COMMON FREQUENCY BROADCASTING 



593 



tendencv of increasing the number of isochronously operated stations 

 is to improve markedly the satisfactoriness of the program received 

 at a point distant from all the stations of the chain. 



On the other hand in a small area midway between the stations, 

 which received but a mediocre service originally since it lay outside the 

 normal service area of either station, the reception with simultaneous 

 operation was somewhat further impaired. The conditions that exist 

 in this no-man's land between any two stations operating on a common 

 frequency seem worthy of a detailed consideration, especially since 

 wide publicity has been given to the misconception that the mainte- 

 nance of the carrier in perfect synchronism at the transmitters would 

 entirely eliminate this area of impaired reception. It will be shown be- 

 low that fundamentally the degree of isochronism merely determines 

 the rate at which alternate strips of bad and good quality reception are 

 swept across this territory. The attainment of exact isochronism 

 would only mean that these strips would tend to be fixed in space and 

 that a certain proportion of the listeners would then receive bad quality 

 all the time instead of getting their share of the good with the bad. 



A smoothed curve of the daytime field strengths from WOC and 

 WHO existing in the middle area on a line between the stations is 

 shown in the lower part of Fig. lO.^"^ The range of variation that the 



Z UJ 



- 1 



0.4 

 



^1 ^^i^ ©MAX 



^ =Odb s = 



+ 0.5 +1.0 -1.0 -0.5 +0.5 +1.0 -1.0 -0.5 



DISTANCE IN WAVE LENGTHS 



Fig. 11— Enlarged sections of the points A, B, and C of Fig. 10 showing in detail the 

 resultant carrier variations with distance. 



resultant carrier level will undergo as the carriers pass in and out of 

 phase is shown in the upper part for the corresponding points along 

 this line. In Fig. 11 enlarged sections showing in detail the variations 

 »i These curves are based on field strength measurements made by Radio In- 

 spectors J. M. Sherman and H. T. Gallaher of the Department of Commerce, and 

 furnished through the courtesy of Mr. H. D. Hayes, U. S. Supervisor of Radio, 

 Chicago, 111. 



