Underground Propagation

Abstract

This article presents both natural and guided propagation of radiowaves in underground structures. By natural propagation, we mean that no special‐purpose cables are strung in underground structures to guide radiowaves, whereas by guided propagation we mean that special‐purpose cables such as leaky feeders are strung in underground structures to guide radiowaves. The imperfect waveguide model is developed for the analysis of natural propagation. It predicts that natural propagation is that of waveguide modes. The fundamental mode is important for long‐distance propagation as it suffers the least attenuation, whereas higher order modes are problematic as they cause fluctuation because of addition or subtraction of these modes. The geometrical optical model is also developed for the analysis of natural propagation. It predicts that natural propagation is achieved via a direct ray and many reflected rays. Only the direct ray and those rays with fewer reflections are significant for long‐distance propagation. The diffuse model is developed for the analysis of guided propagation by coupled‐type leaky feeders. It undertakes to calculate the mean power flow per unit solid angle from a coupled‐type leaky feeder according to Lambert's cosine law. The geometrical optical model is developed for the analysis of guided propagation by radiated‐type leaky feeders. It compares a radiated‐type leaky feeder with an array of magnetic diploes, which radiate energy in a certain direction. Furthermore, the statistical characteristics of both natural and guided propagation are also discussed.