_design%3Astudent-course-diagram-type) — Object Oriented Analysis and Design, chapter 2
- [What is an antenna; its properties](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aantenna-properties) — Basic Electronics Engineering, chapter 5
- [Antenna definition and single wire radiation conditions](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asingle-wire-radiation-conditions) — Propagation and Antenna, chapter 1, asked 4×
- [Lossless one-port properties; Foster I and Cauer I](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Alossless-one-port-foster-cauer-numerical) — Filter Design, chapter 4