--- title: "What is ionosphere; its propagation" subject: "Propagation and Antenna" chapter: 4 key: "semester_6/propagation_and_antenna:ionosphere-wave-propagation" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ionosphere-wave-propagation site: BCT Engineering --- # What is ionosphere; its propagation Propagation and Antenna · Chapter 4: Propagation and Radio Frequency Spectrum - Asked in: 2075 Baisakh, 1+5 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/ionosphere-wave-propagation - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-4 ## Question What is ionosphere? Explain the ionosphere wave propagation showing its different layers. ## Short answer The ionosphere (50–400 km) contains D, E, F1, F2 layers ionised by solar radiation. Sky-wave propagation refracts HF waves back; D absorbs MF, E reflects MF/low HF, F2 reflects HF for long distances. Refraction occurs because n = √(1−81N/f²) decreases with height; critical frequency fc = 9√Nmax, MUF = fc sec θ. ## Answer #### Ionosphere The **ionosphere** is the upper region of the atmosphere, about 50 km to 400 km above the earth, where solar ultraviolet and X-ray radiation ionises the gas, producing free electrons and ions that can bend radio waves back to earth. #### Ionospheric wave propagation In **ionospheric (sky-wave) propagation**, a wave (mainly HF, 3–30 MHz) is sent upward, refracted back by an ionised layer and received hundreds to thousands of km away. ```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 ``` **Layers** - **D (50–90 km):** daytime only; absorbs MF/low HF; reflects VLF/LF. - **E (90–140 km):** reflects MF and lower HF; weak at night; sporadic-E patches. - **F1 (150–250 km):** daytime only; merges with F2 at night. - **F2 (250–400 km):** strongest, day and night; main reflector for long-distance HF. **Mechanism** 1. Refractive index n = √(1 − 81N/f²) falls as electron density N rises with height, so the ray bends away from the normal and turns back. 2. Critical frequency fc = 9√Nmax is the highest frequency returned at vertical incidence; for oblique incidence the limit is MUF = fc sec θi. 3. Waves above the MUF for their angle of incidence escape into space. For f > fc, steep rays pass through the layer, so no sky wave lands inside the **skip distance** (the skip zone). 4. Repeated reflections between earth and ionosphere (multi-hop) give worldwide range. 5. The ionosphere changes with time of day, season and sunspots, so frequencies are changed accordingly and signals may fade. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which layers are present only in daytime? A. F2 and E only B. D and F1 C. Only F2 D. D and F2 Answer: B. F2 persists day and night; E is weak at night. (Key point: Layer persistence) 2. Why does the ray turn back in the ionosphere? A. Refractive index decreases as electron density N rises with height B. Refractive index increases with height C. N falls with height D. The ray is absorbed and re-radiated Answer: A. n = sqrt(1 - 81N/f^2). (Key point: Mechanism) 3. A wave above the MUF: A. Is absorbed by D B. Returns after two hops C. Escapes into space D. Is reflected at vertical incidence Answer: C. Steep rays escape, creating the skip zone up to the skip distance. (Key point: Waves above MUF) 4. Why must operating frequency be changed through the day and seasons? A. Antenna height changes B. Earth radius changes C. Transmitter power changes D. Ionisation changes with time, season and sunspots Answer: D. Otherwise the signal may escape or fade. 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