138 BELL SYSTEM TECHNICAL JOURNAL 



are still in operation that are subject to frequency changes of the 

 order of ± 1,000 cycles during modulation. This condition is not 

 only conducive to impaired fidelity at moderately distant receiving 

 points, but it increases interference and precludes successful common 

 frequency operation. Fortunately, the use of automatic frequency 

 control apparatus in its present form is effective in minimizing this 

 efifect as well as in limiting frequency variations of much longer period. 

 It is probable, therefore, that with the more general use of automatic 

 piezo-electric control, this matter will rapidly cease to be a problem. 



As might be expected, the attention being given to intensive devel- 

 opment has materially stimulated interest in transmission. There is 

 a very evident need for much information of a more quantitative 

 nature than is now available. Data concerning attenuation over 

 city and rural areas as a function of frequency, suitable separations 

 between stations of various powers operating on a common carrier 

 frequency, allowable distances between transmitting stations and 

 nearby populous communities, relative day and night ranges, relative 

 summer and winter ranges, time of the day and season of the year 

 at which the transition occurs, and other questions of a similar nature 

 have become of great practical importance. The problem is rendered 

 particularly difiicult by the range in climatic, topographic, and cul- 

 tural conditions which exist in the United States. Under the cir- 

 cumstances, there are excellent opportunities for important work in 

 this field. 



A significant tendency disclosed by recent measurement work in 

 a number of city areas is public acceptance of and demand for field 

 intensities which a few years ago would have been considered objec- 

 tionably high. For some time it has been more or less generally 

 agreed that a field intensity of 10 mv. per meter would afford a satis- 

 factory high-grade broadcasting service. Recently, however, in spite 

 of increased eftectiveness due to higher degrees of modulation and in 

 spite of continued improvement in the sensitivity of commercial 

 receiving sets, stations establishing field strengths of 10-15 mv. per 

 meter have been greatly handicapped in competing with others capable 

 of producing 30-50 mv. per meter in the same areas. In several 

 densely populated districts measurements have disclosed field inten- 

 sities of 300-500 mv. per meter without any noteworthy number of 

 complaints provided the programs were of a high character. There 

 is little to indicate whether this tendency is the result of a decreased 

 interest in distant stations, a desire for higher standards in reproduc- 

 tion involving lower noise levels, or a combination of these factors 

 with others, but it is evidently a matter which must be given careful 

 consideration in engineering future installations. 



