---
title: "Short note: Antenna, types, properties"
subject: "Electromagnetism"
chapter: 7
key: "semester_3/electromagnetism:short-note-antenna-types-properties"
source: https://www.bctengineer.com/q/semester_3/electromagnetism/short-note-antenna-types-properties
site: BCT Engineering
---

# Short note: Antenna, types, properties

Electromagnetism · Chapter 7: Antennas

- Asked in: 2076 Chaitra, 2 marks; 2076 Asoj, 3 marks; 2075 Asoj, 2 marks; 2074 Asoj, 2 marks; 2073 Shrawan, 2 marks; 2072 Chaitra, 2 marks; 2070 Chaitra, 3 marks; 2069 Chaitra, 2 marks; 2068 Chaitra, 2 marks; 2068 Baisakh, 3 marks (asked 10 times)
- 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/short-note-antenna-types-properties
- Whole chapter: https://www.bctengineer.com/md/semester_3/electromagnetism/chapter-7

## Question

Write a short note on antenna, its types and properties.

## Short answer

An antenna is a transducer converting guided electrical signals to radiated electromagnetic waves. Common types include wire (dipole), aperture (horn), reflector (parabolic), array (Yagi), and microstrip. Key properties: radiation pattern (directivity, beamwidth), gain, radiation resistance, polarization, bandwidth, and effective aperture.

## Answer

An **antenna** is a transducer that converts guided electrical signals (current on a line) into radiated electromagnetic waves in free space, and the reverse. It forms the interface between a transmission line or waveguide and free space.

#### Types

| Class | Examples |
|---|---|
| Wire antennas | Dipole, monopole, loop, helix |
| Aperture antennas | Horn, slot, waveguide opening |
| Reflector antennas | Parabolic dish, corner reflector |
| Array antennas | Yagi-Uda, phased array, log-periodic |
| Microstrip (patch) | Used in mobile phones, GPS |
| Lens antennas | Dielectric lens at microwave frequencies |

#### Properties

1. **Radiation pattern:** plot of radiated field/power against direction (isotropic, omnidirectional, directional); has main lobe, side lobes and back lobe.
2. **Directivity D:** ratio of maximum radiation intensity to the average (isotropic) value, D = 4πU_max/P_rad.
3. **Gain G = ηD:** directivity multiplied by radiation efficiency η = P_rad/P_input.
4. **Radiation resistance R_r:** P_rad = ½I²R_r; for a λ/2 dipole R_r = 73 Ω.
5. **Beamwidth:** angle between half-power points of the main lobe (HPBW); narrow beam means high gain.
6. **Bandwidth:** frequency range over which the antenna keeps its impedance and pattern within limits.
7. **Polarisation:** direction of the E-field: linear, circular or elliptical.
8. **Input impedance:** R_r + loss resistance + jX; matched to the feed line to avoid reflection (low VSWR).
9. **Effective aperture:** A_e = λ²G/4π, the area that captures power from an incident wave.
10. **Reciprocity:** the same antenna has the same pattern and gain for transmitting and receiving.

Example: the half-wave dipole has length λ/2, directivity 1.64 (2.15 dBi), HPBW 78° and an omnidirectional pattern in the plane perpendicular to its axis.

## Check yourself

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

1. What does an antenna do?
   A. Amplifies signals on a line
   B. Converts guided signals into radiated EM waves and the reverse
   C. Stores electromagnetic energy
   D. Filters unwanted frequencies
   Answer: B. It is the transducer between a transmission line or waveguide and free space. (Key point: Antenna function)

2. How are gain and directivity related?
   A. G = D/η
   B. G = D + η
   C. G = ηD, with η the radiation efficiency
   D. G = D^2
   Answer: C. Gain is directivity multiplied by radiation efficiency P_rad/P_input. (Key point: Gain)

3. Which pair is correctly classified?
   A. Horn: wire antenna
   B. Parabolic dish: aperture antenna
   C. Dipole: wire antenna
   D. Yagi-Uda: lens antenna
   Answer: C. Dipole, monopole, loop and helix are wire antennas; horn is aperture, dish is reflector, Yagi-Uda is an array. (Key point: Antenna classes)

4. What is the relation between beamwidth and gain?
   A. A narrow beam means high gain
   B. A narrow beam means low gain
   C. They are unrelated
   D. Wider beam means higher directivity
   Answer: A. Radiation is concentrated in a smaller angle, so directivity and gain rise. (Key point: Beamwidth)

## Related in Electromagnetism

- [Antenna definition and one type](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Aantenna-definition-one-type-properties) — Electromagnetism, chapter 7

## Related in other subjects

- [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
- [Properties for selecting optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-selection-properties) — Propagation and Antenna, chapter 6