GB627048A - Improvements in or relating to concentric conductor electric cables - Google Patents

Abstract

627,048. Transmission cables. STANDARD TELEPHONES & CABLES, Ltd., and BAGULEY, E. Jan. 31, 1946, No. 3123. [Class 36] A communication cable of given outer diameter with three concentric conductors is arranged to give a maximum number of frequency-band channels as determined by the permissible attenuation in the cable length. For an attenuation of X decibels in a length L miles of two concentric-conductor cables, the maximum frequency in megacycles per second (F) is given by the equation where D and d are the diameters in inches of the outer and inner conductors, p the specific resistance (ohms per centimetre cube) and K the specific inductive capacity. For a frequency-band channel of four kilocycles per second the number of channels (N 1 ) is therefore 250 F. With copper conductors (p=1.67Î 10<SP>-6</SP>) and using the above formula, this gives where the logarithm is to base 10 and this D has a maximum value when -=3.6. Now, for d a three-concentric-conductor cable providing two transmission lines and of given outer diameter D with intermediate conductor of diameter d and central conductor of diameter d d 1 the ratio of - is made 3.6, giving a maximum d 1 number of channels N 2 for this inner transmission line of the cable. The total number of channels N 1 + N 2 is therefore where d/d 1 -=3.6. The total number of channels N 1 +N 2 as given by the above expression is a maximum when D/d=3.2 and is greater than the number that would be given if D/d and d/d1 were each made equal to 3.6. If the inner and outer transmission lines of a single threeconductor-cable are to be used for simultaneous out and return communication then the inner transmission line is arranged as before with a ratio d/d 1 =3.6 to give the maximum number of channels therein N 2 and the outer transmission line must give the same number of channels. D This condition N 1 = N 2 gives a value -=1.58. d The inner conductor 4 may be tubular and may contain additional conductors 7, each covered with an insulating layer 8 of a solid polymer of ethylene. A further layer 9 is extruded around these, over which the conductor 4 is formed. Conductors 3 and 1 are applied around further solid insulating layers 5, 6 and the cable is completed by insulation 2, armouring 10 and external covering 11. The intermediate conductor 3 may be constructed to include a thin layer of steel to form a magnetic screen between the two transmission lines.