RADIO BROADCAST COVERAGE OF CITY AREAS 121 



covered for each set of measurements was that of from one hour 

 before sundown to about three hours after sundown. The time of 

 year was the latter part of May, 1926. The curves are plotted from 

 an analysis of the records in terms of mean field strength. The range 

 of variation due to fading is indicated by the shaded portions of the 

 curves. The day and night fields were found to be roughly the same 

 except for WEAF where there is a material drop in the daytime signal 

 between Baltimore and Washington, shown in the WEAF curve. 



Fading was observed to commence somewhere between 50 and 100 

 miles from the stations and the range of the fluctuations was found to 

 increase up to the maximum distance observed. That the field of 

 WEAF was found to be practically as strong at Baltimore as at Phila- 

 delphia is surprising. The data regarding this point are too meager, 

 however, to enable any very definite conclusions to be drawn. The 

 curves are useful principally in enabling the transition to be followed, 

 in a more quantitative way than is done in Fig. 1, from field strengths 

 capable of giving reliable reception, such, for example, as 10,000 

 /iv./m. (microvolts per meter), to those which characterize the un- 

 reliable "distance" reception and are of the order of 100 /xv./m. 



A fact which is of importance to our understanding of these wave 

 phenomena is that "fading," which ordinarily is noticed at distances 

 of the order of 100 miles, may under some conditions become prominent 

 at distances as short as 20 miles from the transmitting station. Such 

 short-distance fading has been experienced in receiving WEAF in 

 certain parts of Westchester County, New York.'^ It appears to be 

 a case where unusually high attenuation, caused by the tall building 

 area of Manhattan Island, has so greatly weakened the directly trans- 

 mitted wave as to enable the effect of the indirect wave component 

 to become pronounced at night. 



In general, the attenuation suffered by the normal surface-trans- 

 mitted waves varies over wide limits depending upon the terrain 

 which is traversed. This is disclosed by the curves of Fig. 3, which 

 show the drop in field strength with distance, for a 5 kw. station, for 

 each of the following conditions : 



a. No absorption, the inverse distance curve (a = 0), 

 h. Sea water, for which the absorption is relatively small 

 {a = 0.0015), 



c. Open country and suburban areas (a = 0.02 to 0.03) as measured 



in the vicinity of New York and Washington, D. C, 



d. Congested urban areas {a = 0.04 to 0.08) as measured for Man- 



hattan Island. 



^ See "Some Studies in Radio Broadcast Transmission," by Ralph Bown, D. K. 

 M artin and R. K. Potter; Proceedings, I. R. E., Feb., 1926. 



