1196 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER, 1953 



were frequency selective to any given degree. For example, 95 per cent 

 of fades which were 40 db deep showed a variation in path loss (fre- 

 quency selectivity) across the observed band 400 mc wide of at least 

 17 db. By extrapolating the curve for 40 db fades (which appears to 

 follow a normal distribution law) it can be estimated that 99 per cent of 

 the 40 db fades had at least 13 db of frequency selectivity within the 

 observed band 400 mc wide. Probably, observation of a wider band 

 would have shown even greater evidence of frequency selectivity. 



During periods when the received signal was materially stronger than 

 mid-day normal there was practically no evidence of frequency selec- 

 tivity. Several periods were observed when the path-loss across the entire 



DIFFERENCE IN PATH LOSS IN DECIBELS 



Fig. 6 -;- Statistical distribution of differences between maximum and minimum 

 path loss in bands 20 mc wide. (Center frequency of 20-mc band random with 

 respect to frequencies corresponding to maxima and minima of path loss). 



