Experiments with Directivity Steering 

 for Fading Reduction * 



By E. BRUCE and A. C. BECK 



Short-wave fading is largely due to phase interference between multiple 

 path signals of varying path length. Fortunately, stable angular differences 

 usually exist between these paths at the point of reception. It is therefore 

 desirable to employ antenna directivity which is "steerable" and suffi- 

 ciently sharp to accept only one of the several paths in order to reduce this 

 fading. 



This paper describes experiments made with a "steerable" directive 

 antenna during reception of transoceanic short-wave signals. The results 

 demonstrate that sharp angular discrimination is a basically sound method 

 of combating fading which is due to phase interference. 



Introduction 



RAPID fading in radio communication has been recognized for some 

 time as being due to the interaction of distinct components 

 having different transmission times. The possibiHty that these com- 

 ponents might arrive from slightly different directions was suggested 

 by various observed facts, among which was the behavior of sharply 

 directive antennas. 



It has been noticed in the past that fading was affected by the direc- 

 tivity of the receiving antenna. An example is given in the oscillo- 

 graph records of Fig. 1 showing observations made by the authors 

 some years ago at Cliffwood, New Jersey. These illustrate a condition 

 of less fading on a large "inverted vee" ^ antenna than on a small non- 

 directional antenna, using telegraph signals received from station GBK 

 in England. Beating the signal with a local oscillator provided the 

 audio frequency which was recorded. The directive antenna output 

 was recorded on the upper trace while the lower strip recorded the 

 output of the substantially non-directive, comparison antenna. 



Such observations as these suggest the possibility of controlling and 

 reducing fading by a systematic use of sharp directivity. The present 

 paper reports some experiments in which changes in fading are cor- 

 related with changes in the directive pattern of a rhombic antenna ^ 

 made by mechanically changing its shape. 



It may be reasoned that, where the total differences in the path 



* Published in April, 1935 issue of /. R. E. Proc. Presented at meeting of I.R.E., 

 April 3, 1935. 



IE. Bruce, "Developments in Short-Wave Directive Antennas," Proc. I. R. E., 

 Vol. 19, pp. 1406-1433, August, 1931; Bell Sys. Tech. Jour., October, 1931. 



195 



