In what ways are real coaxial cables non-ideal?
If the shield were a perfect conductor, currents produced in it by both internal and external fields would flow only along the corresponding surface and not penetrate its thickness at all. However, real cables are made with materials that have nonzero resistivity. This not only causes power loss (heating) within the cable, it also means that all currents penetrate the thickness of the shield to a greater or lesser extent, depending on frequency (skin effect).
Looking in detail at the electric field inside the coax, note that while it has radial symmetry, it is not purely radial at all points along the length of the cable. There is a longitudinal component that is due to the finite rate at which signals propogate through the cable. It is this component that is subject to leakage/interference because of an imperfect shield.
Other non-ideal characteristics of cables include gaps in braided shields, which can enable fields to “fringe” through the gaps. This effect gets worse as the frequency goes up and the gap becomes a larger fraction of the wavelength.
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