256 



BELL SYSTEM TECHNICAL JOURNAL 



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0.3 0.4 0.6 0.8 1.0 1.5 2.0 3.0 4.0 6.0 



DISTANCE FROM TRANSMITTER TO RECEIVER IN MILES 



Fig. 2 — Mass plot of field intensities measured at various distances from the 

 transmitter at Berkeley and Stuart Streets in Boston. The values corresponding to 

 distance less than two miles represent field strengths averaged over one-tenth mile 

 intervals, while those for greater distances represent averages over one-half mile 

 intervals. The open circles indicate fields in the high building area. Residential 

 points outside the city limits have been enclosed in circles. 



residential field strengths correspond to the region beyond Chestnut 

 Hill. 



When attempting to interpret the results of the mass plot of Fig. 2 

 a natural method would be to assume transmission as in free space 

 plus an additional attenuation due to the proximity of the earth and 

 obstacles above the earth's surface. The simpler case of transmission 

 over level terrain in the absence of obstacles will be considered first. 



It has been experimentally determined that the propagation of 

 ultra-short waves over unobstructed paths follows the laws of optics ^* ^ 

 so that the resultant field is composed of a well-defined reflected wave 

 superposed upon a direct wave. Consequently for propagation over 

 level terrain, the explanation is as follows (Fig. 3) : Energy is propa- 

 gated from a transmitter at .4, at a height of hi above the ground, to a 

 receiver at B, at a height of hi above the ground, both directly, as 



' Loc. cit. 



* C. B. Feldman, "The Optical Behavior of the Ground for Short Radio Waves," 

 Proc. I. R. E., Vol. 21, pp. 764-801, June, 1933. 



