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Ionosphere structure and ground wave

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.

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

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:

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

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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