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\$Hearth– Hearth2025-11-28 22:30:57 +00:00Commented Nov 28 at 22:30
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5\$\begingroup\$ use the antenna to tap morse code messages on the containsr wall, no magnetics required \$\endgroup\$jsotola– jsotola2025-11-29 06:23:53 +00:00Commented Nov 29 at 6:23
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1\$\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\$Spehro 'speff' Pefhany– Spehro 'speff' Pefhany2025-11-30 04:25:27 +00:00Commented Nov 30 at 4:25
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2\$\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\$jsotola– jsotola2025-11-30 05:35:33 +00:00Commented Nov 30 at 5:35
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2\$\begingroup\$ I wrote an answer to @uhoh's physics question. Even though it is downvoted, you may want to consider it :) \$\endgroup\$tobalt– tobalt2025-12-03 07:31:14 +00:00Commented yesterday
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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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1\$\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\$Christian– Christian2025-11-30 06:16:44 +00:00Commented Nov 30 at 6:16
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1\$\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\$uhoh– uhoh2025-12-01 00:59:52 +00:00Commented Dec 1 at 0:59
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1\$\begingroup\$ @uhoh thanks for the link, sounds like you've been down a similar rabbit hole of thinking in the past. Hopefully with your bounty we can get to the bottom of it. I have seen some resources which make me think they may be different, but nothing definitive. For example: interferencetechnology.com/… \$\endgroup\$Christian– Christian2025-12-01 09:27:44 +00:00Commented Dec 1 at 9:27
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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\$uhoh– uhoh2025-12-01 01:01:07 +00:00Commented Dec 1 at 1:01
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Yes, a magnetic loop antenna inside a Faraday cage (a fully enclosed metal container) can, with careful engineering, be used to transmit information through the metal wall of the container.
I've seen a few reports of RuBee tags transmitting (apparently at a carrier frequency of 131 kHz) through the wall of a metal drum or other enclosed metal container: (c). Since it happens, it must be possible.
I've also worked with detectors for pipeline inspection gages (PIG detectors). The detectors I'm most familiar with detect big permanent magnets inside the PIG as the PIG is pushed by petroleum oils through an oil pipeline made of thick metal. I've heard stories of a PIG getting stuck in a pipeline, and someone hand-carries the detector that last saw the PIG, walking a couple of miles downstream toward the next detector that hasn't seen the PIG in far too long, walking over pipeline buried a few feet underground, to figure out exactly where along the pipeline was stuck. (The magnets are generally centered in the middle of the PIG, which nearly fills the pipe and so is centered in the pipe a few feet in diameter -- it doesn't need to be "in close proximity to the wall").
(Side note: Sometimes people describe "in" a metal object which sounds like it's inside a Faraday cage, but often the photos clearly show a slot milled on the outside of the object and the rest of the slot filled with RF-transparent epoxy -- which is clearly not what you're asking. ).
While I'm not a physicist, my understanding is that for any electromagnetic signal all ordinary metals have a skin effect that depends on both the material and the frequency of the signal. At around 130 kHz, the skin effect of aluminum is about 0.25 mm and the skin effect of stainless steel is about 1.5 mm. So each additional 0.25 mm thickness of aluminum (or each additional 1.5 mm of stainless steel) reduces the distance that the receiver can detect the transmitter to roughly 1/4 the distance. (So there is some thickness that reduces that distance so small that both the transmitter and reciever must be "in close proximity to the wall"). On the other hand, each time we reduce the carrier frequency by a factor of 100, we can tolerate (at the same transmitter-receiver distance) metal 10 times as thick. (See D. Ciudad, P. Cobos Arribas, P. Sanchez, and C. Aroca: "RFID in Metal Environments: An Overview on Low (LF) and Ultra-Low (ULF) Frequency Systems" ).
I'm interpreting "perfect" to mean "100% completely surrounded by some ordinary metal", which allows some transmissions. If "perfect" means "a superconductor", my understanding is that superconductors completely expel magnetic fields (Meissner effect) as well as electric fields, and so blocks all electromagnetic transmissions.
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\$\begingroup\$ While "seen a few reports" doesn't necessarily equal "it happens", any time there's an exponential decay, there's a possibility of measurable transmission. (The 1/e depth is of order 0.1 mm imperfect Faraday cage so real-world stuff happens. \$\endgroup\$uhoh– uhoh2025-12-03 19:51:57 +00:00Commented yesterday