11.2.1 Introduction

The portion of the electromagnetic spectrum currently used for radio transmissions lies between approximately 10 kHz and 40 GHz. The influence on radio waves of the medium through which they propagate is frequency-dependent. The lower frequencies are greatly influenced by the characteristics of the earth's surface and the ionosphere, while the highest frequencies are greatly affected by the atmosphere, especially rain. There are no clear-cut boundaries between frequency ranges but instead considerable overlap in propagation modes and effects of the path medium.

In the U.S., those frequencies allocated for television-related use include the following:

• 54 72 MHz: TV channels 2 4

• 76 88 MHz: TV channels 5 6

• 174 216 MHz: TV channels 7 13

• 470 806 MHz: TV channels 14 69

• 0.9 12.2 GHz: nonexclusive TV terrestrial and satellite ancillary services

• 12.2 12.7 GHz: direct satellite broadcasting

• 12.7 40 GHz: direct satellite broadcasting

11.2.2 Propagation in Free Space

For simplicity and ease of explanation, propagation in space and under certain conditions involving simple geometry, in which the wave fronts remain coherent, may be treated as ray propagation. It. should be kept in mind that this assumption may not hold in the presence of obstructions, surface roughness, and other conditions which are often encountered in practice.

For the simplest case of propagation in space, namely that of uniform radiation in all directions from a point source, or isotropic radiator, it is useful to consider the analogy to a point source of light, The radiant energy passes with uniform intensity...