A MULTIPLE UNIT STEERABLE ANTENNA 405 



and BsB, and the analysis of Fig. 38 is applicable. In this case voltage 

 addition does not occur since the audio outputs are essentially different 

 owing to the selective fading.^" They add, in general, to a value inter- 

 mediate between the power and voltage sum, although for the more 

 complicated conditions they combine on a power basis. 



The above analysis has been based upon simple two-wave inter- 

 ference and the results might not be applicable to the more complicated 

 and changing conditions of actual transmission. Accordingly, R. L. 

 Dietzold has made a statistical analysis for other types of fading and 

 for three stations as well as for two. The results appear in Fig. 39 

 together with the results of the above graphical analysis for two-wave 

 interference fading. The time sequence of amplitude in the more com- 

 plicated types of fading encountered in practice is not significant; the 

 percentage distribution determines the results. The "four-wave" dis- 

 tribution curve corresponds to four equal waves of random phase. 

 The quadratic distribution curve was deduced experimentally by R. 

 S. Ohl. Except that these different distributions were assumed, the 

 assurnptions were the same as those of Fig. 38. The improvements 

 are expressed in decibels referred to the signal-to-noise ratio for one 

 station or branch with ideal automatic gain control. 



The small effect upon the results of assuming different time dis- 

 tributions lends significance to these calculations. The averaged round 

 numbers are probably about right. 



With no automatic gain control (or with one which acts slowly com- 

 pared with the fading) there is little or no primary gain. With in- 

 finitely fast and stiff gain control action there is a 2- and 2.5-decibel 

 primary gain for two and three stations, respectively. 



A few measurements were made at Holmdel on two-station diversity 

 (antennas 1 and 6 of the MUSA). The thermal noise and rectified 

 carrier technique was used. The results appear in Table VI. 



The measuring technique was exactly the same as used in obtaining 

 the data for Tables III and IV in which voltage addition of the 

 signal was assumed. A 3-decibel improvement is therefore included 

 in the 3.6-decibel figure. This leaves only 0.6 decibel (possibly one 

 decibel or even 1.5 decibels since the measurements are too meager to 

 be reliable to better than one decibel) for primary gain compared with 

 a possible 2.0 decibels. We are inclined to use about one decibel for 

 primary gain. The time constant on the automatic gain control was 

 of the order of 0.06 second in this and all signal-to-noise comparisons. 

 That this time constant was not fast enough to produce the high noise 



^° This refers to speech signals; in the case of telegraph signals the frequency band 

 is so narrow that fading is always essentially non-selective, and voltage addition 

 occurs. 



