A MULTIPLE UNIT STEERABLE ANTENNA 389 



The system described above has been shown mainly to introduce the 

 system shown in, Fig. 32). This diagram shows the audio addition of 

 the outputs of N receivers fed by N antennas. Note that this system 

 has no high-frequency phase shifters in the transmission lines. It is 

 in fact similar to the diversity receiving system described by H. H. 

 Beverage and H. O. Peterson. ^^ For a single wave this is seen to be 

 equivalent to the phased addition at carrier frequency shown in 

 Fig. 32. 



The signal-to-noise improvements shown on Figs. 32 and 2>d> were 

 easily calculated because a single non-fading wave was assumed. In 

 actual practice several fading waves are involved and it is then difficult, 

 if not impossible, to make significant calculations. Later in this 

 section, however, some of the general features of the system shown 

 in Fig. 3?) will be discussed from the point of view of several waves. 



The MUSA system is characterized by the ability to separate waves 

 and it is therefore possible to analyze it in a simple manner for cases of 

 more than one wave. The arrangement in Fig. 34 corresponds to the 

 Holmdel MUSA. The signals from the equally spaced antennas are 

 here phased at the intermediate frequencies. Since random static and 

 first circuit noise give identical results the analysis is given for 

 static only. 



As shown in Case I, if only one wave is present and both branches 

 are phased for it the system functions as in Fig. 32 and it yields the 

 same improvement of 10 log A'^ decibels. If as shown in Case II the 

 second branch is not phased for it (i.e., if the wave falls upon a minor 

 lobe or a minimum of the MUSA directional pattern) less than the full 

 improvement occurs. On the basis of linear audio detectors the re- 

 duction of improvement is 20 log x where x lies between 2 and V2. 

 This quantity refers to the manner in which the noise from sources 

 1,2, • • • A" in Branch A adds with the noise /row the same sources after 

 having been phased differently and perhaps delayed differently in 

 Branch B.^* This involves the audio-frequency band width and 

 method of noise measurement. As will be shown later x is usually not 

 much different from V2. Taking x = V2 the loss in Case II is three 

 decibels. If an audio detector is used which does not demodulate noise 

 when the signal is absent (a square-law detector accomplishes this for 

 practical purposes) this loss disappears, and branches may be phased 

 for temporarily non-existent waves without incurring a penalty. Case 

 III is the important one. It assumes two equal waves. Branch A is 



^^ "Diversity Receiving System of RCA Communications, Inc.," Proc. L R. E., 

 vol. 19, pp. 531-561, April, 1931. 



"The case of .x = 2 (in-phase addition) arises only when the phasing and delay 

 of the two branches are alike. 



