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BELL SYSTEM TECHNICAL JOURNAL 



diffraction fringes, are interposed between the full light and full dark 

 areas. In our radio case the distance from the source to the obstruc- 

 tion and the dimensions of the ol)struction are both very much smaller, 

 in comparison with the \va\e length of the radiation, than for any 

 ordinary case in light, but apparently the phenomenon is of the 

 same general nature. By applying the ingenious principle of sec- 

 ondary sources used by Huyghens we might theoretically determine 

 the distribution of the field beyond an obstruction placed in the 

 path of the advancing radio waves. The basis of this principle is 



Fig. 32 — Theoretical cross-section of radio shadow and associated wave interference 



pattern 



the assumption that each elementary part of the advancing wave 

 may be considered as a tiny transmitter. The effect at any point 

 behind an obstruction, therefore, becomes the resultant effect, con- 

 sidering phase as well as amplitude, of the waves from all these minia- 

 ture sources. 



In ¥\g. 32 the region between vertical lines (a) and (b) represents 

 the geometrical limits of the cross-section of a well defined shadow 

 taken some distance behind the obstruction. An analysis of the 

 resultant field using Hu\ghens' construction would show variations 

 in intensity somewhat as represented by the full line. In other 

 words the shadow will not be distinct but will have alternate maxima 

 and minima within its geometrical limits and similar variations 

 beyond the edges. 



It is very likely, of course, that even in case the foregoing specu- 

 lative analysis of the contour pattern extending north-east of 2XB 

 is fundamentally correct, a great many other influences than that 



