STUDIES IN RADIO BROADCAST TRANSMISSION 181 



each block are, of course, very well connected electrically by means 

 of pipes, cables, etc., with those of adjacent blocks. Between each 

 oscillating circuit (which is pictured as consisting of two buildings 

 with earth connections) there exists a coupling which binds the whole 

 system together more or less flexibly. Thus the obstacle offered by 



steel Structures 





'-'-■■■ '- 'v.^^S^^^^'?'^^^^^^" 



Fig. 27 — Idealized picture of equivalent electrical circuit characteristics of high 



buildings 



a group of buildings might be of a selective nature, and evidently its 

 frequency characteristic may vary with direction. 



Such an aggregate would, in addition to absorbing wave energy. 

 produce a change in velocity or a refraction of the wave front. Some 

 indication of such an effect will be discussed later. Before leaving the 

 subject of shadows, however, let us get a physical picture of their 

 significance. 



From the transmitter a wave front expanding outward and upward 

 encounters an obstruction which we shall assume is near the earth 

 plane. The net result of this encounter is a weakening of the wave 

 over an area near this plane, and piobably a distortion of the energy- 

 bearing fields. We might then imagine this shadow to be a tunnel- 

 like region extending along the earth beyond the obstruction, and as 

 having definite vertical as well as horizontal limits. 



The aerial photograph of Manhattan and adjacent territory, 

 shown on Fig. 28, will give a fairly clear idea of the conditions close 

 to the transmitter. The major obstruction, the location of which 

 has been previously described, is shown in its relation to the line of 

 transmission toward the Riverhead and Stamford testing stations. 



