022 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



but the signal does not drop out completel\\ It appears possible that the 

 outage time normally associated with HF transmission could be greatly 

 reduced bj^ the use of transmitter power and antenna size comparable 

 to that needed in the ionospheric scatter method described below. 



In addition to the auroral zone absorption, there are shorter periods 

 of severe absorption over the entire hemisphere facing the sun. These 

 erratic and unpredictable effects which seem to be associated with erup- 

 tions on the sun are called sudden ionospheric disturbances (SID's) or 

 the Bellinger effect. 



The preceding information is based primarily on F la3'er transmission. 

 The E laj'er is located closer to the earth than the F layer and the maxi- 

 mum transmission distance for a single reflection is about 1,200 miles. 



Reflections from the E layer sometimes occur at frequencies above 

 about 20 mc but are erratic in both time and space. This phenomenon 

 has been explained by assuming that the E layer contains clouds of 

 ionization that are variable in size, densitj^, and location. The maximum 

 frequency returned to earth may at times be as high as 70 or 80 mc.^- 

 The high values are more likely to occur during the sunmier, and during 

 the minimum of the sunspot cyc\e. 



Rapid multipath fading exists on ionospheric circuits and is super- 

 imposed on the longer term variations discussed above. The amplitude 

 of the fast fading follows the Rax-'leigh distribution and echo delaj^s up 

 to several milliseconds are observed. These delays are 10* to 10° times as 

 long as for tropospheric transmission. As a result of these relativelj^ 

 long delays uncorrelated selective fading can occur within a few hundred 

 cycles. This produces the distortion on voice circuits that is characteris- 

 tic of "short wave" transmission. 



Ionospheric Scatter 



The maximum usable frequency used in conventional skywave trans- 

 mission is defined as the highest freciuency returned to earth for which 

 the average transmission is within a few db of free space. As the fre- 

 quency increases above the muf the signal level decreases rapidly but 

 does not drop out completely. Although the signal level is low, reliable 

 transmission can be obtained at frec[uencies up to 50 mc or higher and 

 to distances up to at least 1,200 to 1,500 miles. ^-^ In this case the signal 

 is 80 to 100 db below the free space value and its satisfactory^ use re- 

 quires much higher power and larger antennas than are ordinarily used 

 in ionospheric transmission. The approximate variation in median signal 

 level with frequency is shown in Fig. 18. 



Ionospheric scatter is apparentlj^ the result of reflections from man}^ 



