tories engineers made their mea- surements, they found that radio power came down far faster than Eq. (1.5) suggests. After making many measurements and doing the best statistical analysis they could, they got an exponent of 3.84, as shown in Eq. (1.6): (1.6) In conversation, we refer to this relationship as "38.4 dB per decade," a loss of 38.4 dB (a factor of 7000) for each factor of 10 in distance. Considering how variable different places are and how much other factors affect a radio signal path, we have no problem 7But how are we going to speak and hear our telephones in the vacuum of space? As we recall from conversations back in the early days of cellular vision, circa 1970, using 60 GHz for cellular radio was talked about because 60 GHz was an absorption frequency for atmo- spheric oxygen and would provide excellent cell-to-cell isolation for that reason. Even Bell Tele- phone Laboratories did not have 60-GHz amplifiers that were powerful enough or cheap enough for a telephone market such as cellular that might someday approach 1 million subscribers, so they retreated to the more practical frequency band around 900 MHz. Downloaded from Digital 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 Key Radio Concepts calling the typical terrain loss an inverse fourth power, 40 dB per decade, as shown in Eq. (1.7): (1.7) The higher-than-free-space loss rate is a good thing for wireless capacity rather than a bad thing. Our interferers tend to be further from our receivers than our desired sig- nal transmitters, so a tendency of the signal to deteriorate quickly with distance helps us discriminate between desired and undesired radio waves. While most engineers think and speak about path loss, we find it easier to think of the radio path or even path gain with the understanding that the number is going to be far less than 1 (negative decibels