386 BELL SYSTEM TECHNICAL JOURNAL 



branch receivers could be expected to perform as well as the experi- 

 mental one now performs at its best. The occasions when it might 

 not are the infrequent times when violent flutter fading occurs. 



At least one type of flutter fading appears to be associated with a 

 pronounced scattering which results in a kind of shower of erratic 

 waves arriving over a wide range of directions. Receiving antenna 

 directivity has been found definitely helpful in all except the most 

 violent cases. Apparently when improvements due to directivity 

 occur they occur principally by selecting a more or less normally 

 propagated wave bundle and rejecting the shower of erratic scattered 

 waves. When, in the most violent cases, no reduction of the flutter 

 can be achieved the reason may be that the unit antenna accepts too 

 wide a horizontal range to permit the MUSA to discriminate sufficiently 

 against the shower. (It will be remembered that the MUSA array 

 factor is of the form of a semiconical shell and thus the MUSA will, 

 in general, accept as wide a horizontal range as the unit antenna 

 permits.) 



V. The Signal-to-Noise Improvement of the MUSA 

 Receiving System 



Because of the complicated nature of short-wave transmission and 

 also because of the uncertain state of noise measuring technique, it is 

 not a simple matter to give a satisfactory answer to the question: 

 "What is the signal-to-noise improvement of a MUSA system?" In 

 this section an attempt has been made to simplify the problem by 

 separating the various factors involved. The section begins with an 

 analysis of the problem assuming simple types of wave transmission. 

 This is followed by experimental studies and discussions. 



In discussing the signal-to-noise advantage of a MUSA it is under- 

 stood that a reference receiving system must be adopted, and for this 

 purpose one of the unit antennas connected to an automatic gain con- 

 trolled receiver was chosen. Other types of antennas as, for instance, 

 a simple vertical or horizontal doublet might have been used but other 

 factors not significant to the MUSA would then have been involved. 



Simple Analysis of the Signal-to-Noise Ratio Improvement 



The MUSA differs from other directional antennas in that it is an 

 array of antennas between which there is negligible electromagnetic 

 coupling. This allows (but does not require) a different point of view, 

 not explicitly involving directivity, in considering the signal-to-noise 

 advantage of the array. The following analysis is made from this 

 point of view. In Figs. 32 to 34 antennas are represented by signal 



