620 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



50 



20 



10 



5 



2 

 \.0 

 0.5 



0.2 



0.1 



0.01 0.02 0.05 0.1 0.2 0.5 1.0 2 5 10 20 50 100 

 FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 17 — Typical values of midday ionospheric absorption. 



wave paths require at least two frequencies. A relatively low frequency- 

 is needed to get under the nighttime muf and a higher frequency is 

 needed that is below the daytime muf but above the region of high 

 absorption. This lower limit depends on the available signal-to-noise 

 margin and is commonly called the lowest useful high frequenc3^ 



Frequencies most suitable for transmission of 1000 miles or more will 

 ordinarily not be reflected at the high angles needed for much shorter 

 distances. As a result the range of sky wave transmission ordinarily does 

 not overlap the range of ground wave transmission, and the intermediate 

 region is called the skip zone because the signal is too weak to be useful. 

 At frequencies of a few megacycles the groundwave and skywave ranges 

 may overlap with the result that severe fading occurs when the two 

 signals are comparable in amplitude. 



In addition to the diurnal variations in frequency and in absorption 

 there are systematic changes with season, latitude, and with the nom- 

 inally eleven-year sunspot cycle. Random changes in the critical fre- 

 quency of about ±15 per cent from the monthly median value are also 

 to be expected from day to day. 



The F layer is the principal contributor to transmission bej^ond 1 ,000 to 

 1 ,500 miles and tj'-pical values of the maximum usable frequency can be 

 summarized as follows: The median nighttime critical frequency for F 

 layer transmission at the latitude at Washington, D. C, is about 2 mc 

 in the month of June during a period of low sunspot activity. All fre- 

 quencies below about 2 mc are strongly reflected to earth while the higher 



