Surface wave

Lore & Background

The study of surface waves in radio propagation traces to the work of Arnold Sommerfeld and Jonathan Zenneck, who analyzed wave propagation over a lossy earth. The Zenneck surface wave is a non-radiative, bound-mode guided wave that exists as an exact solution to Maxwell's equations, supported by a planar or spherical interface between two homogeneous media with different dielectric constants. It propagates parallel to the interface and decays exponentially perpendicular to it, a property called evanescence. This wave exists under the condition that the permittivity of one material is negative while the other is positive, as at the interface between air and a lossy conducting medium below the plasma frequency. The electric field strength falls off at a rate of e⁻αᵈ/√d along the interface due to two-dimensional geometrical spreading combined with frequency-dependent exponential attenuation. Evidence does not show that Sommerfeld-Zenneck surface waves are a mode of propagation in radio space wave propagation, as the path-loss exponent is generally between 20 dB/dec and 40 dB/dec. Other types of surface wave include the Norton ground wave, trapped surface wave, gliding wave, and Dyakonov surface waves. Within microwave field theory, the interface of a dielectric and conductor supports surface wave transmission, and some such waves may be considered as single-wire transmission lines.

Reader's Guide

Surface waves are significant in radio propagation because they provide a mechanism for guided wave transmission along the Earth's surface, enabling communication beyond the horizon at certain frequencies. The Norton ground wave is a radiative ground wave that follows the curvature of the Earth, while the Zenneck surface wave is a non-radiative bound mode. The distinction between these types is important for understanding propagation losses and antenna design. In microwave field theory, surface wave transmission occurs at the interface of a dielectric and conductor, and in coaxial cable a transverse-magnetic mode propagates as a surface wave around the central conductor, which in high-impedance coax or on a single central conductor without an outer shield supports low-attenuation, very broadband propagation known as E-Line. The surface plasmon polariton is an electromagnetic surface wave that travels along an interface where one medium has negative permittivity, making it sensitive to boundary changes such as molecular adsorption. The legacy of Sommerfeld and Zenneck's work lies in providing an exact solution to Maxwell's equations for wave propagation over lossy earth, though the practical role of the Zenneck wave in radio propagation remains debated, as evidence does not confirm it as a dominant mode.

Did You Know?

• Surface waves in radio propagation include the Norton ground wave, which is radiative and follows the Earth's curvature.
• The field components of a surface wave diminish with distance from the interface, and the wave does not transmit power normal to the interface.
• In high-impedance coaxial cable, a transverse-magnetic mode propagates as a surface wave, supporting low-attenuation broadband transmission called E-Line.