TRANSATLANTIC RADIO TELEPHONY 129 



loop six feet on a side and wound with 46 turns of stranded wire. The 

 first box contains the beating oscillator and high-frequency detector, 

 the second box of the filter and amplifying apparatus for the inter- 

 mediate frequency and the third box the final detector and amplifier. 

 The shielded box at the left of the picture, which is connected to the 

 loop by means of leads in the copper tube, is the apparatus for intro- 

 ducing the comparison signal of known strength into the loop for 

 measuring purposes. This receiving and measuring set is described 

 more in detail in a paper by Bown, Englund, and Friis in the "Pro- 

 ceedings of the Institute of Radio Engineers for April, 1923." 



Although it was this very selective and reliable method of inter- 

 mediate-frequency reception which was used in London, it is quite 

 possible to receive the single-side-band transmission by means of a 

 regular heterodyne receiving set. Even a self-regenerative set will 

 suffice under some conditions. It is necessary, however, to adjust 

 the frequency of the oscillator very carefully to that of the trans- 

 mitting carrier frequency, otherwise serious distortion of the received 

 speech will result. Also it is, of course, necessary that the tuning 

 be not too sharp if ordinary tuned circuits and not filter circuits are 

 employed. One might expect that some difficulty would be experi- 

 enced in maintaining the frequency at the receiving end in sufficiently 

 close agreement with the sending frequency. In the tests no par- 

 ticular difficulty was experienced, the oscillators at the two ends 

 being so stable that only an occasional slight readjustment of the 

 receiving oscillator was required. With the development of more 

 stable oscillators, doubtless, the frequency with which readjustments 

 are required, will be further reduced. If serious distortion of the 

 received speech is to be avoided the two frequencies must be within 

 about 50 cycles, an accuracy of 0.1 per cent at 50,000 cycles. 



Transmission Advantages of the System 



Since the present experiment represents the first use of the single- 

 side-band eliminated carrier type of system some further discussion 

 of the characteristics and advantages of the system is appropriate. 



The importance of the system in conserving frequency range will 

 be appreciated when it is realized that the total frequency range 

 available for transatlantic telephony is distinctly limited. Just 

 what the most suitable range is has not been accurately determined 

 but it seems limited to below 60,000 cycles (5000 meters) because of 

 the large attenuation suffered during the daylight hours by frequencies 

 higher than this. On the lower end of the frequency scale, trans- 



