RADIO EXTENSION LINKS TO TELEPHONE SYSTEM 633 



Following the success in applying single sideband to one of the short- 

 wave channels consideration has been given to utilizing the position 

 of the vacated sideband or the contiguous position for a second channel. 

 The resultant method is called "twin channel single sideband." 

 Referring to the diagram in the upper right-hand corner of Fig. 13, 

 Channel A is the single sideband which has just been discussed while 

 the second single sideband is now placed in B position so as to give two 

 separate conversations with the same transmitter. This immediately 

 brings to mind the question, "Is the advantage of single side band 

 lost by using the two sideband positions for two separate channels?" 

 Odd as it may seem it is only slightly affected. If those two sideband 

 positions are used for a single channel the frequencies in the two 

 sidebands appear simultaneously, and in corresponding positions. The 

 transmitter must handle both simultaneously and the receiver must be 

 broad enough to receive both bands. However, when two separate 

 conversations are placed in the two separate sideband positions 

 similar frequencies do not appear simultaneously in both bands except 

 at such remotely occasional times that their mutual interference is 

 small or not noticeable, and at the same time the receiver for each 

 channel is tuned for only one sideband, thereby keeping down the 

 noise. By using the two positions for two separate channels it is 

 possible to get on a statistical basis two single sideband circuits each 

 8 db better in signal-to-noise ratio than would be obtained using the 

 same two sideband positions for one channel alone. It produces a 

 remarkable increase in efficiency of use of a circuit. 



Now it also happens that addition of the second channel requires 

 a surprisingly small amount of apparatus. Looking at this same 

 figure, Channel B is indicated on the left providing input B to 

 modulator IB. This modulator modulates the same frequency as 

 modulator lA but crystal filter B selects the lower sideband in this 

 case, which sideband is now delivered to modulator 2 along with that 

 from filter A. These two parts are all the apparatus necessary to add 

 to this transmitter to convert it from one channel to two channels. 

 It is thus to be observed that by suitable application of single sideband 

 to the short-wave channels it has been possible to multiply their 

 number by two and increase each one 8 db in signal-to-noise ratio. 

 This twin channel single sideband has been applied to two of the 

 three short-wave transoceanic circuits, to the San Francisco-Honolulu 

 circuit, and undoubtedly will be applied to other circuits in the future. 



Referring again to Fig. 5, there will be seen an element marked 

 "Directive Antenna." Directive antennas have been used very little 

 in broadcasting. They are coming into greater use with short-wave 



