Influence of twist-pitch random non-uniformity on the radiated immunity of twisted-wire pairs

Random non-uniformity of the twist pitch play an important role in radiated immunity of twisted-wire pairs (TWPs). In this work, a numerical prediction model based on transmission-line theory is used to investigate the response of a TWP with non-uniform twisting. The TWP is connected to balanced loads, is placed above an ideal ground plane, and illuminated by a plane-wave electromagnetic field. Unlike in previous works, where non-uniform TWPs were simply described as a cascade of individual twists of helical shape with different pitch, here the TWP is represented as a bifilar helix with continuous variation of the twist pitch. The modeling approach is therefore more consistent with the physical structure of a real TWP, and exploits a random function which relates the local value of the twist-pitch with the position along the TWP. It is shown that twist-pitch non uniformity does not affect the common-mode component of the response to the external field, whereas it influences the differential-mode voltage, whose sensitivity to twist-pitch non uniformity can be conveniently analyzed by resorting to a statistical approach.