Radio Extension Links to the Telephone System 



By R. A. HEISING 



TO the general public, the word radio means broadcasting, and a 

 radio set means a radio receiver for listening thereto. The 

 average man never has any direct contact with a radio transmitter, 

 nor with the radio telegraph which preceded the radio telephone and 

 so utilizes the all inclusive word "radio" for that one part of it which 

 he sees, buys, and uses. 



The radio engineer is not quite in that class. There is, however, 

 much in radio with which he is unacquainted. The radio field has 

 become so broad and extensive that it is physically impossble for 

 anyone to keep abreast of the whole art. Since the application of 

 radio to telephone links is a specialized field, undoubtedly much of 

 its history and technical developments is known only sketchily or not 

 at all to many engineers. This paper, therefore, is planned to cover 

 this field briefly, show its general development, and describe in prin- 

 ciple a number of devices developed for use therein, most of which 

 are seldom if ever used in the field of broadcasting. 



The radio telephone was not one of those devices that an inventor 

 springs upon an unexpecting world. On the contrary it was expected, 

 and was the object of search and investigation for years before a prac- 

 tical form appeared. Because the wire telephone followed the wire 

 telegraph, technical men expected the radio telephone to follow the 

 radio telegraph as soon as the latter had been practically demon- 

 strated. Telephone men developed an interest in it as soon as it was 

 suggested. Telephony over large bodies of water, over difficult terrain, 

 and to moving conveyances was difficult or impossible for wire 

 telephony, and the telephone man was intrigued by the possibility of 

 providing his circuits without the use of conducting wires. 



During the first few years of this century, several radio telephone 

 systems were technically demonstrated but were found impractical. 

 In 1912 an important step occurred. The audion, invented by 

 DeForest, was brought to the attention of the American Telephone 

 and Telegraph Company. It appeared to have possibilities making it 

 superior to the mechanical and the arc repeaters for wire telephone 

 lines. A telephone repeater, or amplifier, was a main object of search 

 at that time by telephone men. The audion became the subject of 

 study in the Research Department of the American Telephone and 



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