---
title: "Define antenna and EMW propagation"
subject: "Basic Electronics Engineering"
chapter: 5
key: "semester_2/basic_electronics_engineering:define-antenna-emw"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/define-antenna-emw
site: BCT Engineering
---

# Define antenna and EMW propagation

Basic Electronics Engineering · Chapter 5: Communication System

- Asked in: 2067 Chaitra, 4 marks
- Video: [How does an Antenna work? | ICT #4](https://www.youtube.com/watch?v=ZaXm6wau-jc) — Sabin Civil Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/define-antenna-emw
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-5

## Question

Define antenna and electromagnetic wave (EMW) propagation.

## Short answer

An antenna converts electrical signals to electromagnetic waves and vice versa. EM waves consist of oscillating perpendicular electric and magnetic fields traveling at 3×10^8 m/s without a medium. Propagation: ground wave follows Earth surface (<2 MHz); sky wave reflects from ionosphere (2-30 MHz); space wave is line-of-sight (>30 MHz).

## Answer

#### Antenna

An **antenna** is a transducer (a metallic conductor or structure) that converts electrical signals into electromagnetic waves for radiation into free space (transmitting antenna), and converts the incoming electromagnetic waves back into electrical signals (receiving antenna). It is the interface between the transmission line and free space. Examples are a dipole, a monopole (whip), a loop, a Yagi-Uda array, and a parabolic dish.

Its important parameters are gain, directivity, radiation pattern, bandwidth and impedance.

#### EMW propagation

Electromagnetic waves are fields of mutually perpendicular electric and magnetic components that travel at c = 3 × 10⁸ m/s in free space without any medium. EMW propagation means the way the wave travels from the transmitting antenna to the receiving antenna. There are three modes.

1. **Ground wave:** follows the earth's surface, for frequencies below about 2 MHz (AM broadcasting).
2. **Sky wave:** reflected back from the ionosphere, for 2 to 30 MHz (short-wave, long-distance).
3. **Space wave (line of sight):** straight path between antennas, for above 30 MHz (FM, TV, microwave, satellite).

```text
      ionosphere  ~~~~~~~~~~~~~~
          sky wave /          \
 Tx ===== space wave ======== Rx
  \____ ground wave _________/
```

## Check yourself

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

1. What does an antenna do?
   A. Converts electrical signals to EMW and EMW back to electrical signals
   B. Amplifies audio signals
   C. Generates the carrier
   D. Stores energy
   Answer: A. It interfaces the transmission line with free space. (Key point: Antenna)

2. Which mode follows the earth's surface below about 2 MHz?
   A. Sky wave
   B. Space wave
   C. Ground wave
   D. Direct wave above 30 MHz
   Answer: C. Ground waves are used for AM broadcasting. (Key point: Ground wave)

3. Which frequency range uses sky wave?
   A. 2 to 30 MHz
   B. Below 2 MHz
   C. Above 30 MHz
   D. Above 1 GHz only
   Answer: A. HF is reflected by the ionosphere. (Key point: Sky wave)

4. Which mode is line of sight above 30 MHz?
   A. Ground wave
   B. Sky wave
   C. Space wave
   D. Surface wave
   Answer: C. FM, TV, microwave and satellite use space waves. (Key point: Space wave)

## Related in Basic Electronics Engineering

- [Electromagnetic waves and propagation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Aemw-propagation) — Basic Electronics Engineering, chapter 5, asked 2×

## Related in other subjects

- [Propagation of electromagnetic energy](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Apropagation-em-energy) — Surveying I, chapter 2
- [Ionospheric wave propagation and layers](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aionospheric-propagation-layers) — Propagation and Antenna, chapter 4, asked 3×
- [Short note: Medium wave propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Amw-propagation-short-note) — Propagation and Antenna, chapter 4, asked 3×
- [Path loss in radio wave propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Apath-loss-derivation) — Propagation and Antenna, chapter 5, asked 2×