Radiowave Propagation: Physics and Applications

5

DIRECT TRANSMISSION

5.1 INTRODUCTION

In general, the propagation of a transmitted signal may be affected by interactions with the environment in which the signal propagates (such as reflection and refraction as will be discussed in Chapter 6). In many systems of practical importance, interactions with the environment do not cause significant changes in the signal propagation. The term “direct transmission” is meant to designate such situations; this propagation mechanism is dominant for many satellite communication and radar systems.

In this chapter, the Friis transmission formula for prediction of signal strength under ideal direct transmission conditions is derived. Calculation of attenuation due to atmospheric gases is then discussed, on both vertical and slant paths. A model for predicting rain attenuation and the expected system outage time for a given “rain margin” follows. Methods of improving system performance through diversity and the scintillation phenomena that occur on paths through the atmosphere are then discussed. Finally, a derivation of the look angle from a specified point on the Earth's surface to a geostationary satellite is given in the appendix of this chapter because of the importance of the direct transmission mechanism in satellite communications applications.

5.2 FRIIS TRANSMISSION FORMULA

Consider an isotropic radiator, which is a conceptual antenna that radiates energy equally in all directions. Such antennas do not exist in practice; nevertheless, ...