ULTRA-SHORT-WAVE PROPAGATION 269 



A separation of the field strengths into those obtaining in the high- 

 building area (indicated by open circles in Fig. 2) from those in the 

 lower-building area (indicated by solid dots) shows that the attenuation 

 is somawhat greater in the former. 



No effect of the elevated railway structures on the average field 



was observed. 



Noise Measurements 



For reception in the car, by far the greatest interference is that 

 caused by the electrical systems of passing automobiles. Special tests 

 to determine whether or not street cars produced any noise gave a 

 negative result. That is, under conditions where automobile noise 

 was a limitation to reception, trolley noise was unaudible. While no 

 special tests were made of the noise from elevated trains, at no time 

 was it found objectionable. 



With the receiver on top of the building at Berkeley and Stuart 

 Streets, the predominating interfering noise was caused by an electrical 

 substation next door. When the antenna was lowered approximately 

 to the roof level the noise from the elevator motors and switching 

 equipment in the pent house near by was well above any other noise. 

 Upon raising the antenna to its proper position the elevator noise was 

 reduced to a negligible amount compared with the power station noise, 

 because of the combined effect of the directivity of the antenna and 

 increased distance. The resulting noise was of approximately the 

 same magnitude, indicating that the elevator switching noise was 

 reduced by a fairly large factor in raising the antenna. This fact has 

 an important bearing on reception of signals on the roofs of ofifice 

 buildings, since elevator switching noise is in general the limiting 

 factor. Occasionally an automobile, started in the street below, 

 would produce measurable interference. At Seaverns Street, however, 

 the most objectionable noise was caused by the ignition systems of 

 automobiles which were accelerating in low gear in the street below. 



Acknowledgment 



The authors wish to acknowledge the cooperation of the New 

 England Telephone and Telegraph Company and the Graybar Electric 

 Company, and also Commissioner E. A. Hultman and Signal Director 

 T. A. J. Hayes, both of the Boston Police Department. 



APPENDIX 

 Methods of Calibrating 

 In order to obtain an absolute calibration of the measuring equip- 

 ment, the field strength at a distant point in space, at the same height 



