--- title: "Sky wave and ionospheric layers" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:sky-wave-ionospheric-layers" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/sky-wave-ionospheric-layers site: BCT Engineering --- # Sky wave and ionospheric layers Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2080 Asoj, 2+6 marks - Video: [Sky Wave Propagation (Basics, Ionosphere Layers, Multi Hope Propagation, Issues & Parameters)](https://www.youtube.com/watch?v=AhgFZBQLLxA) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/sky-wave-ionospheric-layers - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4 ## Question What is the sky wave? Explain the ionospheric wave propagation showing its different layers. ## Short answer Sky wave travels upward and is refracted back by the ionosphere, used for HF. The ionospheric layers are D (absorbs), E (reflects MF/low HF), F1 (daytime, some reflection), and F2 (main reflecting layer, highest electron density). Propagation depends on critical frequency, MUF, and skip distance; multi-hop enables global coverage. ## Answer #### Sky wave A **sky wave** is a radio wave that travels upward from the transmitting antenna and is bent (refracted) back to earth by the ionosphere, reaching receivers far beyond the horizon. It is the main mode for HF (3–30 MHz) long-distance communication. #### Ionospheric wave propagation and layers The ionosphere (≈ 50–400 km) is ionised by solar UV and X-rays. Its free electrons lower the refractive index n = √(1 − 81N/f²), so an upgoing wave bends away from the normal and gradually turns back to earth. ```text height (km) 400 |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ F2 (250-400) 300 | /\ day & night 220 |~~~~~~~/~~\~~~~~~~~~~~~~~~~~~ F1 (150-250) | / \ day only 110 |~~~~~/~~~~~~\~~~~~~~~~~~~~~~~ E (90-140) 70 |::::/::::::::\::::::::::::::: D (50-90) | / sky \ day only 0 |__/____________\_____________ earth Tx skip Rx ``` | 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 | **How a sky-wave link works** 1. A wave of frequency *f* enters the layer at angle θi. It returns if f ≤ fc sec θi, where fc = 9√Nmax is the critical frequency. The limiting value is the MUF. 2. Steep rays at frequencies above fc pass through; the first ray that returns lands at the **skip distance**. Between the ground-wave limit and this point lies the **skip zone**. 3. The returned wave reflects from earth and may go up again (**multi-hop**), so worldwide coverage is possible. 4. The D layer absorbs lower frequencies by day; at night D disappears and F1 merges with F2, so lower frequencies are used and F2 carries the traffic. 5. Signals fade due to multipath and changing ionisation; OWF ≈ 0.85 MUF is chosen for reliability. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A sky wave is: A. A wave guided along the ground B. A wave sent upward and refracted back to earth by the ionosphere C. A wave travelling directly between antennas D. A wave reflected only by the ground Answer: B. It reaches receivers far beyond the horizon, mainly at HF. (Key point: Sky wave definition) 2. Why does an upgoing wave bend back to earth in the ionosphere? A. Refractive index n = sqrt(1 - 81N/f^2) falls as electron density rises, bending the ray away from the normal B. The refractive index rises with height C. Electrons absorb the wave and re-emit downward D. Gravity pulls the wave back Answer: A. Gradual decrease of n with height turns the ray back. (Key point: Refraction mechanism) 3. A wave of frequency f entering at angle theta_i returns if: A. f >= fc sec(theta_i) B. f <= fc cos(theta_i) C. f <= fc sec(theta_i) D. f = 0.85 fc Answer: C. The limit is the MUF; above it the wave escapes. (Key point: Return condition) 4. The skip zone lies between: A. Two successive hops B. The transmitter and the ionosphere C. The D and E layers D. The end of the ground-wave range and the first sky-wave return point Answer: D. Steep rays above fc pass through, so no signal arrives in this zone. (Key point: Skip zone) ## Related in Propagation and Antenna - [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× - [Space wave and sky wave characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aspace-wave-sky-wave) — Propagation and Antenna, chapter 4