Microwave Engineering: Land & Space Radiocommunications

CHAPTER 4

Microwave Transhorizon Systems

4.1 ENGINEERING OF TRANSHORIZON SYSTEMS

4.1.1 Introduction

In radio links established beyond the horizon, on paths generally much longer than line of sight, antennas are never visible to each other except possibly during a reduced percentage of time under extreme conditions of superrefraction. The large-scale average structure of the troposphere induces a bending of the radio waves, in relation to the general refractivity conditions, and the small-scale local structure produces the scattering that is responsible for the unexpected propagation a long way beyond the horizon. The mechanisms of propagation that then come into play at frequencies higher than 30 MHz are the diffraction on the obstacles on the surface of the Earth and the diffusion or scattering due to the presence of heterogeneities in the troposphere. The attenuation due to diffraction increases quickly with the distance, the height of the obstacles, and the frequency, as shown in the Sections 1.9 and 3.4.2, and gives way gradually to tropospheric scatter; it is thus necessary to analyze the share of each one of these mechanisms in order to predict the total propagation loss.

The propagation by tropospheric scatter, also called forward scatter or usually troposcatter, results mainly ...