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20. Abstract (Cont'd) 



at these sites for positive and non-positive gradient profiles. Positive gradient (winter) profiles generally resulted 

 in at least 10 dB less loss at 50 km range than did non-positive gradient profiles. Optimum propagation for 

 positive gradient surface ducts varied from 200 to 1000 Hz. 



The reflectivity of the bottom models varied greatly. Where propagation was relatively good by bottom 

 reflected paths, optimum frequencies were around 500 Hz. The greatest differences between propagation 

 losses calculated for the positive and non-positive profile cases are seen where such reflectivity is poor. 



S/N 0102- LF- 014- 6601 



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