Theoretical Investigation and Experiment of Shielding Efficiency of a Multi-Braided Coaxial Cable

We determine the different voltages, as the crosstalk, which could appear on a multi­ braided coaxial cable when a disturbing cur­ rent flows on the outer braid. We compare the shielding efficiency of a classical coaxial cable to the one of a multi-braided cable. In that case, we suppose that insulating layers are situated between the braids, theoretical results point out resonances which appear when the concentric transmission lines made by the different shields are short-circuited. Measu­ rements on a two layer shield confirm the pre­ sence of these resonances. In order to avoid them, a first solution consists in connecting the characteristic impedances to both ends of the successive lines made by two adjacent bra­ ids. An other mean to damp down the resonances is to replace the insulant between the braids by a dielectric having an appropriate conduc­ tivity. Theoretical results show that the mul­ ti-braided coaxial cable gives a best immunity to disturbances than a single braid, but the improvement depends on the total length of the line and on the working frequency band.