Surface wave propagation pros and cons

Surface wave propagation is the mode in which a vertically polarised wave travels along the surface of the earth, following its curvature. The earth acts as a lossy guiding boundary: the wave induces charges and currents in the ground, which keep it attached to the surface. It is the main mode for VLF, LF and MF (up to about 2–3 MHz).

     wave front tilts forward (wave tilt)
       |  /  /  /  /
       | /  /  /  /    → direction of travel
  Tx  /|/  /  /  /
 ====|==================================
  ~~~~ induced earth currents (loss) ~~~~

Explanation

• Vertical polarisation is necessary; a horizontally polarised wave is short-circuited by the conducting ground and dies out quickly.
• Wave tilt: energy is lost into the ground, so the wave front tilts forward. The E-field gains a horizontal component that drives power down into the earth. Tilt is larger for poor ground.
• Attenuation: field strength is approximately

E = A · E₀ / d

where E₀/d is the unattenuated (inverse-distance) field and A (< 1) is the ground attenuation factor, found from Sommerfeld/Norton curves using the "numerical distance". A falls faster with:

• higher frequency (loss increases rapidly with f),
• lower ground conductivity σ and permittivity εr,
• longer distance.
• Diffraction: the wave bends around the earth's curvature by diffraction, more easily at long wavelengths, which is why low frequencies travel far.
• Ground type: sea water (σ ≈ 4 S/m) gives the longest range; moist soil is medium; dry and rocky land is poor.
• Range: VLF/LF up to 1000+ km; MF ≈ 100–300 km; HF only tens of km.

Advantages

1. Stable, reliable signal with no fading (no ionospheric dependence).
2. Works day and night, in all seasons.
3. Not limited by line of sight; follows earth's curvature.
4. VLF waves penetrate sea water, useful for submarine communication.
5. Simple receiving arrangements; good for local and regional broadcast (AM).

Disadvantages

1. Usable only at low frequencies (below ≈ 3 MHz), so bandwidth is very small.
2. Requires very tall vertical antennas (λ/4 is 75 m at 1 MHz) and extensive ground systems.
3. High transmitter power needed because of ground losses.
4. Range falls sharply with frequency and over poor soil.
5. Atmospheric noise is high at LF/MF.