A MULTIPLE UNIT STEERABLE ANTENNA 339 



If, however, the directivity can be varied or "steered" to meet the 

 various conditions imposed by nature, a new field is opened in which a 

 new order of antenna sharpness and gain is possible. In addition to 

 the gain in signal-to-noise ratio afforded by directivity, a reduction in 

 selective fading is possible if the sharpness is increased to the point 

 where a separation of differently delayed waves is achieved. As early 

 as 1927, Edmond Bruce ^-^ found remarkable reductions in short- 

 wave fading by using a receiving antenna having an extremely sharp 

 directional pattern. The successful employment of sharp directivity 

 is, of course, predicated upon considerable stability of wave directions. 

 The experiments reported by R. K. Potter ^ in 1930 suggested that 

 short waves are propagated in a more or less orderly manner and that 

 stable wave directions might exist. Later experiments,^ made in co- 

 operation with the British Post Office, using pulse transmission to 

 resolve angles in time, gave confirming data and demonstrated clearly 

 the physical facts upon which is based the system to be described in the 

 present paper. These fundamental facts, outlined in the paper de- 

 scribing the experiments just mentioned, are recapitulated here because 

 a clear understanding of their nature and significance is an essential 

 introduction to the subject in hand. In the pulse tests it was found 

 that: 



"1. To the extent that we have been able to resolve the propagation 

 into separate (vertical) angles, the separate angles are found not to be 

 erratic; they vary slowly. 



" 2. There appears to be at least a qualitative relation between angle 

 and delay; the greater the delay the greater the angle above the hori- 

 zontal. 



"The existence of the many waves of different delay, which is 

 known to make fading selective with respect to frequency, greatly 

 impairs the quality of a short-wave radio telephone circuit. . . . The 

 experimental facts, tentatively established, that individual wave angles 

 are fairly stable and that waves of different delay invariably possess 

 different vertical angles, make this problem hold considerable promise. 



"The simple antennas described . . . are suitable for angle determi- 

 nation because of their ability to reject a single wave but they are not 



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

 vol. 19, pp. 1406-1433, August, 1931; 5e// Sys. Tech. Jour., vol. 10, pp. 656-683, 

 October, 1931. 



^ E. Bruce and A. C. Beck, "Experiments with Directivity Steering for Fading 

 Reduction," Proc. I. R. E., vol. 23, pp. 357-371, April, 1935; Bell Sys. Tech. Jour., 

 vol. 14, pp. 195-210, April, 1935. 



*R. K. Potter, "Transmission Characteristics of a Short-Wave Telephone 

 Circuit," Proc. I. R. E., vol. 18, pp. 581-648, April, 1930. 



^ Friis, Feldman, and Sharpless, "The Determination of the Direction of Arrival 

 of Short Radio Waves," Proc. I. R. £., vol. 22, pp. 47-78, January, 1934. 



