562 BELL SYSTEM TECHNICAL JOURNAL 



Developments Essential to Growth 



The first long distance radio telephone circuit operated (and it still 

 operates) between the United States and England with long-wave 

 transmission at about 5000 meters. We did not then, and we do not 

 today, know how any considerable amount of intercontinental radio 

 telephony could have been accomplished with circuits of this kind. 

 The frequency space available in the long-wave range would accom- 

 modate comparatively few channels. The high attenuation to over- 

 land transmission and the high noise level at these wave-lengths pre- 

 clude their satisfactory use for very great distances or in or through 

 tropical regions. The discovery that short waves could be transmitted 

 to the greatest terrestrial distances and could be satisfactorily received 

 in the tropics came at a most opportune time. 



Short-wave transmission not only released the limitations on distance 

 and location inherent to long waves but also opened up such a wide 

 range of frequency space as to give opportunity for an extensive growth 

 in numbers of both radio telegraph and radio telephone circuits. Short 

 waves further encouraged the growth of radio telephony by making it 

 cheaper. Thus, it became possible to make directive antenna struc- 

 tures of moderate size which increased the effectiveness of transmission 

 many times, thereby reducing the transmitter power required for a 

 given reliability of communication. Short waves were the indis- 

 pensable element without which material growth could not have oc- 

 curred, but there were other significant things. 



An important desideratum in telephony is privacy. Commercial 

 radio telephony would have been severely hampered if privacy systems 

 had not been developed to convert speech into apparently meaningless 

 sounds during its radio transit. 



Another item of great aid in promoting growth was the development 

 of methods of accurate stabilization of transmitted frequencies. The 

 first effect of this was to eliminate the extreme distortion which charac- 

 terized early short-wave telephone transmission and which was found 

 to be due to parasitic phase or frequency modulation effects in the 

 transmitters. As the number of radio communication facilities, both 

 telegraph and telephone, grew, accurate stabilization of frequency be- 

 came a necessity in order to permit effective utilization of the available 

 frequency space without mutual interference between stations. 



Later Technical Advances 



The "rhombic" antenna is mechanically simple and electrically 

 nearly aperiodic, covering a wide wave-length range efficiently. It 



