Numerical assessment of radiated susceptibility of twisted-wire pairs with random nonuniform twisting

The influence of twist-pitch nonuniformity on the radiated susceptibility of a twisted-wire pair (TWP) running above ground and illuminated by a plane-wave field is investigated via a transmission-line model. The geometrical representation of the TWP involves a bifilar helix with a variable pitch function, which readily allows for the description of any unwanted/unknown deviations from the ideal twist geometry. Basic examples are used to show that such a deformation plays a fundamental role in the pickup of differential-mode (DM) noise, thus substantiating the need for the proposed prediction model. By describing the pitch random variations as a spatial stochastic process, a realistic representation of the twisting features is obtained (both for TWPs with imperfect uniform twists and for TWPs intentionally manufactured with random nonuniform twists). Repeated runs are used to characterize the sensitivity of the induced DM noise to random nonuniform twisting. Especially, it is shown that the twist-pitch nonuniformity, sometimes intentionally used to reduce pair-to-pair crosstalk in TWP cables, may however lead to a severe decrease of the TWP immunity to external fields