132 BELL SYSTEM TECHNICAL JOURNAL 



sion characteristic or, if sufficiently severe, actually cause the repeater 

 to sing. This general situation has made it necessary to introduce 

 in the circuits at such points "crosstalk suppression" filters in the 

 non-J pairs and longitudinal choke coils in all pairs. 



Staggering 

 In addition to the various steps which are taken in order to reduce 

 crosstalk by improving the line conditions, the type J system may 

 include a feature which has been used in the type C system, — the 

 staggering of the channel bands used on neighboring pairs. The ad- 

 vantage of staggering results from the facts that (a) the sensitivity of 

 the ear and the power of the voice vary over the audible range, {b) 

 the efficiencies of transmitter and receiver also tend to vary over the 

 frequency range, (c) part of a channel band may lie opposite "dead" 

 frequency range on an adjacent pair, and (d) by controlling the arrange- 

 ment of the sidebands the crosstalk may be made unintelligible even 

 if not inaudible. The staggering feature is readily provided in the 

 type J system by a suitable choice of carrier frequency for the second 

 group modulator and first group demodulator. With the staggered 

 systems the highest frequency used would be about 143 kilocycles. 



Attenuation Problem 

 In what has preceded in the discussion of line problems, the emphasis 

 has been confined chiefly to the question of the smoothness of a line 

 from an impedance standpoint in order to keep down reflection effects 

 and, correspondingly, to improve the operation from a system-to- 

 system crosstalk standpoint. There is also the problem of the higher 

 attenuation incident to the use of higher frequencies. Between 

 30,000 cycles and 140,000 cycles, the normal wet weather attenuation 

 for a 165-mil open-wire pair, for example, rises from about 0.13 to 0.28 

 db per mile, — an increase of approximately 2:1. Repeaters on the 

 type J system, if applied on the basis of approximately the same output 

 level and minimum level requirements, must be spaced at about one- 

 half the interval of the type C systems. Normal spacings for type J 

 systems would therefore be expected to range from 75 to perhaps 

 100 miles where no large amount of intermediate cable existed. 



However, another problem, not present to a similar degree at the 

 lower frequencies, tends in many cases to have a controlling effect 

 on this spacing, that is, sleet or ice on the wires. With ice, frost, or 

 snow on the wires, the wet weather attenuation may be exceeded by 

 very large amounts. The additional attenuation is due primarily to 

 the coating on the wires themselves rather than the coating on the 



