6 BELL SYSTEM TECHNICAL JOURNAL 



rather than outside interference, the coaxial is smaller and cheaper. 

 As the frequency is made higher, the shielding from outside interference 

 afforded by the surrounding cylinder increases, so that at very high 

 frequencies mechanical considerations alone control and the coaxial is 

 clearly cheaper than the balanced shielded pair. 



In the frequency range up to about a million cycles, however, inter- 

 ference from outside sources, including natural static and radio, must be 

 considered in determining the thickness of the surrounding cylinder and 

 the cost comparison is not so clear. It will be evident that as regards 

 shielding, the balanced pair is at a large advantage because the two 

 sides of the circuit are designed to be electrically similar. By proper 

 care in manufacture this balance can readily be made sufficient to in- 

 sure adequate shielding with a surrounding lead tube of thickness de- 

 termined solely by mechanical considerations. 



With a coaxial structure, however, it appears likely that to keep 

 interference within proper bounds, a simple lead tube must be made 

 considerably thicker than required by mechanical considerations, so 

 that such a structure would probably be more expensive than a lead 

 shielded pair. However, by adding other materials an adequately 

 shielded coaxial unit can be constructed which will have a considerably 

 thinner outside wall. 



For example, there is described in the Espenschied-Strieby paper ^ a 

 coaxial unit in which the inside diameter is minimized by first using 

 copper tapes, the thickness of wall is minimized by adding thin iron 

 tapes and the whole is made waterproof by a thin surrounding lead 

 tube. This results in a unit of smaller inside and outside diameters 

 than those of a lead tube surrounding a balanced pair of like attenua- 

 tion. Since, however, the wall of the coaxial is thicker and the struc- 

 ture more complicated, the costs of the two units are estimated to be 

 not greatly different when they are designed for the frequency range 

 up to a million cycles. A minor advantage for the balanced pair re- 

 mains, however, in that, whatever may be the top frequency, there is no 

 limitation as to the lowest frequency permissible for interference 

 reasons. 



In the above discussion of new cable construction the amplifiers and 

 transmission regulators required have not been mentioned. If similar 

 conducting and insulating materials are used, shielded balanced pairs 

 and coaxials h^ive similar transmission-frequency characteristics. The 

 variations with temperature are also similar. The factors which limit 

 the overall amplifications are also the same. There is only one impor- 

 tant point of difference between the amplifiers required for the two 

 systems. This is the necessity for input and output transformers to be 



