RADIO PROPAGATION FUNDAMENTALS 



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10. 



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8:00 10:00 12:00 



Fig. 16 — Typical diurnal variation of critical frequency for January at lati- 

 tude 40 degrees. 



me. The magnitude of the absorption varies with the angle of the sun 

 above the horizon and is a maximum about noon. The approximate 

 midday absorption is shown on Fig. 17 in terms of db per 100 miles of 

 path length. (On short paths this length is the actual path traveled, not 

 the distance along the earth's surface.) 



Long distance transmission requires that the signal be reflected from 

 the ionosphere at a small angle instead of the perpendicular incidence 

 used in obtaining the critical frequency. For angles other than directly 

 overhead an assumption which seems to be borne out in practice is that 

 the highest frequency for which essentially free space transmission is 

 obtained is /^/sin a, where a. is the angle between the radio ray and iono- 

 spheric layer. This limiting frequency is greater than the critical fre- 

 quency and is called the maximum usable frequency which is usually 

 abbre^'iated muf. The curved geometry limits the distance that can be 

 obtained with one-hop transmission to al^out 2,500 miles and the muf at 

 the longer distances does not exceed 3 to 3.5 times the critical frequency. 



The difference between day and night effects means that most sky- 



