RADIO BROADCAST COVERAGE OF CITY AREAS 125 



obtaining at night in certain parts of Westchester was due largely to a 

 rapid frequency modulation of the broadcast transmitter. The fre- 

 quency fluctuation of the transmitted band apparently caused the 

 direct and indirect transmissions to slip in and out of phase rapidly. 

 The use of a master oscillator control for insuring stability of fre- 

 quency greatly improved matters, but evidence still remains of what 

 might be called the normal night-time fading. 



Another interesting effect which stands out in this map is the high 

 attenuation of the wave-front transmitted over Long Island as com- 

 pared with that which pursues the path of Long Island Sound and that 

 of the ocean front to the south. The field over the eastern half of the 

 island is contributed to by the water-transmitted waves from either 

 side, giving rise to interference patterns similar to those in Westchester 

 County. 



A question which naturally arises is that of how strong a field, as 

 measured in this way, is required for satisfactory reception. It is too 

 early in the art to answer this question very definitely, for it depends 

 first upon the standard of reception which is assumed, with respect to 

 quality of reproduction and freedom from interference; and second 

 upon the level of the interference. The interference, both static and 

 man-made, varies widely with time and with location. It is therefore 

 obviously impossible to give anything more than a very general inter- 

 pretation of the absolute merit of field strength values. Observations 

 made by a number of engineers over a period of several years in the 

 New York City area, having in mind a high standard of quality and 

 of freedom from interference, indicate the following: ^ 



L Field strengths of the order of 50,000 or 100,000 juv./m. appear 

 to be about as strong as one should ordinarily desire. Fields much 

 stronger than this impose a handicap upon those wishing to receive 

 some other station. 



2. Fields between 50,000 and 10,000 /xv-im. represent a very desir- 

 able operating level, one which is ordinarily free from interference 

 and which may be expected to give reliable year-round reception, 

 except for occasional interference from nearby thunder storms. 



3. From 10,000 to 1000 juv./m. the results may be said to run from 

 good to fair and even poor at times. 



4. Below 1000 /^v./m. reception becomes distinctly unreliable and 

 is generally poor in summer. 



5. Fields as low as 100 fxv.jm. appear to be practically out of the 

 picture as far as reliable, high quality entertainment is concerned. 



^ See also the paper by A. N. Goldsmith, " Reduction of Interference in Broadcast 

 Reception," Proceedings, I. R. E., October, 1926. 



