LONG-WA]'E RADIO TRANSMISSION PHENOMENA 5 



"fault" in the otherwise uniform daylight ionized layer surface. The 

 form of fault presumably is a ring generated by the intersection of the 

 shadow cone of the occulting layer with the si)herical reflecting layer 

 surface. As the fault passes over the reflected radio ray apex, reception 

 of the optimum ra>' may be impeded, either because of scattering due 

 to an irregular reflecting surface,^ or on account of the passage of the 

 ionization density through a value defined by the Brewster angle, 

 thereby changing the mode of transmission from daylight metallic 

 reflection to night-time refraction.'' 



One of the objects of this analysis was to find the angular altitudes 

 of the sun at some fixed point on the transmission path, coinciding 

 with the phases of the sunset cycle of field variation at the receiving 

 station. It was expected that with these data available over the 

 period of a year, it would not be difficult to locate the salient point on 

 the transmission path affected by the cessation of the sun's rays by 

 means of Sumner lines of position ^^■^^ provided, of course, that the 

 physical structure of the layers showed negligible variation. From 

 the above-mentioned data it would also be possible to compute the 

 distance between the occulting and reflecting layers ^^ and to accurately 

 predict the time of occurrence of the sunset minimum. As shown in 

 Tables I-IV, annual and fortuitous variations in the time of occurrence 

 of the phases of the phenomenon prevent the satisfactory application 

 of this method to long path effects, and when applied to the Rocky 

 Point-Houlton path the point so located apparently is situated a 

 considerable distance to the southwest of the most southerly path 

 terminal. This presumably indicates that, if the phenomena take 

 place at a fixed location on the transmission path, there must either 

 be an annual variation of considerable magnitude in the effective 

 distance between the reflecting and occulting layers, or the mechanism 

 involved must be considerably more complex than that initially 

 assumed. 



Frequency Range of Phenomena 



The characteristic diurnal sunset cycle was found on all frequencies 

 studied during this investigation, the scope of which was limited to 

 frequencies between 18 and 68 kilocycles. Available published data 

 taken by other investigators indicate that the pronounced sunset 

 minimum characteristic of long-wave transmission is not observed at 

 broadcast frequencies.* '^^ This may be due to the failure of trans- 

 mission to improve after the sunrise drop, giving an all day minimum, 

 or, if the minimum is due principally to interference bands, the fineness 

 of band pattern at high frequencies may prevent the phenomenon from 

 being recognized. 



