600 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY ]957 



Fading Phenomena 



Variations in signal level with time are caused by changing atmos- 

 pheric conditions. The severity of the fading usually increases as either 

 the frecj^uency or path length increases. Fading cannot be predicted ac- 

 curately but it is important to distinguish between two general types: 

 (1) inverse bending and (2) multipath effects. The latter includes the 

 fading caused by interference between direct and ground reflected waves 

 as well as interference betw^een two or more separate paths in the atmos- 

 phere. Ordinarily, fading is a temporary diversion of energy to some 

 other than the desired location; fading caused by absorption of energy 

 is discussed in a later paragraph. 



The path of a radio wave is not a straight line except for the ideal 

 case of a uniform atmosphere. The transmission path may be bent up or 

 down depending on atmospheric conditions. This bending may either 

 increase or decrease the effective path clearance and inverse bending 

 may have the effect of transforming a line of sight path into an obstructed 

 one. This type of fading may last for several hours. The frequency of its 

 occurrence and its depth can be reduced by increasing the path clear- 

 ance, particularly in the middle of the path. 



I 



100 



5 10 15 20 25 30 35 40 



SIGNAL LEVEL IN DECIBELS BELOW MEDIAN VALUE 



Fig. 4 — Typical fadii g characteristics in the worst month on 30 to 40 mile 

 line-of -sight paths with 50 to 100 foot clearance. 



