658 BELL SYSTEM TECHNICAL JOURNAL 



fined to a small physical channel which is shielded from outside dis- 

 turbances. The system, in effect, comprehends a frequency spectrum 

 of its own and shuts it off from its surroundings so that it may be used 

 again and again in different systems without interference. 



This new type of facility has not yet been commercially applied. It 

 is, in fact, still in the development stage. Sufficient progress has 

 already been made, however, to give reasonable assurance of a satis- 

 factory solution of the technical problems involved. This progress 

 is outlined below under three general headings: (1) the coaxial line and 

 its transmission properties, (2) the wide band repeaters, and (3) the 

 terminal apparatus. 



The Coaxial Line 



An Experimental Verification 



One of the first steps taken in the present development was in the 

 nature of an experimental check of the coaxial conductor line, de- 

 signed primarily to determine whether the desirable transmission prop- 

 erties which had been disclosed by a theoretical study could be fully 

 realized under practical conditions. For this purpose a length of 

 coaxial structure capable of accurate computation was installed near 

 Phoenixville, Pa. Figure 3 shows a sketch of the structure used and 

 gives its dimensions. It comprised a copper tube of 2.5 inches outside 

 diameter, within which was mounted a smaller tube which, in turn, 

 contained a small copper wire. Two coaxial circuits of different sizes 

 were thus made available, one between the outer and the inner tubes, 

 and the other between the inner tube and the central wire. The 

 instal ation comprised two 2600-foot lengths of this structure. 



The diameters of these coaxial conductors were so chosen as to ob- 

 tain for each of the two transmission paths a diameter ratio which 

 approximates the optimum value, as discussed later. The conductors 

 were separated by small insulators of isolantite. The rigid construc- 

 tion and the substantial clearances between conductors made it pos- 

 sible to space the insulators at fairly wide intervals, so that the dielec- 

 tric between conductors was almost entirely air. The outer conductor 

 was made gas-tight, and the structure was dried out by circulating 

 dry nitrogen gas through it. The two triple conductor lines were 

 suspended on wooden fixtures and the ends brought into a test house, 

 as shown in Fig. 4. 



The attenuation was measured by different methods over the fre- 

 quency range from about 100 kilocycles to 10,000 kilocycles. In- 

 vestigation showed that the departures from ideal construction occa- 

 sioned by the joints, the lack of perfect concentricity, etc., had remark- 



