---
title: "Types of propagation"
subject: "Data Communication"
chapter: 4
key: "semester_5/data_communication:propagation-types"
source: https://www.bctengineer.com/q/semester_5/data_communication/propagation-types
site: BCT Engineering
---

# Types of propagation

Data Communication · Chapter 4: Transmission Media

- Asked in: 2072 Kartik, 5 marks
- Video: [Wave Propagation Introduction | Antenna and Wave Propagation | Hindi |](https://www.youtube.com/watch?v=v29xL6HZCfY) — Dopamine
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/propagation-types
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-4

## Question

Explain about different types of propagation.

## Short answer

Ground-wave propagation follows Earth's curvature, used below 2 MHz for AM radio. Sky-wave propagation refracts signals (2-30 MHz) off the ionosphere for long-distance HF communication. Line-of-sight propagation (above 30 MHz) requires direct visibility between antennas, used for FM, TV, mobile, and microwave links; maximum distance depends on antenna height.

## Answer

Radio waves travel from transmitter to receiver in three main ways, depending on frequency.

#### 1. Ground-wave propagation
```text
  Tx ~~~~~~~~~~~~~~> Rx  (follows Earth's curvature)
 ====================================
```
The wave follows the curvature of the Earth's surface. It is used for frequencies below about 2 MHz (VLF, LF, MF, such as AM radio and navigation). The range can be hundreds to thousands of km, depending on power; losses grow with frequency.

#### 2. Sky-wave (ionospheric) propagation
```text
          ionosphere -------------
            /           \
          /               \
 Tx ---/                   \--- Rx
===================================
```
Waves of 2 to 30 MHz (HF, shortwave) go up and are reflected (refracted) back by the ionosphere, so they can travel around the globe in multiple hops. It is used for shortwave broadcasting and long-distance amateur radio, and it is affected by the time of day and solar activity.

#### 3. Line-of-sight (LOS) propagation
```text
   Tx antenna            Rx antenna
     |  ------------------->  |
    _|_                      _|_
```
Above 30 MHz (VHF, UHF, microwave, satellite), waves travel in a straight line and are not bent back by the ionosphere. The transmitting and receiving antennas must see each other, so the range is limited by the Earth's curvature. The radio horizon of one antenna of height h (in metres) is d = 3.57√(K·h) km with K = 4/3, so two antennas of equal height can be at most about 7.14√(K·h) km apart. Towers or satellites are used to extend the range. LOS is used for FM, TV, mobile, microwave links and satellites.

## Check yourself

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

1. Ground-wave propagation is used for which frequencies?
   A. 2 to 30 MHz
   B. Below about 2 MHz (VLF, LF, MF such as AM radio)
   C. Above 30 MHz
   D. Only above 1 GHz
   Answer: B. Ground waves follow the Earth's curvature at low frequencies. (Key point: Ground wave)

2. How does sky-wave propagation reach distant places?
   A. The ionosphere reflects/refracts 2-30 MHz waves back, giving multiple hops around the globe
   B. Waves follow the ground surface
   C. Waves travel in a straight line between towers
   D. Satellites relay the signal
   Answer: A. HF waves are bent back by the ionosphere. (Key point: Sky wave)

3. Why are towers or satellites needed in line-of-sight propagation?
   A. The ionosphere absorbs them
   B. They travel only through the ground
   C. Waves above 30 MHz are not bent by the ionosphere and range is limited by Earth's curvature
   D. They need repeaters because of heavy attenuation in air only
   Answer: C. Antennas must see each other; curvature limits range. (Key point: LOS)

4. Which pairs mode with use correctly?
   A. Ground wave - FM and TV; sky wave - AM
   B. LOS - shortwave; ground wave - satellites
   C. Sky wave - satellites; LOS - AM radio
   D. Sky wave - shortwave broadcasting; LOS - FM, TV, microwave
   Answer: D. Shortwave uses the ionosphere; FM/TV use LOS. (Key point: Matching)