RADIO PROPAGATION FUNDAMENTALS 



621 



frequencies are either greatly attenuated or are lost in outer space. The 

 approximate maximum usable frequency for other conditions is greater 

 than 2 mc by the ratios shown in Table I. 



Table I 



When all of the above variations add "in phase," transmission for 

 distances of 2,500 miles or more is possible at frequencies up to 40 to 60 

 mc. For example, using the table, 2,500-mile transmission on an early 

 December afternoon in one year out of five can be expected on a fre- 

 quency of about 42 mc, which is 3 X 3.5 X 2 = 21 times the reference 

 critical frequency of 2 mc. Peaks of the sunspot cycle occurred in 1937 

 and in 1947-1948 so another peak is expected in 1958-1959. 



The maximum usable frequency also varies with the geomagnetic 

 latitude but, as a first approximation, the above values are typical of 

 continental U. S. Forecasts of the muf to be expected throughout the 

 world are issued monthly by the National Bureau of Standards.^'' • *^ 

 These estimates include the diurnal, seasonal, and sunspot effects. 



Another type of absorption, over and above the usual daytime absorp- 

 tion, occurs both day and night on transmission paths that travel through 

 the auroral zone. The auroral zones are centered on the north and south 

 magnetic poles at about the same distance as the Arctic Circle is from 

 the geographical north pole. During periods of magnetic storms these 

 auroral zones expand over an area much larger than normal and thereby 

 disrupt communication by introducing unexpected absorption. These 

 conditions of poor transmission can last for hours and sometimes even 

 for days. These periods of increased absorption are more common in 

 the polar regions than in the temperate zones or the tropics because of 

 the proximity of the auroral zone and are frequently called HF "black- 

 outs." During a "blackout," the signal level is decreased considerably 



