 Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Radio Engineering Concepts Radio Engineering Concepts voltage levels U1 and U2. The assumption being made in Eq. (1.4) is that the electri- cal load, which we call impedance, is not changing as the voltage changes from U1 to U2. In the world of radio, we sometimes do not care about an extra 30 dB. So long as S/N ratios or bit error rates are low enough, an extra factor of 1000 in a signal strength does not matter. Other times we are concerned about 0.1 dB because a 2 percent difference in a particular factor is crucial to system capacity. 1.7 The Radio Path Our physics textbooks make it clear that electromagnetic fields and electromagnetic ra- diation radiate through open space in a vacuum with the same energy in the same spherical angle. Since the surface area of a spherical angle increases as the square of the distance from the source at the center, this tells us that the intensity (power) p of a radio wave decreases as the square of the distance r: (1.5) (The constant term a takes into account all the factors besides distance.) When we all live in an unobstructed vacuum, we can start using this kind of propagation model. 7 As usual, real life is far more complicated than Physics 101 would have us believe. Let us start with the two basic assumptions in Eq. (1.5): no air and no obstructions. At frequencies up through 1 GHz and distances up to 10 km, it is pretty safe to ig- nore air losses. As we move up in frequency, the absorption of air becomes more im- portant, but cellular systems are operating at short enough distances that we believe that it is a minor factor in these systems, even on rainy days. 8 Terrain obstruction is more important to our wireless system design than air and weather. Our planet's surface is curved and hilly and covered with varying kinds plant life and buildings. When Bell Telephone Labora