---
title: "Ionosphere structure and ground wave"
subject: "Propagation and Antenna"
chapter: 4
key: "semester_6/propagation_and_antenna:ionosphere-structure-ground-wave"
source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ionosphere-structure-ground-wave
site: BCT Engineering
---

# Ionosphere structure and ground wave

Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum

- Asked in: 2078 Chaitra, 4+4 marks
- Video: [wave propagation | sky wave propagation | ground wave propagation | propagation of wave | antinna](https://www.youtube.com/watch?v=mrhSpmCNCLk) — Mind of Engineer 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ionosphere-structure-ground-wave
- Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4

## Question

Describe briefly the structure of the ionosphere and its uses for radio frequency spectrum. Write the characteristics of ground wave propagation.

## Short answer

The ionosphere (50–400 km) has D, E, F1, F2 layers. D reflects VLF/LF, E reflects MF at night, F2 reflects HF for long-distance sky-wave; VHF+ penetrate. Ground wave travels along the surface, must be vertically polarised, usable below ~3 MHz, range depends on ground conductivity, and is stable with no fading.

## Answer

#### Structure of the ionosphere and its use for the RF spectrum
The **ionosphere** is the part of the upper atmosphere, about 50–400 km high, ionised by the sun's UV and X-rays. It contains several layers:

| Layer | Height | Electron density (per m³) | Present | Main effect |
|---|---|---|---|---|
| D | 50–90 km | 10⁸–10¹⁰ | Day only | Absorbs LF/MF sky wave; reflects VLF |
| E | 90–140 km (≈110) | ≈10¹¹ | Mostly day (weak at night) | Reflects MF and lower HF; fc ≈ 3–4 MHz |
| Sporadic Eₛ | ≈110 km | Patchy, high | Irregular | Occasional VHF long-distance |
| F1 | 150–250 km (≈200) | ≈2–5 × 10¹¹ | Day only | Some HF reflection, absorption; fc ≈ 4–5 MHz |
| F2 | 250–400 km | ≈10¹² (highest) | Day and night | Main HF reflector; fc ≈ 5–12 MHz; longest hops |

At night D and F1 disappear and E weakens, leaving mainly F2.

**Uses for the radio spectrum:**
- **VLF/LF:** reflected by the D layer's lower edge; the earth and ionosphere form a waveguide giving stable worldwide navigation and time signals.
- **MF:** absorbed by D in daytime; reflected by E at night, so AM broadcasts travel far at night.
- **HF (3–30 MHz):** refracted by E and F2 layers; makes long-distance and worldwide sky-wave communication (SW broadcast, maritime, military, amateur).
- **VHF and above:** pass through the ionosphere (except occasional sporadic E), so they are used for satellite and space links, and local LOS services.

#### Characteristics of ground wave propagation
1. Wave travels along the earth's surface, following its curvature by diffraction.
2. Must be **vertically polarised**; horizontal polarisation is shorted by the ground.
3. Attenuation increases rapidly with frequency, so it is useful only up to about 2–3 MHz (VLF, LF, MF).
4. Range depends on ground conductivity and permittivity: longest over sea water, shortest over dry, rocky land.
5. **Wave tilt:** the wave front leans forward, sending energy into the ground.
6. Stable signal with no fading; independent of time of day and season.
7. Needs tall antennas, good ground systems and high power.
8. Ranges: VLF/LF up to 1000+ km, MF about 100–300 km.

## Check yourself

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

1. The ionosphere is formed by:
   A. Solar UV and X-ray ionisation of the upper atmosphere (about 50-400 km)
   B. Cosmic rays hitting the troposphere
   C. Lightning in the stratosphere
   D. Earth's magnetic field only
   Answer: A. The free electrons refract radio waves. (Key point: Ionosphere definition)

2. MF behaviour in the ionosphere is:
   A. Reflected by F2 in daytime
   B. Absorbed by D in daytime, reflected by E at night
   C. Passes through at all times
   D. Reflected by D in daytime
   Answer: B. So AM broadcasts travel far at night. (Key point: MF use)

3. VHF and above are used for satellite links because:
   A. They are reflected by F2
   B. They are absorbed by D
   C. They pass through the ionosphere
   D. They travel as ground wave
   Answer: C. Except for occasional sporadic E, they are not returned. (Key point: VHF and above)

4. Ground wave attenuation:
   A. Falls with frequency
   B. Is least over dry rocky land
   C. Is independent of ground type
   D. Rises rapidly with frequency and is least over sea water
   Answer: D. So it is usable only up to about 2-3 MHz. (Key point: Ground-wave characteristics)

## Related in Propagation and Antenna

- [Short note: Radio frequency spectrum](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arf-spectrum-short-note) — Propagation and Antenna, chapter 4, asked 3×
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- [What is ionosphere; its propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aionosphere-wave-propagation) — Propagation and Antenna, chapter 4
- [RF spectrum; MW and SW propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arf-spectrum-mw-sw) — Propagation and Antenna, chapter 4
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