Information is electronically represented as some energy that is changing, therefore we measure an on state (1) and an off state (0), to create a (0,1) binary representation of the information.

Now a static magnetic field by definition has a frequency of 0 Hz, so frequency modulation can't happen. But what if we modulate the amplitude? (Tesla), would that consist as a static field?

  • Can an amplitude modulation work to send information from an electronic device that creates a magnetic field with constant frequency?
  • What if the device is already electrically shielded, as this is a follow up to my previous question. If we already have a device that is sufficiently electrically shielded, can information leak out from the magnetic field via amplitude modulation?

I am specifically interested how can / if possible / information leak out from an electrically shielded device by modulating a magnetic field.

EDIT: I made the question more precise. Referring specifically to the relationship between amplitude modulation / frequency modulation, and a static magnetic field. I believe a magnetic field with 0 frequency is only static if the amplitude is constant as well, reading the answers below.


closed as unclear what you're asking by Andy aka, laptop2d, Wesley Lee, Chris Stratton, DoxyLover Mar 14 '17 at 6:41

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    \$\begingroup\$ "modulate the amplitude" means changes over time. You're contradicting yourself if you call that field "static". \$\endgroup\$ – Marcus Müller Mar 13 '17 at 0:13
  • \$\begingroup\$ Stopping electric fields is quite easy. Stopping magnetic fields is far more difficult. There will always be some amount magnetic leakage. \$\endgroup\$ – vini_i Mar 13 '17 at 0:22
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    \$\begingroup\$ Changing magnetic field unavoidably induces an electric field. That causes a current in the electric shielding. That current cancels the changes of the magnetic field. A substantial amount of magnetic field leaks out only if the electric shield is thinner than the penetration depth (see the skin effect). That depth decreases as the frequency increases. \$\endgroup\$ – user287001 Mar 13 '17 at 0:25
  • \$\begingroup\$ If you put a bar magnet in a cardboard box, does it leak information? Not very much, but if you then observe the box with a compass needle, you can probably deduce the orientation of the magnet inside, so it's up to your to make an application determination if that constitutes "leaking information". Note that something like rotating the magnet with a motor would satisfy the definition of "modulation" so clearly very little information can be from a static field. But if the orientation of the magnet correlates with something that should be secret, recognize it has in fact been leaked. \$\endgroup\$ – Chris Stratton Mar 13 '17 at 2:24
  • \$\begingroup\$ @ChrisStratton moving the device is not relevant here. The guys above said that if the magnetic field is changing the eddy current in the shield, if well shielded, blocks that penetration, so frequency modulated magnetic field is blocked from leaking. And user287001 confirmed that amplitude modulation is essentially the same leakage. So essentially both freq. and ampl. modulation is a changing magnetic field? And a good electrical shield blocks both types of signals? \$\endgroup\$ – user138887 Mar 14 '17 at 0:11

A static field (indeed, any static symbol) conveys no information, by Shannon's theorem. This is because the probability of the state is 1, there is only one 'symbol', and if it is unchanging, the frequency bandwidth is zero.

Once one talks of amplitude modulation, however, the field is NOT static, there is a bandwidth, and a signal that can carry information. Every loop-shaped antenna picks up modulated magnetic field, so we know that kind of receiver is workable. In a sense, a permanent magnet with a shutter (driven vanes of soft steel, perhaps?) makes such an amplitude modulated signal. Such a modulator, in my auto, senses crank position from the passage of a notched rotor next to a magnet and sense coil.

The crank sensor might be less sensitive if electrically shielded, but would still work. Magnetic fields are hard to shield.

  • \$\begingroup\$ But doesn't the electric shield block that signal? If you say that the field is not static, and guys said above that non-static fields generate eddy current in the shield. So if we have a device inside a shield, wouldn't the eddy current created on the shield cancel out the magnetic field changes in order to "block the information from leaking out" the shield? \$\endgroup\$ – user138887 Mar 14 '17 at 0:22
  • \$\begingroup\$ An conductive shield does waste some energy when the B field changes, but it lets the change occur. It would take a superconductor to completely exclude it, and a lot of copper to slow the B field penetration to blind the crank sensor. At 60 Hz, the 'skin depth' in copper is 8mm. \$\endgroup\$ – Whit3rd Mar 14 '17 at 0:26
  • \$\begingroup\$ So the energy turns into heat? Sorry I'm confused. The heat is not a problem. I am interested in whether like a magnetometer outside the shield could pick up the changes in the amplitude of the magnetic field? \$\endgroup\$ – user138887 Mar 14 '17 at 0:31
  • \$\begingroup\$ Magnetic field is both generated by a static source (a magnet), and by dynamic currents in metal (as, in a 'shield'). The kind of shield that would work against electric fields at a wide range of frequencies (thin foil), only is effective at very high frequencies or very great thicknesses, for magnetic fields, because the 'shielding' depends on transient eddy currents that oppose the magnetic field change. The thin shield has resistor heat losses, which cause the current to die with time, and allow B field to progress through the shield thickness. \$\endgroup\$ – Whit3rd Mar 14 '17 at 0:38
  • \$\begingroup\$ Is that "very great thickness" are you referring to the "skin depth". \$\endgroup\$ – user138887 Mar 17 '17 at 7:58

A static magnetic field can carry information, but being static, that information cannot change.

Commonest information carried by a static magnetic field : this way is North, that way is South.

  • \$\begingroup\$ The orientation information is not relevant to my question. If we have a device in a shield, like a radio or phone, it's not like it's going to move, so that information is irrelevant. I was asking about information transmitted by the magnetic field generated by the device , if possible to penetrate the electric shield. \$\endgroup\$ – user138887 Mar 14 '17 at 0:20

Depends on your definition of static.

The presence of a magnetic field itself can carry information.

It's permeation through space-time can also carry information .

If you believe in monopoles and their interaction / entanglement, there are more possibilities then...

  • \$\begingroup\$ It can only carry information if it can be controlled/modulated. But as pointed out above by other people, if we modulate the magnetic field, the information turns into an electric field, which can be shielded. So a static magnetic field should have only 1 state, and that has 0 shannon entropy. That was my question. \$\endgroup\$ – user138887 Mar 14 '17 at 0:16

Of course information can be encoded in static magnetic fields — that's the whole idea behind all forms of magnetic recording, from the stripe on your credit card, to audio and video tapes, to the trillions of bits on a hard drive. Remember, a magnetic field is bipolar, which means that it has a definite orientation, and information can be encoded in how that orientation relates to some other reference (frequently another magnetic field).

In most cases, it's easiest to read out the data electronically by introducing motion between the recorded information and a sensor of some sort — swiping your card, moving a tape past a read/write head, spinning a hard disk — but it isn't necessary. For example, you can read out magnetic patterns optically using a "developer" fluid that contains tiny magnetic particles. There are also the mechanisms used in magnetic core memories and Hall-effect devices (such as shaft angle encoders, which can be read out regardless of whether the shaft is moving).

  • \$\begingroup\$ That is not what I was referring to. I wasn't referring to encoding information in magnetic form. I was referring to leaking / transmitting information from an electrically shielded environment via static magnetic fields, if possible. \$\endgroup\$ – user138887 Mar 14 '17 at 0:16
  • \$\begingroup\$ In that case, you're going to have to be a lot more clear about exactly what you mean by "static". \$\endgroup\$ – Dave Tweed Mar 14 '17 at 4:25

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