27 - Radiowave propagation

Abstract

This chapter discusses radiowave propagation in the frequency bands from 10kHz to 100GHz and the influence of the earth, the atmosphere, and the ionosphere on such transmissions. Electromagnetic energy radiates outwards from the source, usually an antenna, at approximately the speed of light and it is attenuated and influenced by the medium through which it travels. Radiowaves may be propagated in one or more of five modes, depending upon the medium into which they are launched and through which they pass. These modes are free space propagation, where radiowaves are not influenced by the earth or its atmosphere; ground wave propagation, where radiowaves follow the surface of the earth; ionospheric propagation, where radiowaves are refracted by ionized layers in the atmosphere; tropospheric propagation, where transmission is line of sight with some atmospheric refraction occurring; and scatter propagation, where natural phenomena such as tropospheric turbulence or ionized meteor trails are used to scatter radio waves. The primary influence on the atmosphere is radiation from the sun that causes the ionized layers to form and create global climatic and regional weather patterns.