A MULTIPLE UNIT STEERABLE ANTENNA 385 



To a considerable extent, the magnitude of the quality improvement 

 ascribed to the MUSA in the preceding paragraphs depends upon the 

 fact that double side-band signals were employed. For, with double 

 side-band signals the selective fading caused by the interference of the 

 differently delayed waves results not only in selective fading of the 

 audio output, but also produces non-linear distortion when the carrier 

 fades selectively. This non-linear distortion sounds much like over- 

 modulation, and when it occurs in its more violent forms it completely 

 ruins the quality and intelligibility. With single side-band trans- 

 mission it is possible to demodulate with such a strong carrier that 

 non-linear distortion is virtually eliminated. The fading of the audio 

 output is sometimes more selective than with double side-band but 

 the resulting quality is substantially better. 



Single side-band transatlantic signals were not available during the 

 trial of the MUSA system. However, as mentioned in Section III, 

 receiving equipment was available which rejects one side band and 

 reduces the percentage modulation by a factor of ten or more. It was 

 found that this equipment, applied to the one-antenna system, resulted 

 in substantially reduced non-linear distortion and that the quality 

 could be still further improved by the reduction of selective fading 

 afforded by the MUSA. With MUSA reception there was apparently 

 no quality improvement in going from double to single side band. 



Summarizing Discussion 



In this section the general performance of the experimental MUSA 

 has been described in a necessarily qualitative manner. Motion pic- 

 ture oscillograms were shown to illustrate the performance under fairly 

 typical transatlantic conditions. An investigation of propagation from 

 Halifax in which the MUSA was employed to identify ionosphere re- 

 flections was included to supplement the rather fragmentary evidence 

 available in motion picture oscillograms. The improved quality ob- 

 tained with MUSA reception was discussed from several points of 

 view. The evaluation of the MUSA has been general; it serves partly 

 to introduce the following section which deals specifically with the 

 signal-to-noise ratio evaluation. 



Before closing this section it is appropriate to discuss conditions with 

 which the experimental MUSA could not adequately cope. 



On numerous occasions the fact that only two branches are provided 

 has definitely handicapped the performance. More often, however, 

 the need for greater angular discrimination or resolving power has 

 been apparent. Except on infrequent occasions a MUSA two to three 

 times the length of the experimental one and equipped with three 



