adio waves, like light waves and all other forms of electro-magnetic radiation, normally travel in straight lines. Obviously this does not happen all the time, because long-distance communi- cation depends on radio waves traveling beyond the horizon. How radio waves propagate in other

than straight-line paths is a complicated subject, but one that need not be a mystery. This chapter, by Emil Pocock, W3EP, provides basic understanding of the principles of electromagnetic radiation, the struc- ture of the Earth’s atmosphere and solar-terrestrial interactions necessary for a working knowledge of radio propagation. More detailed discussions and the underlying mathematics of radio propagation physics can be found in the references listed at the end of this chapter.

FUNDAMENTALS OF RADIO WAVES

Radio belongs to a family of electromagnetic radiation that includes infrared (radiation heat), visible light, ultraviolet, X-rays and the even shorter-wavelength gamma and cosmic rays. Radio has the longest wavelength and thus the lowest frequency of this group. See Table 21.1. Electromagnetic waves result from the interaction of an electric and a magnetic field. An oscillating electric charge in a piece of wire, for example, creates an electric field and a corresponding magnetic field. The magnetic field in turn creates an electric field, which creates another magnetic field, and so on.

These two fields sustain themselves as a composite electromagnetic wave, which propagates itself into space. The electric and magnetic components are oriented at right angles to each other and 90× to the direction of travel. The polarization of a radio wave is usually designated the same as its electric field. This relationship can be visualized in Fig 21.1. Unlike sound waves or ocean waves, electromagnetic

waves need no propagating

medium, such as air or water.

This property enables electro- magnetic waves to travel through the vacuum of space.