TRANSATLANTIC RADIO TELEPHONY 131 



If, after having suppressed the carrier, both side-bands were trans- 

 mitted, their reception would require perfect synchronism between 

 the carrier resupplied at the receiving end and that eliminated at the 

 sending end, a condition which is practically impossible to meet 

 without transmitting some form of synchronizing channel, which is, 

 indeed, much the same as transmitting the carrier itself. If the 

 receiving carrier is not synchronized, the two side-bands will interfere 

 with each other upon being detected. By eliminating one side-band, 

 this interference is prevented and reception may be carried on, using 

 a locally supplied frequency which is only approximately equal to 

 that of the suppressed carrier. The two may differ by as much as 

 50 cycles before the quality of the received speech is greatly impaired. 

 The importance to the carrier suppression method of eliminating 

 one side-band will, therefore, be appreciated. The present system 

 eliminates one side-band while still in the low-power stage. While 

 it would be possible to do this selecting after they have both been 

 raised to the full transmitting power, this would require the use of a 

 filter of high-power carrying capacity, which would make the filter 

 very costly and also render the system inflexible to change of wave 

 length. The present system overcomes both of these difficulties by 

 filtering our one side-band at low-power levels and by the use of the 

 double modulation method. 



Another very important reason for the transmission of a frequency 

 band as narrow as is possible lies in the difficulty of constructing an 

 antenna to transmit more or less uniformly at these long waves a 

 band of frequencies which is an appreciable fraction of the main 

 carrier frequencies. For example, in the ordinary method of trans- 

 mission an antenna which was intended to transmit a 30,000-cycle 

 carrier and its two speech side-bands would need to be designed to 

 transmit all the frequencies from 27,000 cycles to 33,000 cycles, a band 

 which is equal to 20 per cent of the carrier frequency. This band is 

 considerably wider than that given by the resonance curve of a highly 

 efficient long wave antenna. To accommodate both side-bands would 

 require flattening out the resonance curve either by damping, which 

 means sacrifice in power efficiency, or by special design of the antenna, 

 possibly throwing it into a series of interacting networks and causing 

 it to become a rather elaborate wave filter. The importance, from 

 the antenna standpoint, of narrowing the frequency band required to 

 be transmitted is, therefore, evident. 



It is extremely important that the received signal be affected as 

 little as possible by changes in the transmission efficiency of the 

 medium. The voice frequency currents produced at the receiving 



