416 BELL SYSTEM TECHNICAL JOURNAL 



decibels more consistently than the present MUSA yields eight 

 decibels. 



It may be worth while here to point out that as the number of an- 

 tennas in a MUSA system is increased there is no tendency for static 

 to become subordinate to thermal noise (set noise) or vice versa when 

 static, like thermal noise, adds on a power basis. Only to the extent 

 that transmission-line loss increases with the number of antennas will 

 the ratio of thermal noise to static increase. 



A type of transmission sometimes occurs for which the experimental 

 MUSA gives only small signal-to-noise improvement. We refer to 

 the highly scattered propagation associated with flutter fading, dis- 

 cussed at the close of Section IV. In such cases signal-to-noise im- 

 provement is not highly significant, however, since at least in the worst 

 cases, the distortion renders the circuit worthless. Thus, increasing 

 the transmitting power is likewise ineffective. On the other hand the 

 experimental MUSA can accomplish something by rejecting some of 

 the scattered waves which appear to be responsible for the flutter 

 fading. This is accomplished without a corresponding loss of signal- 

 to-noise ratio since, of course, noise is rejected, too. Fortunately, 

 flutter fading does not seem to be associated prominently with greatly 

 depressed field intensity so the failure to secure signal-to-noise im- 

 provement with flutter fading does not appreciably penalize the MUSA 

 as a means of extending operation through periods of depressed field 

 conditions. 



VI. Recapitulation 



The MUSA receiving system described in this paper is the culmina- 

 tion of some four years effort to determine the extent to which receiving 

 antenna directivity may be carried to increase the reliability of short- 

 wave transatlantic telephone circuits. ^^ Fundamental experimental 

 studies of wave propagation were made with particular emphasis upon 

 how the waves arrive. Based upon the results of these studies a 

 system was evolved in which a new technique of phasings was required. 

 The result is a steerable antenna whose signal-to-noise advantage is 

 seven to eight decibels compared with the largest fixed antenna that 

 can be employed effectively. By analyzing this improvement and 

 comparing the various contributing factors with theory, it is possible 

 to estimate that a system three times larger than the experimental 

 one will yield an additional four to five decibels, and will perform 

 better consistently. In addition to the signal-to-noise improvement a 



5' Potter and Peterson, "The Reliability of Short-Wave Radio Telephone Cir- 

 cuits," BellSys. Tech. Jour., vol. 15, pp. 181-196, April, 1936. 



