BACKGROUND 



139 



1300 



1200 



1100 



1000 



0^ 900 



800 



2 X 



700 



600 



500 



400 



300 



200 



100 



4 5 6 



Br, in Milliradians 



10 



Figure 4.35. Refradiviiy gradients needed for radio ducts. 



the approximate upper limit of radiosonde-observed gradients: a ground- 

 based layer 100 m thick attains this gradient at 8.3 mrad while the 

 maximum observed gradient is intercepted by the 30-m layer curve at 

 ^0 = 4.5 mrad. A third example was calculated for an elevated layer 0.5 

 km above the ground and 100 m thick by assuming normal refraction 

 between the ground and the base of the layer and solving for the necessary 

 ducting gradient within the layer. The large values of n gradient neces- 

 sary for this case explain why elevated ducts were not observed. Al- 

 though the preceding examples were calculated for a ground transmitter, 

 the combinations of dp, An/Ar, and Ar are very nearly the same as would 

 be obtained for any other transmitter height within the first 3 km above 

 the surface. 



