11.4 SURFACE-WAVE COMMUNICATIONS

When both the transmitting and receiving antennas are on, or very close to, the ground (that is, h<<λ), particularly at low frequencies where the wavelength is large compared with practical antenna heights, the direct and ground-reflected components of the space wave interfere destructively near the surface. Under these conditions, the dominant received component is the surface wave. As antenna height increases, the relative contribution of the surface wave decreases and propagation transitions progressively to space-wave dominance.

A surface wave travels along the Earth’s surface and is sustained by the interaction between the electromagnetic field and currents induced in the conducting ground. Because it is bound to the ground–air interface, it follows the curvature of the Earth and can extend beyond the geometric horizon. Unlike sky-wave propagation, the surface wave does not rely on ionospheric refraction but remains confined to the boundary region near the ground. It is not simply the lower half of a spherical radiation pattern, but a distinct boundary-guided mode concentrated close to the surface, continuously supported by induced conduction currents, while the upward-radiated portion of the original field either dissipates into space or is absorbed in the lower ionosphere.

On poorly conducting ground, induced currents are weak and resistive losses high, leading to rapid attenuation and reduced communication range. The rate of attenuation depends on both ground conductivity and wave polarization.