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BELL SYSTEM TECHNICAL JOURNAL 



or mask the atmospheric boundary reflections. Several observations 

 which can be ascribed to such boundaries have nevertheless been 

 published.^' * Obviously, only boundaries lying considerably higher 

 than those discussed here will give the path differences to produce the 

 same type fading at these longer waves. At the same time the ap- 

 parent diffuseness of a boundary will fall off with increase in wave- 

 length, thus removing the restriction of reflection to near grazing 

 incidence angles only. 



6.0 



2.0 



80 



70 60 50 40 



RESULTANT FIELD, DECIBELS BELOW FREE SPACE 



Fig. 16 — Calculated field strength curves showing the eflfect of air boundary height 

 and density on the reflected radiation component, for the Highlands East Moriches 

 circuit. Transmission path 112 kilometers, over sea water, wave-length 4.7 meters, 

 polarization both vertical and horizontal. Vertical antennas 42 and 5 meters high, 

 horizontal antennas 45 and 9.5 meters high, respectively. 



This tentative mechanism also explains several other observed 

 features. Thus, for a given type of boundary instability, the fading 

 rate will increase as the wave-length decreases. Furthermore, since 

 the slope of the "i? " curve increases as the wave-length decreases, the 



