atmosimikkm: stk vtification and refraction 



205 



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Figure 22. Ravs with a ground-based duct. 



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-»- DISTANCE x 



Figure 23. Rays with an elevated duct. 



departs from the transmitter at angle ai measured 

 from the horizontal. As the ray proceeds outward 

 and downward it is bent less and less, corresponding 

 to the decreasing distance between the M and 1 

 lines. Finally it reverses and rises to the height 

 indicated. Ray 1 must therefore oscillate between 

 the heights determined by the crossing of the M and 

 1 lines. Ray 1' starting upward at the same angle a x 

 oscillates between the same height limits as ray 1. 

 Rays 2 and 2' emerging at angle a p are the limiting 

 rays which are trapped in the duct between the 

 heights h, and h„. Beyond the horizon ray 3 and 

 below the duct lies the diffraction region for this 

 case. Ray 4 emerging at an angle greater than a v is 



not trapped but after reflection passes entirely 

 through the duct. 



Ground-based ducts are likely to be found along 

 coasts where warm, dry air from over land flows out 

 over a colder sea. This situation, for instance, prevails 

 in the summer months along the northeastern coast 

 of the United States. Elevated ducts occur frequently 

 along the southern California coast. 



An illustrative series of theoretical coverage 

 diagrams as obtained by the ray tracing method 

 described are collected in reference 448. A few of 

 these diagrams are reproduced in Figure 24, for a 

 frequency of 200 mc and a transmitter elevation of 

 ~h\ = 100 ft, corresponding to an h\/\ ratio of approxi- 



