iH.i\s.iii..L\nc RADIO ir.i.ri'iioM: i k.ixsmissiox a7\ 



As the sunlit hcinispluTc ri-cfdos soutlnvard aftir the siimiiu'r solstice 

 a tinu- is rtMclu'd. about the niiildli- of August, at vvhirh thf full uinlit- 

 tiiui- valufs arc again ri-alizrd. liiyond this time they are sustained 

 for increasing iH'riiHJs of lime. It is of interest to note that at tiiese 

 two limes of the year, ihe last of April and the middle of August, 

 direct simlighl exists over the darkened hemis(ilure sonie aOO kiU)- 

 melers above the great circle path. 



Kor all of ihe conditions noted above, n.inu-K', sunset, sunrise, .ind 

 summer approach of the transmission path to the northern boundary 

 of the night hemisphere, the path lies in a region wherein the radiation 



Jtn rtb Uv >er Iby .Juw .July tu^ S«l Oct Nm Ok 



F«b >br A(r May Am JiJy <tJ9 Scpl Oct Nov Dk. 



Fig. II — Monthly averages of daylight field strength 



from the sun grazes the earth's surface at the edge of the sun-lit 

 hemisphere. The transmission path also approaches this region 

 during daylight in the winter months, as will be seen by reference 

 to the first position of Kig. 7 for the month of January. The results 

 of measurements for the months of November, December and Janu- 

 ary- for all of the frequencies measured show definite reductions in the 

 daylight field strengths. This reduction is evident in Fig. 8 for the 

 .57-kiloc>cle transmission, but shows up more strikingly in the curves 

 of Fig. 11. The effect of each of these conditions, in which the trans- 

 mission pa^h approaches the region in which the solar emanation is 

 tangential to the earth's surface, will be observed to be that of an 

 increase in the transmission loss. The fact that in one instance this 



