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I have asked previously about the properties & shielding of the "static" magnetic field previously, where I have determined that static fields are hard to shield.

However what I am really interested in, is that is it possible to shield against an alternating magnetic field, in the sense that can the information itself be blocked?

When I refer to shielding I don't just refer to EM shielding but also information shielding.

For example:

  • We put a radio transciever a metalic box to act as a shield.
  • The metalic box can easily shield against the electric field going out or going into the box, therefore we can't send out signals from the box, since the electric field is blocked by the shield.
  • So far so good, the electric field doesn't leak information, however that electric field can have an alternating magnetic field. (static field doesn't matter as pointed out above)
  • So if we put a magnetometer ouside the shield, can the magnetometer pickup the signal from the transciever? It may be electrically shielded, but the magnetometer looks for the magnetic field, and therefore the information can go out via the magnetic field?

Some people have pointed out that eddy currents could be generated by the alternating magnetic field , that would manifest itself in the shield. So the eddy currents would cancel out the change in the alternating magnetic field, and make it static.

So I don't quite understand this phenomena, my point is, I don't care if a static magnetic field goes out from the box, which will inevitably do according to the answers on my earlier question.

  • So a static magnetic field will go out, but that's no issue (since it can't carry information)
  • But will the alternating magnetic field succesfully be stopped by the eddy currents, or will the magnetometer pickup the information leaking out from the box?

In other words, can information be sent out from an electrically shielded environment via alternating magnetic fields? How effective is an electrical shield against information blocking that may be carried by magnetic waves?

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    \$\begingroup\$ "the eddy currents would cancel out the change in the alternating magnetic field, and make it static." Not quite true. Eddy currents can only occur during the change of the external magnetic field. Once the field goes static again the eddy currents cease and the new static field will pass the material. Also, the eddy currents cannot cancel the field change completely, they only counteract it to a certain degree (unless the shielding is super-conducting). \$\endgroup\$ – JimmyB Mar 18 '17 at 14:34
  • \$\begingroup\$ @JimmyB what do you mean by "during the change of the external magnetic field", you mean the field outside the shielded environment? Also if the eddy currents block some of the field change, is it possible to determine how efficient they are, relative to the efficiency of the electric shield itself, in it's "information blocking" role? I assume the magnetic field is always harder to "informationally" contain than the electric. \$\endgroup\$ – user138887 Mar 18 '17 at 14:38
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    \$\begingroup\$ Imagine I instantly (=high frequency) flip the direction of the m. field. This quick change will induce eddy currents, but only for a short while because the actual change of the field lasts for only a moment. The the eddy currents cease and the new, flipped, now static field penetrates the matierial unhindered by eddy currents. Hence, eddy currents act only as a low-pass filter for magnetic fields, not more. \$\endgroup\$ – JimmyB Mar 18 '17 at 14:42
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can information be sent out from an electrically shielded environment via alternating magnetic fields?

Yes.

How effective is an electrical shield against information blocking that may be carried by magnetic waves?

Depending on the thickness of the shielding and the frequency, anywhere from very effective to totally useless. The thinner the shield and the lower the frequency, the less effective it is at attenuating the magnetic field. Whether it will be enough to 'block' the signal also depends on distance and sensitivity of the receiver, and the nature of the signal.

Real shields don't completely block the electric field either. If you put a sensitive receiver close enough to a 'totally' shielded high frequency transceiver you could probably pick up enough rf to get some information from it.

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A superconducting material will block both DC and AC fields totally, because it allows a current to flow that exactly cancels the impinging field. Materials with nonzero resistivity will be less effective, because the energy of the induced current gets dissipated as heat, causing the current to die out. This is why conductive boxes provide better shielding at higher frequencies.

Magnetic materials (high permeability) block magnetic fields because they readily form an internal field that cancels the impinging field. Unless the permeability is infinite, there will still be some field that escapes, but modern practical materials offer very high attenuation values.

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Because they are each intertwined you can think of magnetic signals in the same way as electric signals, but in a 90 degree rotation.

You can literally understand a shield as very much like an inductor in a signal line. As you are aware, an inductor "BLOCKS" sudden transitions or high frequency electrical voltage signals.

In the same way, a shield BLOCKS sudden changed in magnetic field and attenuates the level of alternating fields.

enter image description here

Just like a step voltage "decays" across an inductor in a signal line, a step magnetic field will also decay through a shield.

In an electrical inductor you have a coil of wire around a ferrous material and the inductance effect is caused by the energy you need to store to build up that magnetic field.

Similarly, in a magnetic inductor, you need to build up an electric field around the "magnetic Current". Literally electrons spinning around in imaginary wires buried inside the shield material. This is what we call "eddy currents".

So, to answer your question fully. Yes, low frequency magnetic "Signals" can be sent out of a metal box.

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    \$\begingroup\$ "...can be sent out of a metal box, as long as the skin-depth of the metal for that frequency isn't far smaller than the thickness of the box." For example, b-fields at 60Hz easily escape an aluminum box even if it's 1/8" thick. But if it's 1ft thick, the 60Hz won't escape (since skin depth is a bit over 1/4" for 60Hz for aluminum.) Yet thin aluminum easily blocks SW, VHF, microwave, etc. \$\endgroup\$ – wbeaty Aug 3 '17 at 8:33
  • \$\begingroup\$ @wbeaty yes indeed, good add, size DOES matter. \$\endgroup\$ – Trevor_G Aug 3 '17 at 13:14
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Doing lab measurements of magnetic shielding, using 1"square loops (actually 50 Ohm resistors, to avoid shorting the Signal Generator driving the primary loop and to reverse terminate the secondary high-freq waveform) into 1" loops into coax, I measured 150 nanoseconds DELAY thru the 1.4mil (35 micron) 1ounce/foot^2 copper foil.

The output amplitude was ---- this from memory; the labbook is far away, right now ---- about 5 milliVolts. Given a 5 volt drive to the primary-loop, the secondary-loop produced only 5 milliVolts. With 150 nanosecond delay. From a square-wave drive [20nS Tr,Tf?] to the primary-loop.

Thus 1,000:1 attenuation and 150nS delay. The output waveform has a "diffusion" waveshape, not a healthy "S" risetime. This response, "diffusion", is the standard waveshape predicted by the differential-equations for heat spreading in one-dimension, for ink diffusing out in water, and for magnetic fields attempting to propagate THRU (not along, but THRU) metals.

The delay is consistent with what Jackson computes in his E&M book, for that thickness of copper foil.

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