STUDIES IN RADIO BROADCAST TRANSMISSION 165 



ordinary transmitted band the various frequencies are treated alike 

 by the medium and the simple assumption that they follow the same 

 route with the same velocity is justified. If none of these assump- 

 tions is correct but the departure is not large the effect will be merely 

 to introduce slight irregularities into the spacing interval and the 

 general nature of the result will not be changed. 



Let us now examine more closely the record, a part of which is 

 shown in Fig. 13. A portion of this has been condensed into the 

 curves of Fig. 14. One unit along the time axes of these curves 

 represents a 25-second interval. 



To obtain these curves the amplitude of the signal has been scaled 

 off and plotted, ignoring all the minor irregularities. From this 

 record the relative fading characteristics of these single frequency 

 signals 500 cycles apart are more easily seen, and it is possible to 

 contrast the time of occurrence of points of minimum signal for any 

 pair of them. 



For the frequency difference of 500 cycles (610.5-610 and 610- 

 609.5) these times are obviously quite different but there is no clearly 



Fig. 15 — Daytime record of carrier and side-band signals 



discernible relation between them. The curves for 1000-cycle dif- 

 ference (609.5-610.5), however, show a striking relation in that the 

 maxima and minima of the two are opposed fairly regularly over the 

 entire 33-minute interval covered by the plot. This means that 

 when one frequency has a minimum amplitude the other has a maxi- 

 mum and vice versa. Certainly this suggests a wave interference 

 involving only two major paths whose difference in length is such 

 that the spacing interval is 2,000 cycles. The path difference appears 

 to be changing somewhat irregularly but at an average rate of the 

 order of one wave length (or approximately 500 meters) per minute. 

 Before speculating further on the numerical values which may be 

 derived from this part of the data we had perhaps best consider some 

 other records of a somewhat different kind which are better adapted 

 to provide such values. But first let us reiterate that these are 

 night-time effects. 



