---
title: "What is an antenna; its properties"
subject: "Basic Electronics Engineering"
chapter: 5
key: "semester_2/basic_electronics_engineering:antenna-properties"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/antenna-properties
site: BCT Engineering
---

# What is an antenna; its properties

Basic Electronics Engineering · Chapter 5: Communication System

- Asked in: 2075 Baisakh, 2+2 marks
- Video: [Antennas Part I: Exploring the Fundamentals of Antennas - DC To Daylight](https://www.youtube.com/watch?v=ks8RQ9CD72Q) — element14 presents
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/antenna-properties
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-5

## Question

What is an antenna? Explain any two properties of the antenna.

## Short answer

An antenna is a transducer that converts guided electrical signals to radiated electromagnetic waves and vice versa. Directivity is the ratio of radiation intensity in the best direction to the average intensity; gain equals efficiency times directivity. The radiation pattern plots field strength versus direction, showing directional or omnidirectional behavior.

## Answer

An **antenna** is a transducer that converts an electrical signal on a transmission line into an electromagnetic wave radiated into space (transmitting), and converts a received electromagnetic wave back into an electrical signal (receiving). Examples: dipole, monopole, loop, Yagi-Uda, parabolic dish.

#### Two important properties

1. **Directivity and gain:**
   - Directivity is the ratio of the radiation intensity in the best direction to the average radiation intensity over all directions.
   - Gain = efficiency × directivity, usually in dB relative to an isotropic radiator (dBi). A high-gain antenna radiates strongly in one direction, increasing the range. A half-wave dipole has a gain of 2.15 dBi.

2. **Radiation pattern:**
   - It is a graph showing how the radiated field strength varies with direction around the antenna.
   - Omnidirectional antennas (a vertical monopole) radiate equally in the horizontal plane; directional antennas (Yagi-Uda, dish) have a main lobe and small side lobes.

Other properties: bandwidth, input impedance, polarization, beamwidth, efficiency.

## Check yourself

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

1. An antenna is best defined as:
   A. A transducer between an electrical signal on a line and an EM wave in space
   B. A device that only amplifies signals
   C. A device that stores electromagnetic energy
   D. A filter that removes noise from a signal
   Answer: A. The answer states this: A transducer between an electrical signal on a line and an EM wave in space. (Key point: Definition of antenna)

2. Directivity is the ratio of:
   A. Output power to input power of the transmitter
   B. Radiation intensity in the best direction to the average over all directions
   C. Antenna length to wavelength
   D. Power radiated to the power lost as heat
   Answer: B. The answer states this: Radiation intensity in the best direction to the average over all directions. (Key point: Directivity)

3. How is antenna gain related to directivity?
   A. Gain = directivity / efficiency
   B. Gain = directivity + wavelength
   C. Gain = efficiency x directivity
   D. Gain is independent of directivity
   Answer: C. The answer states this: Gain = efficiency x directivity. (Key point: Gain)

4. A vertical monopole has an omnidirectional pattern, which means it:
   A. Has one main lobe and small side lobes
   B. Radiates only along its axis
   C. Does not radiate at all
   D. Radiates equally in the horizontal plane
   Answer: D. The answer states this: Radiates equally in the horizontal plane. (Key point: Radiation pattern)

## Related in other subjects

- [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×
- [Antenna arrays and two-element expression](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Adefine-array-two-isotropic) — Propagation and Antenna, chapter 2
- [Realizability conditions and lossless two-port properties](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Arealizability-lossless-two-port) — Filter Design, chapter 4
- [Antenna definition and one type](https://www.bctengineer.com/question.md?key=semester_3%2Felectromagnetism%3Aantenna-definition-one-type-properties) — Electromagnetism, chapter 7