262 BELL SYSTEM TECHNICAL JOURNAL 



has been given to the points which are well below the curve, in ac- 

 cordance with the view of the experimenters that these points repre- 

 sent particularly unfavorable receiving locations. 



Trevor and Carter ^^ have made a similar interpretation of the data 

 presented by Jones '° which shows that for the larger distances the 

 field strength was inversely proportional to the square of the distance. 

 If this inverse-square-of-distance curve were extended to shorter 

 distances it would be found that most of the nearby points would lie 

 somewhat below it. This is presumably because of the lack of favor- 

 able receiving locations in the high-building area. While the empirical 

 formula arrived at by Jones may represent his data satisfactorily, the 

 physical picture assumed of a free space field times an absorption 

 factor is untenable since it requires a radiated power approximately 

 20 db below that measured. Undoubtedly, the power radiated is not 

 in error by this amount, since Trevor and Carter obtained a satisfactory 

 numerical check on the basis of the other picture by using the value 

 of power radiated as given by Jones. 



It is possible, of course, to represent any data by an inverse-distance 

 factor times an exponential factor for a limited range of distances. 

 An attempt to do this with the data of Fig. 2 by making the empirical 

 curve agree with the experimental inverse-square-of-distance curve at 

 1 and 4 miles results in a curve that agrees well with the data between 

 0.6 and 5.0 miles but is 11 and 22 db low at 0.2 and 12.0 miles, re- 

 spectively. Even if these rather large discrepancies at the limits of the 

 curve were neglected it would still be impossible to interpret the data 

 in terms of the free space field times an exponential absorption factor, 

 because of the fact that the empirical curve so determined requires a 

 radiated power 35 db below that measured ; this is untenable since the 

 over-all uncertainty in the absolute value of the measurements is only 

 a few decibels. 



It should be pointed out that each point of Fig. 2 represents the 

 average field over an interval of either a tenth or a half mile depending 

 upon whether the transmission path involved was less or greater than 

 two miles. Within each interval the field varied by five to fifteen 

 decibels because of the local wave interference pattern, as is shown by 

 the samples of the graphs taken with the recorder which are presented 

 in Figs. 6 and 7. Fig. 6 is an example of the record taken in the 

 business district of Boston at a distance of about one and a half miles 

 from the transmitter near the region A shown in Fig. 8. The maxima 

 and minima are spaced very closely and differ by ten to fifteen decibels. 

 This was characteristic of the type of record obtained at the shorter 

 distances. At the greater distances the magnitude of the local varia- 

 tions was less, as illustrated by Fig. 7, which is a sample of the record 



