electromagnetism - Can a "static" magnetic field carry Information?

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.

Answer

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.