LONG-WAVE RADIO TRANSMISSION PHENOMENA 3 



Measurements made at Houlton on telegraph traffic from Tuckerton, 

 Nauen, Ongar and Rocky Point are believed to be subject to relative 

 errors as great as or even greater than ±2.5 db. Comparisons be- 

 tween a local oscillator and telegraph traffic ordinarily are made by 

 means of a cathode ray tube, and errors are occasioned both by the 

 difficulty of reading the tube scale and by the varying strength of 

 telegraph signals whose intensity is a function of the keying speed 

 probably because of sluggish antenna systems. 



The measurements on special 60-kc. test signals are believed to be 

 sufficiently precise to provide a satisfactory index of the phenomena. 

 The measurements on telegraph traffic provide data for a qualitative 

 estimate of the nature of the various phases of the phenomena but 

 probably are of no great value in fixing the exact time of occurrence of 

 field strength increments smaller than 2 db, which may represent the 

 total variation of some phases of the diurnal cycle. 



General Characteristics of Sunset Effect on Short Paths 



The form of the average diurnal sunset cycle of received field 

 strength, plotted as a function of the sun's angular altitude at some 

 salient point on the great circle transmission path, is shown on Fig. 1 

 for several paths. With the exception of the shortest path of 122 km. 

 between Rocky Point and Chatham, New Jersey, a well defined typical 

 sunset cycle was observed in all cases. If we assume that the sunset 

 dip is due directly or indirectly to the cessation of the active solar rays 

 in the upper atmosphere, the absence of characteristic large field 

 variations at sunset for short paths may be ascribed to a predominance 

 of low elevation transmission which does not enter the layers ionized 

 by the sun. If rectilinear this transmission would pass ^ km. above 

 Chatham on the 122-km. path, due to tangency with the earth's 

 surface. The possibility of transmission between two points 122 km. 

 apart on the earth's surface without a "sky wave," ^ therefore requires 

 that the ray be bent around the spherical contour of the earth by 

 diffraction and atmospheric refraction.^- ^^ 



The daily occurrence of a sunset dip on paths of 700 km. and longer 

 may be explained in several ways, all of which, however, require the 

 existence of a downcoming ray. For example, the sunset phenomena 

 may be due to a "fault" in the reflecting ionized layer, ^ or to inter- 

 ference between two reflected rays or between a reflected ray and a 

 ground wave.®- "• ^* 



Briefly, in accordance with the "fault" hypothesis, the shadow cast 

 upon the ionized reflecting layer by the sun's active rays tangent to 

 the earth's surface, or to an opaque atmospheric layer concentric with 

 the earth which hereafter will be called the occulting layer, produces a 



