STUDIES IN RADIO BROADCAST TRANSMISSION 



171 



been plotted against time as in Fig. 20. The widths of the frequency 

 bands covered in this case are indicated. This picture is essentially 

 a bird's-eye view of band fading records such as are illustrated in 

 idealized form by Fig. 18, the amplitude axis being perpendicular 

 to the page. It reveals the presence of minima spaced at more or 

 less definite frequency intervals, and suggests the presence of other 



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Fig. 20 — Plotted curves which condense a long band fading record so as to bring out 

 the frequency spacing interval of the selective fading 



depressions in regular spacing beyond the scope of our pictures, for 

 when one minimum slides out of sight another appears to take its 

 place from the opposite side of the band. The minima traces shown 

 in broken line w^ere outside the record but were located by extra- 

 polating the sections. 



Other depressions of small amplitude appear to be superimposed 

 upon the major changes but the present data appear inadequate to 

 give reliable information concerning them. These minor depressions 

 seem most evident during periods of rapid change. 



The presence of these major minima in regular array bears a marked 

 similarity to the familiar wave interference case in light and fits in 

 very nicely with the theory detailed in previous paragraphs. Assume 

 for a moment the simple case of two transmission paths producing 

 such an effect and account for the difference in their lengths by pre- 

 suming that one path follows more or less closely along the surface 

 of the earth while the other seeks higher altitudes and in some fashion 

 gets back to earth at the receiving station. 



