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

# Electromagnetic waves and propagation

Basic Electronics Engineering · Chapter 5: Communication System

- Asked in: 2071 Magh, 2+3 marks; 2068 Magh, 2+3 marks (asked 2 times)
- Video: [Radio Wave Propagation Basics - Where do Signals Go - and How?](https://www.youtube.com/watch?v=UgA6zU59KZw) — DXCommander
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/emw-propagation
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-5

## Question

What do you mean by electromagnetic waves? How are they propagated? Explain.

## Short answer

Electromagnetic waves consist of oscillating perpendicular electric and magnetic fields traveling at 3×10^8 m/s without a medium. They propagate from a transmitting antenna to a receiver via three modes: ground wave (below 2 MHz, follows Earth's surface), sky wave (2-30 MHz, reflected by ionosphere), and space wave (above 30 MHz, line-of-sight).

## Answer

**Electromagnetic waves (EMW)** are waves made of oscillating electric and magnetic fields, perpendicular to each other and to the direction of travel. They need no medium and travel at the speed of light in free space (c = 3 × 10⁸ m/s). Radio waves, light and X-rays are examples; they are produced by accelerated charges, such as the current in an antenna.

#### Propagation

A transmitting antenna radiates the wave, which travels to the receiving antenna in one of three ways, depending on frequency.

```text
          sky wave  .--ionosphere--.
                  /                 \
 Tx ----space wave (line of sight)--- Rx
  \_____ground wave (follows earth)___/
```

1. **Ground (surface) wave propagation:** the wave travels along the earth's surface, following its curvature. Used for frequencies below about 2 MHz (LF, MF), such as AM broadcasting. Range is up to a few hundred km, and it depends on the ground conductivity.
2. **Sky wave propagation:** the wave goes up to the ionosphere, which reflects (refracts) it back to earth. Used for HF (2 to 30 MHz), such as short-wave radio. It gives very long distance communication, even across continents, with the possibility of fading.
3. **Space wave propagation:** the wave travels in a straight line, directly or after reflection from the ground. It is used for VHF, UHF and microwaves (above 30 MHz), such as FM, TV, mobile and satellite links. It needs line of sight, so antennas are placed on tall towers. Range is about d ≈ 3.57 √h km for an antenna height h in metres, so tall towers give a wider coverage.

## Check yourself

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

1. Which describes an electromagnetic wave?
   A. Longitudinal sound waves needing air
   B. Perpendicular oscillating E and B fields that travel without a medium
   C. Electric field only, needing a medium
   D. Fields parallel to travel direction
   Answer: B. EMW fields are perpendicular to each other and to travel direction; speed c in free space. (Key point: EMW definition)

2. Which mode suits AM broadcasting (below about 2 MHz)?
   A. Sky wave
   B. Space wave
   C. Satellite wave
   D. Ground wave
   Answer: D. Low frequencies follow the earth's surface. (Key point: Ground wave)

3. How does sky wave propagation give long distance?
   A. The wave follows a straight line to the receiver
   B. The ionosphere reflects HF (2 to 30 MHz) waves back to earth
   C. The wave travels inside the ground
   D. The wave passes through the ionosphere
   Answer: B. Ionospheric reflection allows communication across continents. (Key point: Sky wave)

4. What is needed for space wave propagation and how does range depend on height?
   A. No line of sight; height does not matter
   B. Ionosphere; range proportional to frequency
   C. Line of sight; range falls with height
   D. Line of sight; taller antennas give wider range (d ≈ 3.57√h km)
   Answer: D. Above 30 MHz waves go straight, so tower height extends coverage. (Key point: Space wave)

## Related in Basic Electronics Engineering

- [Define antenna and EMW propagation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Adefine-antenna-emw) — Basic Electronics Engineering, chapter 5

## 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
- [Magnetic material for electromagnetic relays](https://www.bctengineer.com/question.md?key=semester_3%2Felectrical_engineering_material%3Arelay-magnetic-material) — Electrical Engineering Material, chapter 4