# Low frequency magnetic communication

1. May 5, 2008

### enroger

In low freq magnetic communication, the carrier is not EM wave but low frequency varying magnetic field. Most of the energy is transmitted within the mag field, very little result in EM wave. I find this very strange, because in Maxwell equation's plane wave solution, B is always equal to E/c.

According to the link this site don't allow me to post(@\$&^&*%), they do it by having the antenna a lot smaller than wavelength.

My question then is when is E = B/c valid? When not?

2. May 5, 2008

### Staff: Mentor

I suspect that what you're asking about is more commonly called Near field magnetic communication. From what I gather by Googling around a bit, you make the magnetic field around the transmitter oscillate in such a way that the far-field electromagnetic radiation (whose energy falls off as $1/r^2$ is negligible. Almost all of the energy goes into the oscillation of the near-field which falls off as $1/r^6$. This gives a very short range, up to a couple of meters or so, which has advantages in some situations.

3. May 5, 2008

### enroger

Yes, that's it. This is just like an ordinary solenoid. What I don't understand is what determines you have far field or near field emission?

I mean with the plane wave solution B = E/c, and that any solution would be a superposition of plane wave solution, then there should be no near field, everything must be far field right?

Does it have something to do with the size of the antenna compared to wavelength?

4. Jul 23, 2009

### leks.tl

I have a question.
Does anybody research effect of magnetic field to the communication equipment (e.t. access point, subscriber module or something like that)?
Can you refer me on the link where I can find this information, please?

BR

5. Jul 23, 2009

### Pumblechook

Induction or near field .. was soon abandoned in Marconi days.

6. Jul 23, 2009

### Bob S

Could you be talking about the TTE (through the earth) wireless communication devices for communicating with trapped coal miners in mines several hundred meters below the surface? I believe these are near-field communication devices where the ratio of E/H is not important. Specifically the near field of a magnetic loop (dipole) will penetrate more than the near field of an electric dipole, just because of the electric field attenuation due to the electrical conductivity of soil and rocks. Google TTE emergency rescue communication.

7. Jul 23, 2009

### Pumblechook

May be that low frequequency is required where wire dipoles would be very long indeed so loops are used.

The near field soon falls off with distance. It requires a proper EM wave to propagate over any distance even though magnetic loops are used as the aerials. I have used loops on the AM band broadcast band for long distance recpetion which can't be near field..not over a few thousand km.

8. Jul 24, 2009

### leks.tl

Maybe dipole made problem, because near dipole access point broke down to boot mode. Also near dipoles there are a lot of other radio and wireless equipment. Can you refer me on some literature or recommend me literature for this topic please?

9. Jul 25, 2009

### Bob S

do a Google search on low frequency magnetic communication coal mine, and low frequency magnetic communication TTE

10. Jul 25, 2009

### fleem

That equation only applies to the far field. It doesn't mean there is no near field. There is.