384 BELL SYSTEM TECHNICAL JOURNAL 



As distinguished from selective fading, which is greatly reduced by 

 the rejection of all but one wave bundle, general fading is by no means 

 eliminated. The reader may expect, however, that when the MUSA 

 selects one wave bundle from several it restricts the waves accepted 

 to those which have traveled more nearly a common path, and for a 

 given degree of turbulence in the ionosphere, the fading should be 

 slower, since only relative changes among the several waves result in 

 interference fading. Such a tendency no doubt exists and has been 

 noticed occasionally in the operation of the MUSA but rarely has 

 there been a marked effect (excepting certain cases of flutter fading to 

 be described later). This will be understood when it is recalled that 

 even a fifty-microsecond delay interval means that a difference of 500 

 wave-lengths is involved for a wave-length of thirty meters. In order 

 that the fading rate be sharply reduced it is required that the iono- 

 sphere shall preserve this difference, to within a half wave-length, more 

 effectively than it does if larger differences are involved. Since a hall 

 wave-length is only 0.1 per cent of 500 wave-lengths a rather high 

 degree of balance is thus required. Evidently, the turbulence of the 

 ionosphere usually prevents such a balance. 



Using broadcast signals (double side band) from Daventry a 

 thousand or more comparisons were made of the MUSA versus a single 

 antenna and receiver, using the switching arrangement mentioned in 

 Section III. Remarkable improvements were sometimes observed and 

 some improvement was almost always noted. The exceptions were the 

 instances when distortion was not detectable using one antenna, and 

 the rare occasions when particularly violent flutter fading occurred. 



Space diversity reception using two antennas showed a substantial 

 improvement, usually, but failed ever to show the order of improve- 

 ment demonstrated by the two-branch MUSA when two or more wave 

 bundles of comparable amplitude occurred. Figures 23 and 25 sug- 

 gest, by the way in which the audio outputs are seen to combine, that 

 the distortion with MUSA reception is slight compared to that with 

 diversity reception. 



The increased naturalness which results from reducing the distortion 

 is, of course, pleasing to the ear and has some value in telephone 

 circuits on account of the subscribers' satisfaction. In addition, it 

 increases the intelligibility particularly when considerable noise is 

 present. It is impossible to evaluate the increased intelligibility 

 definitely but, in certain cases at least, it permits the signal-to-noise 

 ratio to be two or three decibels lower. From the point of view of 

 picking up short-wave broadcasts for rebroadcasting, a more sub- 

 stantial value can be attached to the MUSA quality improvement. 



