44 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



tors: distance, shadowing and standing wave patterns. The variation 

 \vith distance from the base station follows the theoretical free space 

 loss up to 500 feet or more, as long as the points are within line of sight. 

 Typical values of the free-space loss are shown in Table I. Beyond 

 about one-half mile the median path loss over plane earth increases 

 about 12 db each time the distance is doubled out to distances of 20-30 

 miles.^' ^ 



In addition to the increase in path loss with distance, which is ac- 

 counted for reasonably well by the theory of radio propagation over 

 plane earth, bold features of geography such as mountains and large 

 buildings cause shadow losses that result in irregular coverage patterns. 

 For example the median loss at street level for random locations in 

 New York City is about 25 db greater than the plane earth values com- 

 puted for the distance and antenna heights involved ; the corresponding 

 10 per cent and 90 per cent losses are about 15 and 35 db respectively.^ 



Superimposed on the above effects which vary relatively slowly with 

 location are standing wave patterns whose effect on path loss can change 

 substantially within a foot or so. The standing waves are the result of 

 random additions of multiple reflections from nearby buildings or ter- 

 rain, and the variation in path loss follows the Rayleigh distribution for 

 small changes in distance in urban areas. In other words, there is no 

 theoretical limit on the deviation from the median but in 1 per cent of 

 the possible locations the signal is likely to be more than 8 db above the 

 median value and in 99 per cent of the possible locations the signal level 

 is not expected to be more than 18 db below the median value. 



The motion of the mobile unit through the standing wave patterns 

 causes signal fluctuations or flutter in the received signal. The flutter 



Table I — Free Space Loss Between Dipoles 



> Young, W. R., Jr., Comparison of Mobile Radio Transmission at 150, 450, 900 

 and 3700 Mc. Bell Sys. Tech. Jl., 30, i)|). 1068-1086, Nov., 1952. 



* Aikens, A. J., and L. Y. Lacy, A Tost of 450 -Megacycle. Urban Transmission 

 to a Mobile Receiver. I.R.E., Proc, pp. 1317-1319, Nov., 1950. 



» Bullington, K., Radio Propagation Variations at VHF and UHF. I.R.E., 

 Proc., pp. 27-32, Jan., 1960. 



