WIDE BAND TRANSMISSION OVER BALANCED CIRCUITS 5 



temperature and in part to changes in leakage and capacitance due to 

 varying weather conditions, automatic transmission regulating systems 

 similar in general principles to those already developed for other pur- 

 poses should be adequate to maintain the required stability. 



Crosstalk between different circuits becomes so severe at high 

 frequencies that special transposition treatment or respacing of the 

 wires becomes necessary. Minimizing interference from and to 

 radio systems calls for a high degree of balance which may or may not 

 dictate changes in the wire configuration. Here again it is necessary to 

 distinguish between the requirements for television and for message 

 telephone. Tests indicate that, in view of the more lenient television 

 requirements, several million-cycle television channels can be trans- 

 mitted over different pairs of a single open-wire line without serious 

 disturbance and that to do this it will not be necessary to make radical 

 changes in present wire configurations. 



New Cables 



For new construction, if television is not considered, effective carrier 

 telephone systems may be set up by various methods. One might be a 

 very broad-band method, a good example of which is given in the 

 Espenschied-Strieby paper. ^ Another might be a much narrower band 

 method using conductors similar to those in an ordinary cable. In the 

 one case many telephone channels are obtained from a single pair by 

 dividing up a frequency range, say one million cycles wide, into some- 

 what more than 200 channels. In the other case only 20 odd channels 

 are obtained per pair of wires and use is made of 10 pairs of wires 

 to obtain the same total number. It is too early to say which of these 

 plans might be best under various practical conditions. 



To meet future television needs, it may be necessary to provide for 

 transmission of continuous frequency bands a million cycles in width or 

 wider. It is interesting to compare the coaxial with balanced pairs 

 surrounded by individual shields for such transmission. 



For 6 db loss per mile at a million cycles, it works out that a solid 

 copper coaxial unit with the rubber disc insulation described in the 

 Espenschied-Strieby paper ^ has an internal diameter (inside of shield) 

 of about 0.25 inch. For the same attenuation a pair of wires, each the 

 same size as the central wire in the coaxial unit (70 mils diameter), 

 insulated with rubber discs and with a copper shield, will have an inside 

 diameter under the shield of about 0.4 inch. For outside conductors 

 or shields made of lead the inside diameters become about 0.4 inch for 

 the coaxial and 0.5 inch for the balanced pair. Therefore, if the thick- 

 ness of the outer conductor is determined by mechanical considerations 



