Effects of Partial Shields on Transmission Lines at Low Frequencies

It is often desirable to estimate the degree of shielding achieved between two transmission lines crossing at some specified angle, when one of them is surrounded by a metallic tubing of finite length. A simple analysis that allows a rather accurate estimation of the shielding effectiveness of the tubing at low frequencies where the quasi-static assumption holds is presented. Based upon this quasi-static assumption and the empirical results that the fields generally fall off inversely with the square of the distance from a line and with the aid of Schelkunoff's shielding theory, an equation predicting the shielding effectiveness of the tubing has been derived and confirmed experimentally. Examples are given involving coaxial cables, parallel-wire lines, and copper shields of various lengths crossing at various angles. The discrepancies between calculated and measured values are generally less than 4 dB and in most cases less than 1 dB. The theory has also been applied to cases where the cables approach, but do not cross each other, such as were recently measured sured at the U. S. Navy Underwater Sound Laboratory.[1] Good agreement is obtained in this case also. Other situations involving ferro-magnetic shields and twistedpair lines have also been considered. The restrictions on the applicability of the formula developed have been pointed out explicitly.