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I understand that in the near field of a loop antenna the magnetic field dominates, before it has formed a radiative propagating EM wave. Based on this, and the fact that magnetic fields can penetrate metals, could a magnetic loop antenna in close proximity to the wall of a fully enclosed metal container be used to transmit information through the wall of the container (which would normally block EM if it was in the far field)?

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  • \$\begingroup\$ By "perfect metal" do you mean superconducting? If so, then no, because of the Meissner effect. \$\endgroup\$ Commented 2 days ago
  • \$\begingroup\$ If the metal is steel you might be able to detect a signal. \$\endgroup\$ Commented 2 days ago
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    \$\begingroup\$ use the antenna to tap morse code messages on the containsr wall, no magnetics required \$\endgroup\$ Commented yesterday
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    \$\begingroup\$ @jsotola If you suggest attaching a magnet to the side and pick up the motion by induction in the loop antenna you have a viable answer that fits the question. Just say’n. \$\endgroup\$ Commented 21 hours ago
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    \$\begingroup\$ @Spehro'speff'Pefhany that is actually a really good idea ... turn the metal container into a speaker diaphragm. .. send sound into the container and then reverse the process and use the container as a microphone diaphragm ... or there could be a magnet attached to each side of the container wall and a coil on each side to interact with the magnet \$\endgroup\$ Commented 20 hours ago

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Yes, in principle, for non-superconducting metal.

However the bandwidth will be limited because of eddy currents, with thicker more conductive metals being worse. If the metal happens to be ferromagnetic then it will also act as a short magnetically.

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  • \$\begingroup\$ your thoughts on this comment are welcome \$\endgroup\$ Commented 1 hour ago
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Penetration of the magnetic field would be limited by the skin depth of the metal. So penetration would be significant only at very low frequencies.

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    \$\begingroup\$ I guess the heart of the question I'm trying to answer is would the penetration be greater as the loop is in the near field, than it would in the far field? Say i measure EM some distance away from the box, would the measured EM be greater if the loop is closer to the wall of the box \$\endgroup\$ Commented 19 hours ago
  • \$\begingroup\$ @Christian I may be completely wrong, but I think that the skin depth-based limit applies equally well to oscillating electric and to oscillating magnetic fields, as well as to fully propagating far-field electromagnetic radiation. I've just added a bounty to my 2016 question Are the skin depths for an oscillating electric field or magnetic field both the same as EM wave skin depth? in Physics SE, let's see if something happens. \$\endgroup\$ Commented 1 hour ago

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