RADIO BROADCAST COVERAGE OF CITY AREAS 139 



stations through the effect of local stations. Assume, for example, 

 that the listener receives 50,000 microvolts from a local station and 

 500 microvolts from a distant station to which he desires to listen. 

 There exists a 100 to 1 or 80 TU disadvantage to be overcome. When 

 added to the 60 TU needed for crosstalk clearance, the total selectivity 

 requirement, as measured in terms of detected audio current, becomes 

 140 TU, or a current reduction of the order of 10,000,000 to 1 . The need 

 for a high degree of selectivity is therefore apparent. The imprac- 

 ticability of receiving distant stations removed in frequency from local 

 stations by any such narrow margin as 10 kc. is also obvious. 



The effect of receiving set selectivity in increasing the area over 

 which a station may be received without interference from a second 

 station is illustrated in Fig. 13. The two stations are assumed to be 

 of equal powers so that they deliver equal field strengths to receiving 

 stations along a line midway between them. Receiving sets so located 

 are required to have an amount of selectivity called for by the cross- 

 talk margin itself, say 60 TU. On the desired-station side of this 

 line the selectivity may be less; this is the region where poorly selec- 

 tive receivers can be employed. On the undesired-station side of the 

 center line the selectivity requirements are greater. The non- 

 interference area is pushed up closer and closer to the undesired 

 station as the receiving set selectivity is improved, as Is indicated by 

 areas A and B of the figure. For example, assume that the selec- 

 tivity of the receiving set is such as to give a 100 TU cutoff of an 

 undesired station, offset by 50 kilocycles. Sixty TU of this would 

 be required were the two signals of equal strength, so that 40 TU 

 measures the difference by which the undesired signal may be greater 

 than the desired one. The increased area of reception made possible 

 by this additional 40 TU is indicated by that portion of the lower 

 figure which is to the right of the center line and outside of the area A. 

 Within the area A interference would be suffered. This interference 

 area may be diminished by the use of still greater selectivity. The 

 addition of another 20 TU of selectivity (again as measured in terms 

 of detected audio-frequency current) would reduce the interference 

 area to that within the small area at B. The extent to which the 

 selectivity requirement of the receiving set is determined by its loca- 

 tion, therefore, is apparent. The conditions which obtain in multi- 

 station areas, such as New York City and Chicago, obviously call 

 for a general use of high selectivity sets. 



In locating a new transmitting station It should be possible from a 

 knowledge of the relative field strengths of other stations In the 

 vicinity to predict approximately what the Interference area will be 



