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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 21, wherein we are looking down upon the distorted surface, the 

 minima being traced by the Hght Hnes. Toward the short wave end 

 of the band it is evident that a fading record for a single frequency 

 represented, for example, by a section parallel to the time axis and 

 perpendicular to the page, a — a', would show rapid fading, while a 

 similar record at the long wave end of the range as h — h' would give 

 slow amplitude changes. Such sections representing theoretical 



(Long Wave Fading) 



Fig. 21 — Theoretical diagram obtained by extrapolating band fading records to show- 

 how the rapidit}' of fading might be expected to change with the wave-length 



single frequency fading records are shown at the bottom of Fig. 21. 

 The relative fading rates for long and short wave lengths as indicated 

 by these idealized characteristics, are in accord with general experience 

 In describing the stepped-frequency method of obtaining band 

 fading records allusion was made to distortion which might result from 

 speeding up the process. Suppose that we were to use a very small 

 rotating condenser in parallel with the main condenser of the trans- 

 mitter oscillator for changing the frequency, and that this condenser 

 were capable of changing the frequency sinusoidally about a mean 

 value. Then we could represent the variation in frequency with time 

 as is shown by the curve Ci in (a) of Fig. 22. Now if the energy 



