262 BELL SYSTEM TECHNICAL JOURNAL 



efficiency than is feasible at lower frequencies. At 18,000 kilocycles 

 (about 16 meters) a single half- wave radiator or doublet is only about 

 25 ft. long and it is possible to combine a number of them, driven in 

 phase by a common transmitter, into an antenna array which concen- 

 trates the radiated power in one geographical sector. In that direction 

 the effectiveness may be intensified 50 fold or more (17 db) and waste 

 radiation in other directions reduced materially. Thus, one of the 

 transmitters at Lawrenceville, New Jersey, used in the short-wave 

 transatlantic circuits when supplying 15 kw. radiates in the direction 

 of its corresponding receiving station as effectively as would a non- 

 directive system of about 750 kw. 



The transmitting antennas also give some directivity in the vertical 

 plane, increasing the radiation sent toward the horizon and decreasing 

 that sent at higher angles. It is not yet certain that vertical directivity 

 is always advantageous and this effect has not been carried very far. 



At the receiving station the radiated power has dwindled to a small 

 remnant which must be separated from the static as far as possible 

 and amplified to a volume suitable for use in the wire telephone plant. 

 Here again directive antenna arrays are of value. A receiving antenna 

 system sensitive only in a narrow geographical sector, and that lying 

 in the direction from which the signal arrives, excludes radio noise from 

 other directions and thereby scores a gain of perhaps 40 fold (16 db) 

 in the power to which the signal can be amplified without bringing 

 noise above a given value. It also scores against noise which arises in 

 the tubes and circuits used for amplification, since the combined action 

 of the several antennas of the array delivers more signal to the initial 

 amplifier stage where such noises originate. 



Thus, it is evident that transmitter power, transmitting directivity, 

 receiving directivity, and quiet receiving amplifiers are of aid in pro- 

 viding signal transmission held as far as possible above the radio 

 noise. In a well designed system the relative extents to which these 

 aids are invoked will depend upon economic considerations as well as 

 upon the technical possibilities of the art. 



There is one other type of noise than that provided by Nature which 

 is of particular importance at short waves,- — electrical noise from the 

 devices of man. One of the worst offenders is the ignition system of the 

 automobile. The short-wave transoceanic receiving station at Net- 

 cong. New Jersey, is so located that automobile roads are at some 

 distance, particularly in the direction from which reception occurs. 

 Service automobiles which produce interference cannot be allowed near 

 the antenna systems unless their ignition systems have been shielded. 

 Also, electrical switching and control systems incidental to the power, 



