Transfer impedance of nonmagnetic conduits of various shapes

Metallic conduits provide mechanical support for cables, and may also protect the cables against interference caused by currents induced by external electromagnetic fields. The conduit transfer impedance Zt then is a good measure for the provided protection. We studied Zt by means of conformal transformations and of the method of moments in a quasistatic, two-dimensional (2-D) approach. Practical U-, H-, L-, and T-shaped conduits are discussed. Conformal transformations provide simple engineering expressions for the mutual inductance part of Zt for cable positions inside the conduit. The method of moments formulation results in useful approximations for the crossover frequencies in Zt between DC and high frequency. Measurements on U-shaped conduits confirm the calculations