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
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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