A MULTIPLE UNIT STEERABLE ANTENNA 



407 



three-antenna systems, respectively, but for several bundles of large 

 relative delay the secondary improvement may disappear. 



The results of some recent tests of a three-antenna diversity system 

 on trial at Netcong, N. J., carried out under the direction of F. A. 

 Polkinghorn, showed a signal-to-noise improvement of 3 to 3.5 decibels. 

 Assuming a 3-decibel secondary improvement there remains something 

 of the order of 0.5 decibel for the primary improvement. This is 

 plausible in view of the time constant of one second used on the auto- 

 matic gain controls. Linear audio detectors were used in these tests. 

 As will be discussed later the employment of square-law detectors 

 could be expected to add 0.5 decibel to this figure. Linear detectors 

 are to be preferred, however, on the basis of quality distortion. Table 

 Vn is based upon the theoretical and experimental study of diversity 

 action and gives typical results for space diversity systems. 



TABLE VII 

 Summary of Space Diversity Improvements 



Add 0.5 db to primary improvement when square-law detectors are used. 



Table VII shows that on the average the secondary improvement is 

 larger than the primary improvement. In other words, the advantage 

 which accrues from the similarity of the antenna outputs exceeds that 

 which accrues from their diversification. This result had not been 

 expected. 



• It should be emphasized here that the improvements summarized in 

 Table VII for space diversity systems and in Table V for a MUSA 

 system refer to signal-to-noise ratios only; i.e., quality improvement is 

 not included. 



An important advantage of a MUSA system over a space diversity 

 system is its ability to maintain its improvement when more than one 

 wave bundle occur, and since two or more bundles are common, the 

 advantage is distinctly real. A further advantage not discussed thus 

 far relates to interfering signals as distinguished from static. Unless 

 the interfering signals fall upon the principal lobe of the MUSA array 

 pattern when it is steered to receive the desired signal, important 

 directional discrimination against the interference occurs. Little or no 



