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Surface wave factors; MW and SW

Short answer

Surface wave attenuation rises with frequency, depends on ground conductivity and permittivity; only vertical polarisation works. MW uses ground wave by day and E-layer sky wave at night, range 100–2000 km. SW uses F2-layer sky wave with skip zone, fading; frequency changes diurnally and with sunspot cycle.

Write down the factors which affect the surface wave communication. Explain the major characteristics of MW and SW radio propagation.

Factors affecting surface wave communication

  1. Frequency: attenuation increases rapidly with frequency; surface wave is useful only up to about 2–3 MHz.
  2. Ground conductivity (σ): higher σ means lower loss; sea water (≈ 4 S/m) is best, dry/rocky soil is worst.
  3. Ground permittivity (εr): affects wave tilt and attenuation, especially at higher frequencies.
  4. Polarisation: only vertical polarisation propagates; horizontal is short-circuited by the ground.
  5. Distance and earth curvature: field falls as A/d (A = attenuation factor) and faster beyond the diffraction region.
  6. Terrain and obstacles: hills, forests, buildings and mixed land–sea paths add loss.
  7. Transmitter power and antenna (height, efficiency, ground system).
  8. Atmospheric noise (high at LF/MF) reduces usable range.

Characteristics of MW and SW radio propagation

MW (medium wave) propagation, 300 kHz – 3 MHz (AM broadcast 526.5–1606.5 kHz):

  • Daytime: mainly ground (surface) wave; the D layer absorbs the sky wave. Range ≈ 100–300 km depending on power and soil (longest over sea).
  • Nighttime: D layer disappears, the E layer reflects the sky wave, giving 1000–2000 km range, but also co-channel interference and fading where ground and sky waves mix.
  • Vertical polarisation and tall λ/4 mast antennas with radial ground systems are used.
  • Ground-wave attenuation increases with frequency, so the lower end of the band covers more area.
  • High atmospheric noise; narrow bandwidth (9 kHz channels in ITU Region 3, including Nepal; 10 kHz in the Americas).

SW (short wave) propagation, 3 – 30 MHz (HF):

  • Mainly sky wave via the F2 (and E) layer; ground wave dies within a few tens of km.
  • Very long range with modest power: one F2 hop up to ≈ 4000 km, worldwide by multi-hop.
  • Governed by critical frequency, MUF and skip distance; a skip zone exists between the end of ground wave and the first sky-wave return.
  • Fading due to multipath and changes in the ionosphere; diversity reception is used.
  • Frequency must change with time of day, season and sunspot cycle: higher frequencies by day, lower at night; OWF ≈ 0.85 MUF.
  • Affected by ionospheric storms and sudden ionospheric disturbances (radio blackouts).
  • Horizontal antennas such as dipoles, rhombic and log-periodic arrays are common.

✓ Answer checked by Claude Opus 5.5 (high effort)

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