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BELL SYSTEM TECHNICAL JOURNAL 



proximately north-east across this region. These shift with change 

 in frequency of the transmitted wave. Fig. 30 illustrates the space 

 relations for such a movement. The full line curve shows a partial cross 

 section of the contour map of Fig. 1 taken along a line approximately 

 perpendicular to the direction of transmission 110 wave lengths from 

 the transmitter. This represents relative field strength values for 



Miles WardE from FieldTesting Staticn 



Fig 30 — Cross-section of wave interference pattern showing change with frequency 



610.0-kilocycle radiation. When the frequency is raised to 635.0 

 kilocycles, there occurs a movement of the peaks and depressions as 

 is shown by the broken line of Fig. 30. Apparendy the increased 

 frequency causes these channels to be crowded together. 



If we take sections of the field strength contour pattern in Fig. 1 

 and examine carefully the relative amplitude of peaks and depressions 

 represented by these wavy lines we shall find that the ratio of field 

 strength of the peaks to that in the depressions increases with dis- 

 tance from the transmitter. That is, the channels become more 

 sharply defined as we move away from the transmitter. This ratio 

 is shown approximately by the curves of Fig. 31. If these peaks 

 or depressions were simple shadows they would maintain their relative 

 values at a distance from the source or even tend to "heal" causing 

 the ratio to fall rather than rise as is actually the case. 



Within 14.4 wave-lengths (7.1 km.) of the transmitter the pattern, 

 so apparent beyond 30 wave-lengths, merges into one deep shadow 

 a cross-section of which is shown in Fig. 29. The abscissa of this 

 curve is in degrees measured from the transmitter so that the center 

 of the two most distinct low field strength channels extending north- 



