Shielding Performance of Triply Shielded Coaxial Cables

We determine the shielding effectiveness of a triply shielded coaxial cable and compare it to the performance of a singly shielded cable. Then we apply well-known transmission-line theory to make a parametric study of the shielding performance as a function of the geometrical and electrical parameters of the braids. We suppose that the three braids are homogeneous, in order to use exact expressions relating the transfer impedance to the shield characteristics. We study the influence of the intersheath space on screening effectiveness and on the intersheath resonances. To damp these resonances, a material having a low conductivity could be used between the braids. To improve the shielding at low frequencies, an intermediate copper braid can be replaced by a ferromagnetic material. We take into account the variation of the permeability as a function of frequency and we point out, in this case, the effect of the intersheath space.