TABLE OF CONTENTS Section Editor: Marc Pelegrin
Florent Christophe
Radio waves propagate in the vacuum or through the complex media surrounding the Earth-or other planets-and carry information from the source to the receiver. We will focus in this subsection on radio waves involving man-made transmitters for avionics or astrionics applications-i.e., communications, navigation, surveillance, and radiolocation-but the case of the transmitter as a natural source may also be considered for radioastronomy or Earth observation from space.
Radio waves, inferred from Maxwell equations, first observed by Hertz in 1886, and applied for long range by Marconi in 1906, are a combination of an electric field E and a magnetic field H of periodic time variations, produced by electric charge displacements or electric currents.
In a vacuum, far enough from those electric sources, E and H fields appear as plane waves following expressions:
| (11.1) |  |
| (11.2) |  |
where E 0 and H 0 are orthogonal vectors, their modules being linked by
| (11.3) |  |
The power density transported by such waves is given by:
| (11.4) |  |
The direction of vector E 0 defines the linear polarization of the wave; ? is its frequency, and k is the wave vector indicating the direction of wave propagation, orthogonal to both E 0 and H 0. They are linked through
| (11.5) |  |
where c is the velocity of light in a vacuum, equal to 3 10 8 m/s. Vector r
