--- title: "Ionospheric wave propagation and layers" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:ionospheric-propagation-layers" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ionospheric-propagation-layers site: BCT Engineering --- # Ionospheric wave propagation and layers Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2077 Chaitra, 8 marks; 2076 Baisakh, 6 marks; 2072 Asoj, 6 marks (asked 3 times) - 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/ionospheric-propagation-layers - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4 ## Question Explain the ionospheric wave propagation (operation of ionospheric communication) showing its different layers, with the structure of the ionosphere. ## Short answer The ionosphere (50–400 km) contains D, E, F1, F2 layers with varying electron density. Sky waves reflect by refraction; the highest reflected frequency is MUF = fc sec θi, with fc = 9√Nmax. Propagation enables long-distance HF communication via multiple hops, strongest at night when D layer disappears. ## Answer **Ionospheric (sky-wave) propagation** is the mode in which radio waves (mainly HF, 3–30 MHz) sent upward from the earth are bent back (refracted) by the ionised layers of the upper atmosphere and return to earth far away, giving long-distance communication of thousands of km. #### Structure of the ionosphere The **ionosphere** is the region from about 50 km to 400 km height where ultraviolet and X-ray radiation from the sun ionises gas molecules, producing free electrons and ions. Ionisation depends on sunlight, so it varies with time of day, season, latitude and the 11-year sunspot cycle. It is divided into layers: ```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 | At night the D layer disappears, F1 merges with F2 and E becomes weak, so night-time sky-wave propagation uses mainly F2 (and the residual E layer). #### Operation of ionospheric communication 1. **Refraction, not mirror reflection:** the refractive index of an ionised layer is n = √(1 − 81N/f²) (N in electrons/m³, f in Hz). As the wave goes up, N increases, n falls below 1, and the ray bends away from the normal, gradually turning back down. 2. **Condition to return:** the wave returns if at some height sin θi = n, i.e. f ≤ 9√Nmax sec θi. Higher frequencies need oblique incidence; the highest frequency that returns for a given path is the **MUF** = fc sec θi. 3. **Critical frequency fc = 9√Nmax:** the highest frequency reflected at vertical incidence. 4. **Skip distance and skip zone:** for f > fc, waves at steep angles escape; the nearest point where the sky wave returns is the skip distance. Between the end of the ground wave and this point no signal is received (skip zone). 5. **Multi-hop:** the returned wave reflects from earth and goes up again, so signals travel round the world in several hops (one F2 hop ≈ up to 4000 km). 6. **Virtual height:** the height from which a mirror reflection would give the same delay; used in calculations. 7. **Absorption** is largest in the D layer and at low frequency (∝ 1/f²), which is why MF sky wave is weak in daytime. 8. **Frequency choice:** the optimum working frequency (OWF ≈ 0.85 MUF) is used; frequency is changed between day (higher) and night (lower). **Merits:** very long range with low power; cheap. **Demerits:** fading, skip zone, dependence on sun and time, ionospheric storms, limited bandwidth. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which layers are present only during the day? A. D and F1 B. F2 only C. E and F2 D. None, all layers persist at night Answer: A. D and F1 disappear at night (F1 merges with F2), E becomes weak, and F2 remains day and night. (Key point: Ionosphere layers day and night) 2. Why does a wave sent into the ionosphere return to earth? A. Mirror reflection at a sharp surface B. As N increases with height, n = √(1 − 81N/f²) falls below 1 and the ray bends away from the normal and turns back C. Electrons absorb the wave and re-emit it vertically D. The earth's magnetic field pulls it down Answer: B. Increasing electron density decreases the refractive index, gradually bending the ray downward. (Key point: Refraction mechanism) 3. Which pair is correct? A. fc = 9√Nmax; MUF = fc sec θi B. fc = 81√Nmax; MUF = fc cos θi C. fc = √Nmax; MUF = fc/sec θi D. fc = 9Nmax; MUF = fc tan θi Answer: A. Critical frequency is the highest vertically reflected frequency, 9√Nmax, and the secant law gives MUF = fc sec θi. (Key point: Critical frequency and MUF) 4. What is the skip zone, and where is absorption largest? A. Region with no signal between the end of the ground wave and where the sky wave first returns; absorption is largest in the D layer B. Region of strongest signal; absorption largest in F2 C. Region under the ionosphere with fading only; absorption largest in E D. Region beyond 4000 km; absorption largest at high frequency Answer: A. Between ground wave range and skip distance nothing is received. Absorption is greatest in the D layer and falls as 1/f². (Key point: Skip zone and absorption) ## Related in Propagation and Antenna - [Sky wave and ionospheric layers](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Asky-wave-ionospheric-layers) — Propagation and Antenna, chapter 4 - [Ionosphere layers and their importance](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aionosphere-layers-importance) — Propagation and Antenna, chapter 4, asked 2× - [Ionosphere structure and ground wave](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aionosphere-structure-ground-wave) — Propagation and Antenna, chapter 4 - [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 - [Ionosphere layers and characteristics](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aionosphere-layers-characteristics) — Propagation and Antenna, chapter 4 - [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× ## Related in other subjects - [Slow backward wave structure and LNA](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aslow-wave-structure-lna) — RF and Microwave Engineering, chapter 5 - [Full wave bridge rectifier operation](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Afull-wave-bridge-rectifier) — Basic Electronics Engineering, chapter 2, asked 4×