For direct current, the transmission line equations^ are:




5- 1

where I =


current at sending end,

current at receiving end.

voltage at sending end.

voltage at receiving end,

resistance, per ft. of conductors,

conductance, per ft, of insulation, and

length of cable, in feet.



r =


g = x =


The term Vr can be eliminated between these two, yielding


I = Ir ^cosh^/r g x — sinh л/Fg x tanh — Jrg x^ + V^§" tanh — Jrg x




When practical values of r. g, and x are inserted in the above, it will be noted that the second term is small compared to the first, resulting in


I = Ir cosh-Jr g x + tanh x




and that this can be further simplified to I = Ir + Vgx.




Since Tr is the detector current and hence the signal, and I is the meter reading, then Vgx is the error. But Vgx is just the leakage current through the insulation due to the applica­tion of the voltage V, and this is the only source of error introduced by the cable. This error is often minimized by using triaxial cable and maintaining a minimum potential difference between the center conductor and the inner shield — in other words, by using standard guard techniques.

It should be cautioned that the conductance per foot of the insulation is a function of tempera­ture. radiation field, and total dose, and is always greater than the value obtained at room temper­ature at zero radiation field.