A SINGLE-SIDEBAND SHORT-WAVE SYSTEM 



503 



missions for 9-minute intervals. A reconditioned carrier was used at 

 the receiver most of the time on account of the time required to syn- 

 chronize the local oscillator when changing from double to single- 

 sideband reception. The signal-to-noise ratio as well as the articula- 

 tion was found to be the same for either reconditioned or local carrier 

 except when the fields were very low, at which times the local carrier 

 was found to be more satisfactory. Since it was convenient to use a 

 slightly different degree of modulation on double-sideband than on 

 single-sideband transmissions and the filter on the single-sideband 

 receiver passed only 1.2 db less noise than the double-sideband receiver 

 rather than the theoretically possible 3 db, a theoretical difference of 

 8.1 db instead of 9 db in signal-to-noise ratio was to be expected. 



Each point shown on Fig. 9 represents the signal-to-noise ratio 

 which was observed on the single-sideband system at a particular 



50 



< 40 



30 



tr> 20 



-10 10 20 30 



DOUBLE SIDEBAND FIELD STRENGTH IN DECIBELS ABOVE 



40 

 l/iV 



m 



50 



Fig. 10 — Plot of signal-to-noise ratios on single sideband vs. field strength. 



period plotted against the average of the preceding and succeeding 

 values of signal-to-noise ratio measured on the double-sideband 

 system. It will be seen that when the signal-to-noise ratio on the 

 double-sideband system was 10 db the average signal-to-noise ratio 



