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Sky wave and ionospheric 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.

What is the sky wave? Explain the ionospheric wave propagation showing its different layers.

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.

 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
LayerHeightElectron density (per m³)PresentMain effect
D50–90 km10⁸–10¹⁰Day onlyAbsorbs LF/MF sky wave; reflects VLF
E90–140 km (≈110)≈10¹¹Mostly day (weak at night)Reflects MF and lower HF; fc ≈ 3–4 MHz
Sporadic Eₛ≈110 kmPatchy, highIrregularOccasional VHF long-distance
F1150–250 km (≈200)≈2–5 × 10¹¹Day onlySome HF reflection, absorption; fc ≈ 4–5 MHz
F2250–400 km≈10¹² (highest)Day and nightMain 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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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