52 



FACTORS INFLUENCING TRANSMISSION 



do not apply. This is especially true i'or the field 

 near or below the optical horizon. The discussion of 

 nonstandard propagation is beyond the scope of this 

 volume. 



High-angle coverage is generally independent of 

 refraction and is therefore also unaffected by the 

 variations of Min the lowest layers of the atmosphere. 



419 Direct Determination of k 



In Figure 9 the reciprocal of k is plotted as ordinate 

 against the vertical gradient of relative humidity as 

 abscissa. The values shown refer to a standard 

 temj)ei-aturc gradient of — 0.65 degrees Centigrade 

 per 100 meters; unless the temperature gradient 

 differs greatly from this value, the corresponding 

 values of k will not be much affected. The curves 

 given refer to 100 per cent relative humidity at 

 ground level, and an auxiliary table is provided 



GROUND REFLECTION 



' Ground Reflection and Coverage 



The reinforcement of the direct ray by the ground- 

 reflected raj'' is of great importance both in radar 

 and in communication work. In favorable cases, 

 the reflected ray may be of comparable intensity 

 to the direct ray and thus the received intensity 

 may be appro.ximately doubled in places where the 

 two rays have the same phase. This means, in many 

 cases, the possibility of an appreciable increase in 

 range relative to that in free space. 



4.2.2 



Complexity of Reflection Problem 



In order to facilitate the discussion of the problems 

 encountered in reflection, it is necessary to analyze 

 the complex phenomena into simpler constituents. 



REL HUMIDITY GRADIENT 

 % PER 100 UETEBS 



+ 3 



+2 



4-1 



10 



-1 



-6 



-8 



-9 



-10 



-12 



-13 



+ 5 +4 



Figure 9. Grapli 1/k vei'.siis RH (relative humidity) grarlient and temperature for 100 per cent RH at ground. Add 



rreotion tabulated to obtain l/k for RH at ground less than 100 per cent. 



at the top of the graph which gives figures to be 

 added for other values of the relative humidity. 

 The sensitivity of k to moisture gradients in warm 

 chmates is obvious from these data. 



First of all, the incident radiation field is resolved 

 into nearly plane wave components, each forming a 

 narrow pencil of rays striking the reflecting surface 

 within a small area which we shall call the reflection 



