. The Bell System technical journal . ac-curately but it is important to distinguish between two general types:(1) inverse bending and (2) multipath effects. The latter includes thefading caused by interference between direct and ground reflected wavesas well as interference betw^een two or more separate paths in the atmos-phere. Ordinarily, fading is a temporary diversion of energy to someother than the desired location; fading caused by absorption of energyis discussed in a later paragraph. The path of a radio wave is not a straight line except for the idealcase of a uniform atmosphere. The
. The Bell System technical journal . ac-curately but it is important to distinguish between two general types:(1) inverse bending and (2) multipath effects. The latter includes thefading caused by interference between direct and ground reflected wavesas well as interference betw^een two or more separate paths in the atmos-phere. Ordinarily, fading is a temporary diversion of energy to someother than the desired location; fading caused by absorption of energyis discussed in a later paragraph. The path of a radio wave is not a straight line except for the idealcase of a uniform atmosphere. The transmission path may be bent up ordown depending on atmospheric conditions. This bending may eitherincrease or decrease the effective path clearance and inverse bendingmay have the effect of transforming a line of sight path into an obstructedone. This type of fading may last for several hours. The frequency of itsoccurrence 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 mileline-of-sight paths with 50 to 100 foot clearance. RADIO PROPAGATION FUNDAMENTALS 601 Severe fading may occur over water or on other smooth paths becausethe phase difference between the direct and reflected rays varies withatmospheric conditions. The result is that the two rays sometimes addand sometimes tend to cancel. This type of fading can be minimized, ifthe terrain permits, by locating one end of the circuit high while theother end is very low. In this way the point of reflection is placed nearthe low antenna and the phase difference between direct and reflectedrays is kept relatively steady. Most of the fading that occurs on rough paths with adequate clear-ance is the result of interference between two or more rays travelingslightly different routes in the atmosphere. This multipath type of fad-ing
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