Is the signal carried by an “ideal” coaxial cable immune from external electric and magnetic fields? Explain.
Yes, an “ideal” coaxial cable forms a perfect shield against
the influence of external fields.
First, consider the fields generated by the signal in the
first place. The electric field exists only between the center conductor and
the surrounding shield, within the dielectric of the cable. Both the center
conductor and the shield have magnetic fields associated with the currents flowing
within them, but it’s easy to show that the net magentic field outside
the shield is zero.
Thus, the signal in the coaxial cable produces neither type of field outside the shield. Applying the principle of reciprocity, it’s impossible for an external field of either type to affect the signal.
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