DIRECTIVITY STEERING FOR FADING REDUCTION 197 



lengths are small, variations can result only in the carrier and side 

 bands fading in and out together or in other words "general" fading. 

 In such cases, there either may or may not be appreciable angular 

 separation between the multiple waves at the point of reception. How- 

 ever, there is little question that, where multiple waves cause a "selec- 

 tive" fade over a speech band which is, of course, a very small per- 

 centage of the carrier frequency, a material path length difference must 

 exist. Where this is the case, it is difficult to conceive of wave routes 

 which do not possess appreciable angular separation between them at 

 the place of reception. The truth of this latter point is of vital im- 

 portance in this discussion. 



The hope of success in fading reduction through directivity rests on 

 the possibility of a continuous, stable angular separation between the 

 interfering waves during times when fading is really troublesome. 

 Fortunately this possibility is reasonably existent; therefore it should 

 be possible to reject all but one of the interfering paths, by means of 

 sharp directivity, with a consequent reduction in selective fading. 



Description of Equipment 



Tests have shown ^ that a greater degree of angular spread between 

 the multiple waves exists in the incident vertical plane than in the 

 horizontal plane. It might be expected, then, that such a scheme as 

 that illustrated in Fig. 2 would be worth trying. Here the steep edge 



.WVVWWWWWWV 



Fig. 2 — Edge system for achieving fading reduction with 

 moderate antenna directivity. 



of a moderately sharp directional characteristic is moved just far 

 enough into the wave cluster, assumed directively stable, to accept the 

 first wave. Obviously it is possible to approach the wave cluster from 

 the bottom as illustrated or we may approach the cluster from above. 



^H. T. Friis, C. B. Feldman, and W. M. Sharpless, "The Determination of the 

 Direction of Arrival of Short Radio Waves," Proc. I. R. E., Vol. 22, pp. 47-78, Janu- 

 ary, 1934. 



