The coaxial transmission line confines the electromagnetic field 

 to its interior, as does the hollow-tube line, and therefore eliminates 

 interference with external circuits at high frequencies. At low frequen- 

 cies the skin depth may exceed the thickness of the outer conductor, 

 causing not only energy loss but also cross talk, i.e., interference 

 caused by currents induced in communication lines through shields . For 

 frequencies over 10 kHz, the skin depth is less than about 0.01 inch. 

 Therefore, for coaxial cables in which carrier frequencies lie in the 

 band above 100 kHz — which is the case for transatlantic telephone lines — 

 the thickness of the outer conductor need, ideally, be onlv greater than 

 0.01 inch. 



Coaxial cables, like all transmission lines, have four basic elec- 

 trical parameters which, in turn, determine the four secondary electrical 

 parameters commonly used to characterize the cable. The basic parameters 

 are the per-unit-length quantities, R, L, G, and C (resistance, induc- 

 tance, conductance, and capacitance); the parameters which caracterize 

 the cable are attenuation, phase shift, characteristic impedance, and 

 propagation velocity. 



The two most important secondary electrical parameters are the at- 

 tenuation and the characteristic impedance. Attenuation gives the cable 

 loss in dB per unit length and must be considered in designing the cable. 

 Characteristic impedance must be known in order to design terminal and 

 repeater equipment such that optimum impedance matching is achieved. The 

 phase shift and the propagation velocity are related through the frequen- 

 cy; velocity is the speed at which a signal, a pulse, for example, travels 

 along the coaxial cable. Practical lower and upper limits of velocity are 

 about 4000 miles/second and 100,000 miles/second, respectively. The 

 characteristic impedance is the impedance measured at the input between 

 the central and outer conductor of an infinitely long cable. For a cable 

 of finite length, impredance is experimentally determined by measuring 

 the input impedance, first with the far end open and then with the far 

 end shortened. 



Dielectric sizing may require extremely tight design tolerance be- 

 cause positive and negative diameter excursions of a certain magnitude 

 would not be sufficiently self-cancelling to hold the net attentuation 

 to a low enough value. For example, in the manufacture of SD telephone 

 cable — a bottom- lying armorless submarine cabled, 14 having a dielectric 

 O.D. of 1 inch — control of the extrusion process is not able to hold 

 diameter variations below about ±0.005 inch. A supplementary sizing 

 operation is required (involving actual cutting of the polyethylene 

 dielectric) to hold diameter variations under ±0.0001 inch. 



Direct measurement of R, L, G, and C, under different conditions of 

 temperature and pressure (controlled in laboratory tests), yield experi- 

 mental temperature and pressure coefficients which, then, can be used to 

 find the effect on attenuation. For the SD ocean cable mentioned earlier, 

 it turns out that the temperature coefficient accounts for over 75 per- 

 cent of the effect on attenuation. Pressure change causes the pressure 

 coefficient to affect over 50 percent of the attenuation. 



The characteristic impedance of most coaxial-electro-mechanical sub- 

 marine cables lies between about 40 and 70 ohms. Besides being essential 

 to the design of terminal and repeater equipment, knowledge of the 



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